28.5B: Classes of Echinoderms - Biology

28.5B: Classes of Echinoderms - Biology

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Learning Objectives

  • Differentiate among the classes of echinoderms

The phylum echinoderms is divided into five extant classes: Asteroidea (sea stars), Ophiuroidea (brittle stars), Echinoidea (sea urchins and sand dollars), Crinoidea (sea lilies or feather stars), and Holothuroidea (sea cucumbers).

The most well-known echinoderms are members of class Asteroidea, or sea stars. They come in a large variety of shapes, colors, and sizes, with more than 1,800 species known so far. The key characteristic of sea stars that distinguishes them from other echinoderm classes includes thick arms (ambulacra; singular: ambulacrum) that extend from a central disk where organs penetrate into the arms. Sea stars use their tube feet not only for gripping surfaces, but also for grasping prey. Sea stars have two stomachs, one of which can protrude through their mouths and secrete digestive juices into or onto prey, even before ingestion. This process can essentially liquefy the prey, making digestion easier.

Brittle stars belong to the class Ophiuroidea. Unlike sea stars, which have plump arms, brittle stars have long, thin arms that are sharply demarcated from the central disk. Brittle stars move by lashing out their arms or wrapping them around objects and pulling themselves forward. Of all echinoderms, the Ophiuroidea may have the strongest tendency toward 5-segment radial (pentaradial) symmetry. Ophiuroids are generally scavengers or detritivores. Small organic particles are moved into the mouth by the tube feet. Ophiuroids may also prey on small crustaceans or worms. Some brittle stars, such as the six-armed members of the family Ophiactidae, are fissiparous (divide though fission), with the disk splitting in half. Regrowth of both the lost part of the disk and the arms occur, yielding an animal with three large arms and three small arms during the period of growth.

Sea urchins and sand dollars are examples of Echinoidea. These echinoderms do not have arms, but are hemispherical or flattened with five rows of tube feet that help them in slow movement; tube feet are extruded through pores of a continuous internal shell called a test. Like other echinoderms, sea urchins are bilaterans. Their early larvae have bilateral symmetry, but they develop fivefold symmetry as they mature. This is most apparent in the “regular” sea urchins, which have roughly spherical bodies, with five equally-sized parts radiating out from their central axes. Several sea urchins, however, including the sand dollars, are oval in shape, with distinct front and rear ends, giving them a degree of bilateral symmetry. In these urchins, the upper surface of the body is slightly domed, but the underside is flat, while the sides are devoid of tube feet. This “irregular” body form has evolved to allow the animals to burrow through sand or other soft materials.

Sea lilies and feather stars are examples of Crinoidea. Both of these species are suspension feeders. They live both in shallow water and in depths as great as 6,000 meters. Sea lilies refer to the crinoids which, in their adult form, are attached to the sea bottom by a stalk. Feather stars or comatulids refer to the unstalked forms. Crinoids are characterized by a mouth on the top surface that is surrounded by feeding arms. They have a U-shaped gut; their anus is located next to the mouth. Although the basic echinoderm pattern of fivefold symmetry can be recognized, most crinoids have many more than five arms. Crinoids usually have a stem used to attach themselves to a substrate, but many live attached only as juveniles and become free-swimming as adults.

Sea cucumbers of class Holothuroidea are extended in the oral-aboral axis and have five rows of tube feet. These are the only echinoderms that demonstrate “functional” bilateral symmetry as adults because the uniquely-extended oral-aboral axis compels the animal to lie horizontally rather than stand vertically. Like all echinoderms, sea cucumbers have an endoskeleton just below the skin: calcified structures that are usually reduced to isolated microscopic ossicles joined by connective tissue. In some species these can sometimes be enlarged to flattened plates, forming armor. In pelagic species, such as Pelagothuria natatrix, the skeleton and a calcareous ring are absent.

Key Points

  • Sea stars have thick arms called ambulacra that are used for gripping surfaces and grabbing hold of prey.
  • Brittle stars have thin arms that wrap around prey or objects to pull themselves forward.
  • Sea urchins and sand dollars embody flattened discs that do not have arms, but do have rows of tube feet they use for movement.
  • Sea cucumbers demonstrate “functional” bilateral symmetry as adults because they actually lie horizontally rather than stand vertically.
  • Sea lilies and feather stars are suspension feeders.

Key Terms

  • ossicle: a small bone (or bony structure), especially one of the three of the middle ear
  • fissiparous: of cells that reproduce through fission, splitting into two
  • ambulacrum: a row of pores for the protrusion of appendages such as tube feet.

Developmental transcriptomics of the brittle star Amphiura filiformis reveals gene regulatory network rewiring in echinoderm larval skeleton evolution

Amongst the echinoderms the class Ophiuroidea is of particular interest for its phylogenetic position, ecological importance and developmental and regenerative biology. However, compared to other echinoderms, notably echinoids (sea urchins), relatively little is known about developmental changes in gene expression in ophiuroids. To address this issue, we have generated and assembled a large RNAseq data set of four key stages of development in the brittle star Amphiura filiformis and a de novo reference transcriptome of comparable quality to that of a model echinoderm—the sea urchin Strongylocentrotus purpuratus. Furthermore, we provide access to the new data via a web interface:


We have identified highly conserved genes associated with the development of a biomineralised skeleton. We also identify important class-specific characters, including the independent duplication of the msp130 class of genes in different echinoderm classes and the unique occurrence of spicule matrix (sm) genes in echinoids. Using a new quantification pipeline for our de novo transcriptome, validated with other methodologies, we find major differences between brittle stars and sea urchins in the temporal expression of many transcription factor genes. This divergence in developmental regulatory states is more evident in early stages of development when cell specification begins, rather than when cells initiate differentiation.


Our findings indicate that there has been a high degree of gene regulatory network rewiring and clade-specific gene duplication, supporting the hypothesis of a convergent evolution of larval skeleton development in echinoderms.

Multi-celled animals (Metazoa)


5 pages with photos of echinoderms


There are 5 related classes in the phylum Echinodermata (the Latin name means "spiny-skinned"). For a detailed list with all classifications click here:

Characteristics of Echinoderms

Echinoderms are characterized by radial symmetry, several arms (5 or more, mostly grouped 2 left - 1 middle - 2 right) radiating from a central body (= pentamerous). The body actually consists of five equal segments, each containing a duplicate set of various internal organs. They have no heart, brain, nor eyes, but some brittle stars seem to have light sensitive parts on their arms. Their mouth is situated on the underside and their anus on top (except feather stars, sea cucumbers and some urchins).

Echinoderms have tentacle-like structures called tube feet with suction pads situated at their extremities. These tube feet are hydraulically controlled by a remarkable vascular system. This system supplies water through canals of small muscular tubes to the tube feet (= ambulacral feet). As the tube feet press against a moving object, water is withdrawn from them, resulting in a suction effect. When water returns to the canals, suction is released. The resulting locomotion is generally very slow.

Ecology and range of Echinoderms

Echinoderms are exclusively marine. They occur in various habitats from the intertidal zone down to the bottom of the deep sea trenches and from sand to rubble to coral reefs and in cold and tropical seas.

Behavior of Echinoderms

Some echinoderms are carnivorous (for example starfish) others are detritus foragers (for example some sea cucumbers) or planktonic feeders (for example basket stars).

Reproduction is carried out by the release of sperm and eggs into the water. Most species produce pelagic (= free floating) planktonic larvae which feed on plankton. These larvae are bilaterally symmetrical, unlike their parents (illustration of a larvae of a sea star below). When they settle to the bottom they change to the typical echinoderm features.

Echinoderms can regenerate missing limbs, arms, spines - even intestines (for example sea cucumbers). Some brittle stars and sea stars can reproduce asexually by breaking a ray or arm or by deliberately splitting the body in half. Each half then becomes a whole new animal.

Echinoderms are protected through their spiny skins and spines. But they are still preyed upon by shells (like the triton shell), some fish (like the trigger fish), crabs and shrimps and by other echinoderms like starfish which are carnivorous. Many echinoderms only show themselves at night (= nocturnal), therefore reducing the threat from the day time predators.

Echinoderms serve as hosts to a large variety of symbiotic organisms including shrimps, crabs, worms, snails and even fishes.

Sea stars (starfish)

Characteristics of sea stars (or starfish)

Sea stars are characterized by radial symmetry, several arms (5 or multiplied by 5) radiating from a central body. Mouth and anus are close together on the underside, the anus is at the center of the disc together with the water intake (madreporite). The upper surface is often very colorful. Minute pincer-like structures called pedicellaria are present. These structures ensure that the surface of the arms stay free from algae. The underside is often a lighter color.

There are a few starfish that have 6 or 7 arms, for example Echinaster luzonicus or Protoreaster, some even more like the elven-armed sea star (Coscinasterias calamaria). Others normally have 5 arms but now have more arms, because after an injury an arm divided and grew into two arms.

Ecology and range or sea stars

The starfish lives everywhere in the coral reef and on sand or rocks.

Behavior of sea stars

The ability of an organism to grow a body part that has been lost

The spontaneous self amputation of an appendage when the organism is injured or under attack. The autotomized part is usually regenerated.

Is asexual reproduction in which an outgrowth on the parent organism breaks off to form a new individual

Self-division into two parts, each of which then becomes a separate and independent organisms (asexual reproduction)

The majority of sea stars are carnivorous and feed on sponges, bryozoans, ascidians and molluscs. Other starfishes are detritus feeders (detritus = organically enriched film that covers rocks) or scavengers. Some starfish are specialized feeders, for example the crown-of-thorns that feeds on life coral polyps.

Starfish have no hard mouth parts to help them capture prey. The stomach is extruded over the prey, thus surrounding the soft parts with the digestive organs. Digestive juices are secreted and the tissue of the prey liquefied. The digested food mass, together with the stomach is then sucked back in. This method can be observed, if you turn around a starfish, that sits on prey or sand - you will see the stomach retreating.

Starfish are well known for their powers of regeneration. A complete new animal can grow from a small fragment such as a arm. In some species (Linckia multifora and Echinaster luzonicus) one of the arms will virtually pull itself away, regenerates and forms a new animal. Autotomy (self amputation) usually is a protective function, losing the body part to escape a predator rather than being eaten. But here it serves as a form of asexual reproduction. In other species of sea stars (Allostichaster polyplax and Coscinasterias calamaria) the body is broken into unequal parts (= fission) then the missing limbs regenerate.

Predators of starfishes

Tritonshorn - Charonia tritonis

Harlequin Shrimp - Hymenocera elegans

Harlequin Shrimp is carrying a sea star - Hymenocera elegans

The crown-of-thorns (Acanthaster planci) is one of the largest and the most venomous starfishes. It can reach 50 cm diameter and has numerous (10 to 20) spiny arms with formidable thorn like toxic spines. Don't touch them! A species of small cardinalfishes (Siphamia fuscolineata) and a commersal shrimp (Perliclimenes soror) live among those spines. The crown-of-thorns feed on live coral polyps. They "graze" the corals which are left behind white and dead. Their predators are the giant triton shell (Charonia tritonis) and some puffer fish. Scientist have also found out, that some crown of thorns are deterred from eating the coral polyps by the small crabs living among the coral branches (Trapezia sp). These crabs defend their coral host by breaking them off at the pedicellaria. Other small crabs (Tetralia sp) only pinch the tube feets of the starfish. Crown of thorns prefer corals, that are not hosts to these crabs.

The cushion star (Culcita nouvaeguineae) doesn't look like a starfish at all, more like a large sea urchin without spines. Its pentagonal appearance gives only the slightest indication that this organism is related to other starfish.

Photos of sea stars (photo collection) click for enlargement

Crown-of-thorns starfish (Acanthaster planci)

Spiny Cushion Starfish - Halityle regularis

Necklace Sea Star - Fromia monilis

Starfish / sea star (Nardoa variolata)

Horned Sea Star - Protoreaster nodosus

Egyptian Sea Star - Gomophia egyptiaca

Thyca crystallina - this snail lives on sea stars

Starfish Shrimp - Periclimenes soror

Zenopontonia noverca - Starfish Shrimp

Regeneration of an arm: Luzon Sea Star- Echinaster luzonicus

Starfish Shrimp - Periclimenes soror

Comb Jelly on Starfish - Coeloplana astericola

Feather stars

Characteristics of feather stars

Feather stars also known as crinoids. They are characterized by radial symmetry. The body of a typical feather star is cup-shaped, their numerous feathery arms project from a central disc. Some have five arms, others as many as 200. The arms, called pinnules are coated with a sticky substance that helps to catch food. There are appendages known as cirri attached to the underside of the body with which they cling to to sponges or corals. Both their mouth and their anus are situated on the upper side.

Ecology and range of feather stars

Feather stars are primarily nocturnal but they are seen in the open during the day with their arms rolled up.

Behavior of feather stars

Feather stars can crawl, roll, walk and even swim but usually they cling to sponges or corals. Feather stars are very abundant in areas exposed to periodic strong currents, because they feed on plaktonic food.

Numerous animals live in close association with feather stars. Echinoderms are hosts to various symbiotic animals such as the crinoid clingfish (Discotrema crinophila), the elegant squat lobster (Allogalathea elegans) or the crinoid shrimp (Periclimenes sp.). These animals receive shelter and food (left over) and also feed on microorganisms living on feather stars.

Photos of feather stars (photo collection) click for enlargement

Feather star (Stephanometra sp.) - gallery

Feather star (Lamprometra sp) half open, holding on to sponge with its cirri (appendages)

Rolled up feather star (Himerometra robustipinna) by day

Central body of a feather star with mouth and anus

Pinnules of a feather star (Pontiometra) coated to help catch food

Pontoniopsis comanthi - Comanthus shrimp

Laomenes sp8 - Ffeather star shrimp

Diademichthys lineatus - Clingfish

Myzostoma sp - worm living on feather stars

Brittle stars
Basket star

Characteristics of brittle stars

Brittle stars are close relatives of sea stars. Characterized by radial symmetry with a central body from which five snakelike arms protrude. The arms are highly flexible. There is no replication of internal organs, just one set in the central disk. Compared to starfish, brittle stars have a much smaller central disc and no anus. Wastes are eliminated through the mouth which is situated on the underside center.

On the underside of the body disk there is a splitlike opening at the base of each side of each arm. These ten openings are breathing and reproductive outlets, taking in water for oxygen and shedding eggs or sperm into the sea.

The basket stars are a specialized type of brittle stars. They have a series of complexly branched arms which are used to catch plankton.

Serpent stars are seen coiled snakelike around branches of gorgonians.

Ecology and range of brittle stars

Brittle stars are very cryptic and hide in crevices under corals. Best seen at night time, when they emerge to feed on plankton. Usually at places exposed to strong currents.

Serpent stars feed mostly on small invertebrates like mollusks, worms and crustaceans and are generally found in crevices and beneath rocks or in holes in the sand.

Snake stars (for example Ophiothela danae) are found entwined in the branches of black corals or gorgonians where they feed on the rich mucus of their host, in turn performing cleaning functions.

Behavior of brittle stars

As the name suggests, the arms of the brittle stars are rather liable to break. This is actually an escape mechanism. Those arms regenerate quickly and an entire new organism can regenerate, if the broken arm is attached to a seizable portion of the disk. Brittle stars can reproduce asexually by self-division. Brittle stars are the most active and fastest moving echinoderms.

Brittle stars feed on plankton but also on detritus, coral-shed mucus, bottom detritus (detritus = organically enriched film that covers rocks), mollusks and worms.

Photos of brittle stars (photo collection) click for enlargement

Brittle Star - Ophiothela sp

Many snake stars (Ophiothela danae) on gorgonian

Ophiothrix martensi - Martens brittle star

INFO - Serpent star (Ophiarachna incrassata)

Erna's basket star (Astroboa ernae)

Ophiothela danae - brittle star

Periclimenes lanipes - Basket Star Shrimp
Copyright Johanna Gawron

Periclimenes lanipes - Basket Star ShrimpGarnele

basket star (Astroglymma sculptum)

Sea urchin

Characteristics of sea urchins

Radial symmetrical body with a external chitinous skeleton and a centrally located jaw (called Aristotle's lantern) with horny teeth. The mouth consists of a complex arrangement of muscles and plates surrounding the circular opening. The anus is located on the upper surface. Some sea urchins have a spherical, bulb like cloaca (to store fecal material) that protrudes from the anal opening. It can be withdrawn into the shell.

Depending on the species, movable spines of various sizes and forms are attached to the body. These spines often are sharp, pointed and in some cases even venomous. Pincer like pedicellaria for grabbing small prey. Some pedicellaria are also poisonous.

Ecology and range of sea urchins

Rubble and sand. An abundance of sea urchins can be a sign for bad water conditions.

Behavior of sea urchins

Locomotion by tube feet but also by movement of the spines on the underside of the body. Sea urchins are generally nocturnal, during the day they hide in crevices. However some sea urchins such as Diadema sometimes form large aggregations in open exposed areas. Despite their sharp spines sea urchins are easy game for some fishes, particularly triggerfishes and puffers. A triggerfish grabs the sea urchin with its hard beak like mouth by the spines or it blows some water towards the sea urchin and turns it on its back. The underside of a sea urchin has much shorter spines and those are easily crushed. During the breeding season the body cavity is crammed with eggs or sperms. This is one of the main reasons urchins are so attractive to fish predators (Japanese also like them for the same reason).

Some sea urchins are camouflaged. They hold on with their tube feet onto some bottom debris like rubble or pieces of seagrass and carry them on their back. Some even carry live soft corals or anemones.

Most sea urchins are algal grazers but some feed on sponges, bryozonans and ascidians and others on detritus (detritus = organically enriched film that covers rocks).

The sexes are separate and the young are formed indirectly by the fusion of sperm and eggs released into the water.

Sea urchin cardinalfish

Aeoliscus strigatus - Centriscidae)

Sea urchin shrimp
Mandarinfish, dragonet

Many animals live in symbiotic relation with sea urchins. Even on the poisonous spines of the fire urchin (Asthenosoma varium) small shrimps (Periclimenes colemani) can be found. One shrimp (Stegopontonia commensalis) is striped black and white lengthwise and perfectly camouflaged and lives in spines of the long-spined sea urchin (Diadema setosum). Some cardinalfishes and juvenile shrimpfishes also like to take shelter in-between these spines, but even small cuttlefish hide there. It has been observed, that they change their coloring also to black and white. Some flatworms wrap around the thicker spines of the diadema sea urchin (Echinothrix calamaris).

The mandarin dragonet (Mandarinfish) lives close to congregations of sea urchins and hides among them if threatened.

There are two specialized types of sea urchins with an unusual appearance: the sand dollar is very much flattened with very small spines and the heart urchin which are oval and have bristle like spines. The both bury in sand. The heart urchin "jumps" out of the sand, when disturbed.

Photos of sea urchins (photo collection) click for enlargement

Heart Sea Urchin - Maretia planulata

Sea urchin (Prionocidaris verticillata)

Sea urchin (Astropyga radiata)

Sea Urchin - Diadema setosum

Toxic sea urchin (Asthenosoma pileolus)

Matha's sea urchin (Echinometra mathaei)

Zebracrab (Zebrida adamsii) on sea urchin

shrimp (Stegopontonia commensalis)

Coleman shrimp (Periclimenes colemani)

Comb yellies on seeurchin - Coeloplana sp

Shrimpfish (Aeoliscus strigatus)

Urchin clingfish - Diademichthys lineatus


Characteristics of sea cucumbers

Unlike other echinoderms, holothurians don't have a distinct radial symmetry but are bilateral (distinct dorsal and ventral side). Holothurians are also called sea cucumbers. As their name suggests, they are cucumber shaped with an elongated, muscular, flexible body with a mouth at one end and the anus at the other. Around the mouth there is a number of tentacles (modified tube feet) used in food collecting. Sea cucumbers come in many sizes, from small species only a few centimeter in length to long snakelike animals which may stretch up to 2 meter!

Ecology and range of sea cucumbers

Rubble, rocks and sand. Also seen on some sponges in large aggregations.

Behavior of sea cucumbers

Most species feed on the rich organic film coating sandy surfaces. The crawl over the bottom ingesting sand. The edible particles (organic matter such as plankton, foraminifera and bacteria) are extracted when passing through their digestive tract and the processed sand is expelled from the anus (as worm-like excrements).

Sea cucumbers move by means of tube feet which extend in rows from the underside of the body. The tentacles surrounding the mouth are actually tube feet that have been modified for feeding.

Other holothurians feed on current-borne zooplankton. They bury in sand extruding their featherlike tentacles (Pseudocolochirus violaceus, Neothyondium magnum or Pentacta crassa). The tentacles have the same shape as soft corals or some anenemones. Large congregations of some small species are found on sponges. They apparently feed on substances secreted by the sponges as well as detritus from the surface.

Some species of holothurians have separate sexes others are hermaphrodites. The sea cucumbers hold on to exposed rocks or corals, raise their body to a upright position, rock back and forth and release the sperm and eggs into the sea.

Sea cucumbers have a remarkable capacity for regenerating their body parts. When attacked they shed a sticky thread like structure which is actually parts of their guts. The so called Cuverian threads are toxic (the poison is called holothurin) and can dissuade many potential predators. These structures quickly regenerate. (see photos below)


Encheliophis homei and mourlani / Onuxodon margaritiferae

Holothurians host a variety of symbiotic organisms: crabs, shrimps, worms and even a very unusual fish. The pearlfish (Encheliophis homei and mourlani / Onuxodon margaritiferae) has a long slender, transparent body and lives in the gut cavity of the sea cucumber (Boshida argus, Thelanota ananas, Stichopus chloronotus). They also inhabit some starfish as well as pearl oyster shells. The fish leaves and enters (tail first) through the holothurian's anus. They probably feed on the gonads and other tissues of its host. It is said to leave at night to feed on small fishes and shrimps. Sea cucumbers are used in Asia as a base for soups.

Photos of sea cucumbers (photo collection) click for enlargement

Sea cucumber (Bohadschia argus) with Cuiverian threads

Sea cucumber (Bohadschia argus) with Cuiverian threads

INFO - Emperor Shrimp on Sea Cucumber - Periclimenes imperator on Opheodesoma australiensis

Black Sea Cucumber - Holothuria atra

Pineapple sea cucumber (Thelenota ananas)

INFO - Sea Cucumber - Synaptula media

Sea Cucumber details tentacles (Synapta maculata)

Sea cucumber skin (Thelenota rubrolineata)

Horrid Sea Cucumber - Stichopus horrens

Galathea sp1 - crab living on sea cucumbers

Imperator shrimp on sea cucumber - Periclimenes imperator auf Opheodesoma australiensis

Echinodermata: History, Characters and Classification

Echinodermata form a well defined and successful group of marine animals existing since the Palaeozoic, they live at the bottom of all seas creeping about slowly, though some can swim. They exhibit great diversity of form and habit, and form a peculiar group. The body is made of 10 principal divisions which radiate from a main axis, they are five radii and five inter-radii.

The surface having a mouth is oral or ambulacral, and the opposite surface is aboral or adambulacral. Tube feet project from the ambulacral surface forming radial bands called ambulacra.

In Asteroidea and Crinoidea the tube feet of each ambulacrum project on either side of an ambulacral groove at the bottom of which lies a radial nerve cord, but in other classes the ambulacral groove is closed, so that it forms an epineural canal enclosing the nerve cord. Main axis of the body passes between these two surfaces, and the length of the axis determines the shape of the body.

The axis is short in starfishes with a lower aboral surface in others the axis is long, in sea cucumbers the oral surface with the mouth is anterior and the animal lies with the main axis parallel to the ground, in sea lilies oral surface is uppermost.

In Asteroidea, Ophiuroidea and Crinoidea the body is prolonged into arms in the direction of radii, and ambulacral surfaces are sub-equal, but in Holothuroidea and Echinoidea the ambulacral surface extends over most of the compact body leaving only a small aboral area opposite the mouth.

Many Echinodermata possess a faculty of self-mutilation or autotomy by which they can break off their arms or throw out their internal organs when molested, this faculty along with the capacity for regeneration is most marked in many ophiuroids, some asteroids, some holothurians and some crinoids, but it does nor occur in echinoids.

Echinodermata differ from all other coelomate animals mainly due to their radial symmetry, this symmetry is derived secondarily froma bilateral condition and it distorts all their systems. Some structures are bilateral, but externally the symmetry is never quite perfect because a madreporite or anus or a genital opening makes one of the interradii different from others.

2. History of Echinodermata:

The name Echinodermata (Gr., echinos = hedgehog + derma = skin) appears to have originated with Jacob Klein (1734), who, however, applied it only to echinoids.

Echinodermata are all exclusively marine animals living on the shore but mostly on the bottom of the sea. They are coelomate animals with pentaramous radial symmetry, that is the body can be divided into five parts arranged around a central axis, but the larva is bilaterally symmetrical.

There is no head. They have an endoskeleton of calcareous ossicles made from mesoderm, there are also external spines which may be movable or fixed. A large ciliated enterocoelous coelom forms a perivisceral cavity and several intricate systems, one of which is a water vascular system from which project delicate tube feet. Respiratory organs are minute gills protruding out from the coelom.

There is no definite blood vascular system, it is represented only by lacunar tissue, there are no definite excretory organs. Nervous system forms a ring around the mouth with nerves radiating from it, it is the principal nervous system and is in contact with the ectoderm, in addition there is a deeper nervous system lying in the mesoderm.

Sexes are usually separate but copulation does not take place, the gonads discharge to the exterior and fertilisation takes place in sea water. Echinodermata have no parasitic forms. They possess great powers of regeneration.

Echinodermata have a world-wide distribution and the phylum contains some 5,300 known species and a large number of fossil forms. The phylum is divided into two subphyla, viz., Pelmatozoa and Eleutherozoa, Pelmatozoa, has only one living class Crinoidea, while Eleutherozoa has four living classes, Holothuroidea, Echinoidea, Asteroidea, Ophiuroidea.

3. General Characters of Echinodermata:

1. The echinoderms are exclusively marine and are among the most common and widely distributed of marine animals.

2. They occur in all seas from the intertidal zone to the great depths.

3. Symmetry usually radial, nearly always pentamerous.

4. Body is triploblastic, coelomate with distinct oral and aboral surfaces and without definite head and segmentation.

5. They are of moderate to considerable size but none are microscopic.

6. Body shape rounded to cylindrical or star-like with simple arms radiating from a central disc or branched feathery arms arise from a central body.

7. Surface of the body is rarely smooth, typically it is covered by five symmetrically spaced radiating grooves called ambulacra with five alternating inter-radii or inter-ambulacra.

8. Body wall consists of an outer epidermis, a middle dermis and an inner lining of peritoneum.

9. Endoskeleton consists of closely fitted plates forming a shell usually called theca or test or may be composed of separate small ossicles.

10. Coelom is spacious lined by peritoneum, occupied mainly by digestive and reproductive systems and develops from embryonic archenteron, i.e., enterocoel.

11. Presence of water vascular or ambulacral system is the most characteristic feature. It consists of tubes filled with a watery fluid.

12. Alimentary tract is usually coiled tube extending from the mouth located on the oral surface to the anus on the aboral or oral surface.

13. Circulatory or haemal or blood lacunar system is typically present.

14. Respiration occurs through a variety of structures, i.e., papulae in starfishes, peristomial gills in sea urchins, genital bursae in brittle stars and cloacal respiratory trees in holothurians.

15. Excretory system is wanting.

16. Nervous system is primitive, consisting of networks concentrated into the radial ganglionated nerve cords.

17. Sense organs are poorly developed.

18. Sexes are usually separate (dioecious) with few exceptions. Gonads are simple with or without simple ducts.

19. Reproduction is usually sexual, few reproduce asexually or by regeneration.

20. Fertilisation is external, while few echinoderms are viviparous.

21. Development is indeterminate including characteristic larvae which undergo metamorphosis into the radially symmetrical adults.

4. Classification of Echinodermata:

The classification is adopted from Hyman, L. H. (1955). Only living classes and orders have been described.

Subphylum I. Pelmatozoa:

(Gr., pelmatos = stalk + zoon = animal):

1. Mostly extinct echinoderms.

2. Body is attached by the aboral surface or by an aboral stalk.

3. Mouth and anal aperture present on the oral surface facing upwards.

4. Viscera is enclosed in a calcareous test.

5. Tube feet or podia are primarily food catching and devoid of suckers.

6. Main nervous system is aboral.

7. Pelmatozoa has only one living class.

Class 1. Crinoidea: (Gr., crinon = lily + eidos = form):

1. Both extinct and living forms.

2. Living members are without stalk and free moving but extinct forms attached by a stalk.

3. Body consists of an aboral cup, the calyx and oral cover or roof, the tegmen and strongly pentamerous in structure.

4. Oral surface is directed upwards.

5. Mouth usually central, anus usually eccentric are present on the oral surface.

6. Arms movable, simple, mostly branched, usually five or ten in number with or without pinnules.

7. Ambulacral grooves are open and extend along arms and pinnules to their tips.

8. Madreporite, spines and pedicellariae are present.

9. Sexes are separate. Larva doliolaria.

10. Commonly called sea lilies or feather stars.

1. Extinct and living crinoids.

2. Calyx pentamerous, flexible incorporating the lower arm ossicles.

3. Tegmen leathery containing calcareous particles or small plates.

4. Mouth and ambulacral grooves exposed.

Antedon, Rhizocrinus, Metacrinus.

Subphylum II. Eleutherozoa:

(Gr., eleutheros = free + zoon = animal):

1. Mostly living echinoderms.

2. Stem or stalk absent, usually free living forms.

3. Body structure usually pentamerous.

4. Oral surface bearing the mouth is downward or lying on one side.

5. Anus usually on the aboral surface.

6. Ambulacral grooves usually not for food gathering and the tube feet with suckers are chiefly locomotory organs.

7. Main nervous system is oral.

Class 1. Holothuroidea: (Gr., holothurion = water polyp + eidos = form):

1. Body bilaterally symmetrical, usually elongated in the oral-aboral axis having mouth at or near one end and anus at or near the other end.

3. Endoskeleton reduced to microscopic spicules or plates embedded in the body wall.

4. Mouth surrounded by a set of tentacles attached to water vascular system.

5. Podia or tube feet are usually present and locomotory.

6. Alimentary canal is long and coiled and cloaca usually with respiratory trees.

7. Sexes are usually separate and gonad single or paired tufts of tubules.

8. Commonly called sea cucumbers.

Order 1. Aspidochirota:

1. Podia or tube feet are numerous.

2. Mouth is surrounded by 10-30 mostly 20 peltate or branched oral tentacles.

3. Retractor muscles of pharynx are absent.

4. A pair of well developed respiratory trees is present.

Holothuria, Stichopus, Mesothuria.

1. Numerous podia or tube feet.

2. Mouth is usually ventral and surrounded by 10-20 peltate or branched tentacles.

4. Respiratory tree is absent.

Order 3. Dendrochirota:

1. Podia or tube feet are numerous.

2. Oral tentacles are dendritic or branched or branched like tree branches.

3. Oral retractors are absent.

4. Respiratory trees are present.

Thyone, Cucumaria, Phyllophorus.

1. Podia or tube feet are absent except as anal papillae.

2. Oral tentacles are digitate or finger-shaped.

3. Oral retractors are absent.

4. Respiratory trees are present.

5. Posterior region is generally tapering into a caudal portion.

1. Body vermiform having smooth or warty surface.

2. Podia or tube feet are absent.

3. Oral tentacles are 10-20, simple, digitate or pinnate.

4. Pharyngeal retractors are present in some forms.

5. Respiratory trees are absent.

6. Water vascular system is greatly reduced.

Class 2. Echinoidea: (Gr., echinos = hedgehog + eidos = form):

1. Body is spherical, disc-like, oval or heart- shaped.

2. Body is enclosed in an endoskeletal shell or test of closely fitted calcareous plates covered with movable spines.

3. Outer calcareous plates are distinguished into five alternating ambulacral and five inter-ambulacral areas.

4. Podia or tube feet come out from the pores of ambulacral plates and are locomotory in function.

5. Mouth is centrally placed on the oral surface and surrounded by a membranous peristome. Anus is located at the aboral pole and surrounded by membranous periproct.

6. Ambulacral grooves are absent.

7. Pedicellariae are stalked and three jawed.

8. Sexes are separate. Gonads are pentamerous.

9. Development includes a free swimming echinopluteus larva.

10. Commonly called sea urchins and sand dollars.

Subclass I. Bothriocidaroida:

1. Each inter-ambulacral is with single row of plates.

2. Madreporite radially placed.

4. Includes a single extinct Ordovician genus.

Subclass II. Regularia:

1. Body is globular, mostly circular and sometimes oval in shape.

2. Symmetry is pentamerous with two rows of inter-ambulacral plates.

3. Mouth is centrally located at the oral surface and surrounded by peristome.

4. Anus is centrally placed at the aboral pole surrounded by periproct.

5. Aristotle’s lantern is well developed.

6. Madreporite is ambulacral.

Order 1. Lepidocentroida:

1. Test flexible with overlapping or separated plates.

2. Ambulacral plates continue up to mouth lip.

1. Test is rigid and globular.

2. Two rows of long narrow ambulacral plates and two rows of inter-ambulacral plates are present.

3. Ambulacral and inter-ambulacral plates continue up to mouth lip.

4. Gills and sphaeridia are absent.

5. Five bushy Stewart’s organs are present appended to the lantern.

1. Test is symmetrical and globular.

2. Test composed of two rows each in a ambulacral and inter-ambulacral plates.

3. Ambulacral and inter-ambulacral plates reach up to the margin of peristome.

4. Gills and sphaeridia are present.

5. Teeth of Aristotle’s lantern are devoid of keel.

1. Test is rigid and rarely oval.

2. Epiphyses of lantern are enlarged and meeting above the pyramids.

4. All the four types of pedicellariae are present.

Subclass III. Irregularia:

1. Test is mostly flattened oval to circular.

3. Mouth centrally placed on the oral surface.

4. Anus is displaced posteriorly generally marginal at oral or aboral surface and lies outside the apical system of plates.

5. Podia or tube feet are not locomotory.

Order 1. Clypeastroida:

1. Test is flattened, oval or rounded in shape covered with small spines.

2. Mouth and apical system are usually central and oral in position.

3. Aboral ambulacral areas petaloid.

4. Aristotle’s lantern present.

1. Test is oval or heart-shaped.

2. Four aboral ambulacral areas petaloid, fifth not petaloid.

3. Aristotle’s lantern absent.

Spatangus, Lovenia, Echinocardium.

Class 3. Asteroidea: (Gr., aster = star + eidos = form):

1. Body is flattened, pentagonal or star- shaped.

2. Oral and aboral surfaces are distinct, the oral surface directed downwards and aboral surface upwards.

3. Five to fifty long or short rays or arms radiating symmetrically from a central disc.

4. Mouth is centrally placed at the oral surface surrounded by a membranous peristome.

5. Anus is small and inconspicuous located more or less eccentrically on the aboral surface.

6. Ambulacra form prominent grooves provided with podia or the feet.

7. Ambulacra are restricted to oral surface extending from the peristome to the tips of the arms.

8. Endoskeleton is flexible, made of separate ossicles.

10. Respiration by papulae.

11. Sexes separate, gonads radially arranged.

12. Development includes bipinnaria or brachiolaria larva.

13. Commonly called starfishes or sea stars.

1. Arms are provided with two rows of conspicuous marginal plates.

2. Oral plates are infra-marginal and aboral plates are supra-marginal.

3. Pedicellariae are alveolar or sessile type.

4. Podia or tube feet are arranged in two rows.

5. Mouth frame is well developed and adambulacral type.

Luidia, Astropecten, Archaster, Pentaceros.

1. Arms are generally without conspicuous marginal plates.

2. Aboral skeleton is imbricated or reticulated with single or group of spines.

3. Pedicellariae are rarely present.

4. Podia or tube feet are in two rows provided with suckers.

5. Mouth frame is of adambulacral type.

6. Ampullae single or bifurcated.

Aesterina, Echinaster, Hymenaster, Solaster.

1. Marginal plates are inconspicuous or absent.

2. Aboral skeleton is mostly reticulate with conspicuous spines.

3. Pedicellariae are pedunculate type with a basal piece.

4. Podia or tube feet are arranged in four rows and provided with suckers.

5. Papulae are on both surface.

6. Mouth frame is of ambulacral type.

Brisingaster, Heliaster, Zoraster, Asterias.

Class 4. Ophiuroidea:

(Gr., ophis = serpent + oura = tail + eidos = form):

1. Body is flattened with a pentamerous or rounded central disc.

2. Oral and aboral surfaces are distinct.

3. Arms usually five rarely six or seven are long, slender, smooth or spiny.

4. Ambulacral grooves are absent.

5. Anus and intestine are absent.

6. Madreporite is on the oral surface.

7. Sexes are separate, gonads pentamerous.

9. Development includes a free swimming pluteus larva.

10. Commonly called brittle stars.

1. Arms are simple, mostly five in number, moving chiefly in transverse plane.

2. Arm ossicles articulated by pits and projections.

3. Disc and arms are usually covered with distinct shields or scales.

4. Spines on arms are borne laterally and are directed outward or toward the arm tips, not downwards.

Ophioderma, Ophioscolex, Ophiothrix, Ophiolepie.

1. Arms are simple or branched, long and flexible, capable of coiling around objects and of rolling up in vertical plane.

2. Ossicles of arms are articulated in streptospondylus manner.

3. Disc and arms are without or poorly developed scales or shields.

4. Spines are directed downward often forming hooks or spiny clubs.

5. One madreporite in each inter-radius.

Asteronyx, Astrophyton, Astoporpa.

5. Skeleton in Echinodermata:

The mesoderm forms a skeleton of ossicles lying in the dermis, the ossicles may be few and scattered so that they impart a leathery consistency to the body wall, or they may be united by muscles as a definite skeleton, or they may be firmly jointed to form a shell.

Some ossicles usually project as spines over which the epidermis is lost. Two or three spines may be arranged so as to work as pincers, these form pedicellariae of various types, they occur only in Asteroidea and Echinoidea.

The primitive skeleton had two series of plates forming oral and apical systems, but in present day Echinodermata the apical plates are absent or reduced or replaced by accessory plates, e.g., corona of sea urchins. The oral system forms five plates around the mouth, it is fully developed only in Crinoidea forming a calyx. Lime is deposited not only in the skeleton but it may be found in any organ of the body.

6. Coelom in Echinodermata:

A coelom is formed from paired pouches which arise as lateral out-pushings of the embryonic archenteron, thus, the coelom is enterocoelic. The pouches undergo a constriction so that each forms an anterior and a posterior sac, the posterior sacs grow and form coelomic cavities and the anterior sacs become the rudiments of a water vascular system, they are called hydrocoel sacs.

The left hydrocoel sac acquires a stone canal which communicates with the body wall, the right hydrocoel sac disappears, but recent evidence shows that the right hydrocoel sac is represented by the dorsal sac of the axial sinus. Thus, the entire water vascular system is formed from the left hydrocoel and it assumes a radial arrangement of its parts.

Besides the water vascular system the coelomic cavities form a perivisceral cavity containing the main viscera, a perihaemal system which encloses a vascular system, and its aboral sinus extension enclosing the gonads, and an axial sinus of varied development in different classes, but it has an opening forming a madreporite.

7. Relationship of Echinodermata:

The free Eleutherozoa have been derived from attached pelmatozoic ancestors. Echinodermata show no close relationship to any invertebrates, except with Hemichordata and Pogonophora.

These three phyla have a number of common features, among which are the formation of coelom by enterocoel retention of blastopore as the site of the future anus, in having a dipleurula-like larva at some stage, and in having a heart vesicle which may represent the right anterior coelom. The larvae of some echinoderms (auricularia larva of holothurians) closely resemble the tornaria larva of Saccoglossus.

But they are closely related to hemichordates because of the following reasons:

1. The mesoderm is derived from cells around the lips of the blastopore.

2. They possess a mesodermal endoskeleton, whereas the invertebrates have an ectodermal exoskeleton.

3. The blastopore becomes the adult anus as in chordates, in invertebrates (annelids, molluscs) the blastopore becomes the mouth.

4. Mouth arises as a new structure from the ectodermal stomodaeum as in chordates.

5. The coelom is formed from paired lateral diverticula of the archenteron and is enterocoelic.

The many resemblances between echinoderms and hemicordates Auricularia larva, are neither accidental nor due to convergent evolution, but because the two phyla are closely related and both arose from some common ancestor. Hemichordates are closer to this common ancestor, while echinoderms have deviated greatly because they have arisen as a blind branch from the ancestral type.

Phylogenomic analysis of echinoderm class relationships supports Asterozoa

While some aspects of the phylogeny of the five living echinoderm classes are clear, the position of the ophiuroids (brittlestars) relative to asteroids (starfish), echinoids (sea urchins) and holothurians (sea cucumbers) is controversial. Ophiuroids have a pluteus-type larva in common with echinoids giving some support to an ophiuroid/echinoid/holothurian clade named Cryptosyringida. Most molecular phylogenetic studies, however, support an ophiuroid/asteroid clade (Asterozoa) implying either convergent evolution of the pluteus or reversals to an auricularia-type larva in asteroids and holothurians. A recent study of 10 genes from four of the five echinoderm classes used ‘phylogenetic signal dissection’ to separate alignment positions into subsets of (i) suboptimal, heterogeneously evolving sites (invariant plus rapidly changing) and (ii) the remaining optimal, homogeneously evolving sites. Along with most previous molecular phylogenetic studies, their set of heterogeneous sites, expected to be more prone to systematic error, support Asterozoa. The homogeneous sites, in contrast, support an ophiuroid/echinoid grouping, consistent with the cryptosyringid clade, leading them to posit homology of the ophiopluteus and echinopluteus. Our new dataset comprises 219 genes from all echinoderm classes analyses using probabilistic Bayesian phylogenetic methods strongly support Asterozoa. The most reliable, slowly evolving quartile of genes also gives highest support for Asterozoa this support diminishes in second and third quartiles and the fastest changing quartile places the ophiuroids close to the root. Using phylogenetic signal dissection, we find heterogenous sites support an unlikely grouping of Ophiuroidea + Holothuria while homogeneous sites again strongly support Asterozoa. Our large and taxonomically complete dataset finds no support for the cryptosyringid hypothesis in showing strong support for the Asterozoa, our preferred topology leaves the question of homology of pluteus larvae open.

1. Introduction

Echinoderms are composed of five extant classes, sea urchins (echinoids), starfish (asteroids), sea cucumbers (holothurians), brittlestars (ophiuroids) and sea lilies (crinoids). Although the modern classes appear in a relatively short time interval early in the fossil record (525–480 Ma) [1], extant crown group species in each class have more recent origins: present-day crinoids and echinoids radiated in large part after the Permian/Triassic extinction event for instance (approx. 250 Ma) [2,3], with asteroids also likely to have undergone an evolutionary bottleneck at this time [4]. The resulting long stem lineages leading to the living forms mean that the monophyly of each class is not in doubt. The rapid and ancient appearance of the classes, however, means that the resolution of the relationships between them is challenging.

Dealing first with the uncontroversial aspects of echinoderm class-level phylogeny, consideration of morphology and previous molecular analyses strongly support a monophyletic group of echinoids, asteroids, holothurians and ophiuroids (Eleutherozoa). Within this eleutherozoan clade, most authors also accept a close relationship between the globular echinoids and holothurians (Echinozoa). The major point of remaining dispute revolves around the position of the ophiuroids, with two plausible solutions. According to the Asterozoa hypothesis, ophiuroids are the sister group of the asteroids, whereas the Cryptosyringida hypothesis links the ophiuroids instead to the Echinozoa (figure 1).

Figure 1. The two hypotheses of echinoderm evolution discussed in this work. The Asterozoan hypothesis unites ophiuroids and asteroids on the basis of a five-rayed body plan. The cryptosyringid hypothesis places ophiuroids as sister group to the holothurians and echinoids on the basis of, for example, closed ambulacral grooves (see Introduction).

The relationship of the echinoids and ophiuroids is of particular evolutionary interest, as members of both classes possess a pluteus-type larva (electronic supplementary material, figure S1). These are characterized by a mesodermally derived calcite skeleton, which supports the elongated, ciliated arms that are used for feeding and swimming. Pluteus larvae contrast with non-skeleton-forming, generic dipleurula type that characterizes the early development of crinoids, asteroids and holothurians. The dipleurula has been proposed to represent the larval form ancestral to all ambulacrarians, as it is also shared with hemichordates, the sister group to echinoderms. The gene regulatory network (GRN) involved in the development of the larval skeleton in the sea urchin Strongylocentrotus purpuratus is one of the best understood developmental networks in biology [5], but has yet to be characterized in ophiuroids.

Among morphologists, Hyman was ‘of the opinion that the closer relationship of ophiuroids to echinoids rather than to asteroids, as usually supposed, is not to be doubted’ on the basis of the shared pluteus larva, ambulacral canals enclosed by epineural folds and biochemical criteria [6, pp. 699–700]. Littlewood et al. [7] also found support for this scenario with their most parsimonious tree based on combined larval and adult morphology, although they note that the alternative asterozoan tree was only two steps longer.

The first molecular analyses using 18S rRNA genes found monophyletic echinoderms, but a questionable internal topology [8] subsequent reanalysis of these data using more modern phylogenetic methods supports cryptosyringids [9]. Using 28S and 18S rRNA genes, Mallatt & Winchell [10] found strong support for Asterozoa. An analysis of 13 protein-coding sequences encoded in mitochondrial genomes [11] identified three lineages within the echinoderms—crinoids, ophiuroids and a clade consisting of (asteroids, (echinoids, holothurians)). Inspection of this tree shows that the ophiuroids are particularly rapidly evolving, raising the likelihood that their position as sister to other eleutherozoan classes is the result of a long-branch attraction (LBA) artefact [12]. Janies and co-workers analysed combined molecular (seven loci) and morphological datasets from a large set of echinoderms. They produced trees supporting asterozoans and cryptosyringids (as well as other scenarios) and concluded that the overall phylogeny was sensitive to the choice of analytical methods [13].

Most recently, Pisani and co-workers presented an analysis of seven nuclear protein-coding genes and three nuclear ribosomal RNAs from four of the five echinoderm classes (crinoids, ophiuroids, asteroids and echinoids). Analysis of their full dataset supported the Asterozoa hypothesis (ophiuroids, asteroids) [14]. By careful removal of subsets of data expected to be problematic, they produced, in contrast, a tree supporting the cryptosyringid (ophiuroid, echinoid) clade. Their analysis did not include any holothurian sequences. There is the possibility that this omission might have resulted in an unnecessarily long echinoid branch rendering the dataset more susceptible to branch positioning artefacts.

We have re-examined the question of echinoderm class relationships with a large dataset (219 nuclear protein-coding genes) including representatives from all five extant classes of echinoderms including data from two ophiuroids, two asteroids, two holothurians, four echinoids and a crinoid as well as data from three hemichordates and a cephalochordate as outgroup taxa.

2. Material and methods

Our analysis combined sequences from novel data sources and pre-existing protein and expressed sequence transcript databases, as outlined below.

(a) Sources of data

Amphiura filiformis were collected by Peterson mud grabs at a depth of approximately 40 m close to the Sven Loven Centre for Marine Sciences, Kristineberg, on the Gulmar fjord, Sweden (58°15′ N, 11°25′ E). A normalized cDNA library was constructed from mRNA of arm tissues from A. filiformis (regenerated and intact tissues) and sequenced using Roche 454 GS-FLX Titanium technology.

Adult Amphipholis squamata were collected in Argylle Creek on San Juan Island, Washington, and maintained in flow-through aquaria at Friday Harbor Laboratories. Brooded juveniles at a range of stages of adult body plan formation were dissected out of the bursal sacs of the adult animals and flash frozen in liquid nitrogen. Following total RNA extraction, cDNA library construction was carried out by Express Genomics, Maryland, and sequencing of excised inserts was carried out using Roche 454 GS-FLX technology.

Solaster stimpsoni adults were collected by SCUBA at Eagle Cove San Juan Island, Washington, in April during their reproductive season. Adults were spawned by injection of 1-methyl adenine as described [15]. The positively buoyant embryos were cultured in running seawater tables as described [16] with Sanger EST sequencing as described in [17,18].

Adult Florometra serratissima were collected by SCUBA at Bamfield Marine Station Sciences Centre, British Columbia, Canada. Mature oocytes were dissected from the adult female pinnules using forceps and fertilized with sperm obtained by rupturing testes. Embryos were cultured in glass bowls. Gastrula, early doliolaria and late doliolaria developmental stages were frozen in liquid nitrogen and total RNA prepared using the Ambion RNA aqueous mini kit. RNA samples were pooled and prepared for sequencing using the Illumina TruSeq stranded mRNA sample prep with oligo-dT selection then sequenced using HiSeq 101 bp paired-end reads.

Eucidaris tribuloides transcriptome sequences were obtained from the GenBank Sequence Read Archive (accession no. SRX043529).

All other echinoderm nucleotide sequences were taken from the NCBI est_others database.

(b) Preprocessing of nucleotide sequences

High coverage and short read lengths mean that sequences produced using so-called next generation (i.e. Illumina, 454) technologies benefit from being preassembled using dedicated software.

We assembled a dataset of 46 312 223 2 × 101 bp paired-end sequences from the crinoid F. serratissima using the Inchworm program from the Trinity package [19]. This resulted in a set of 1 681 253 sequences.

Amphiura filiformis reads were assembled in MIRA v. 3.2.0rc3 [20], giving 35 742 sequences. Amphipholis squamata and E. tribuloides reads were assembled using the Newbler assembler, to give 53 028 and 21 969 contigs, respectively [21].

(c) Orthologue identification and alignment construction

We used a set of 12 625 Saccoglossus kowalevskii (Skow) sequences retrieved from the NCBI protein sequence database to search the predicted proteomes of the chordate Branchiostoma floridae (Bflo), retrieved from the JGI and the sea urchin S. purpuratus (Spur) retrieved from the NCBI. We identified trios of sequences that formed consistent reciprocal best matches in each other's proteomes and were therefore likely orthologues. Best matches were identified using blastp [22].

For all Saccoglossus sequences that were members of an orthologous trio (Skow, Bflo and Spur), we used tblastn to search a database of all echinoderm and hemichordate expressed sequences from the NCBI est_others database, with the addition of the Florometra, Amphipholis, Amphiura, Solaster and Eucidaris processed datasets described above and Ptychodera flava sequences taken directly from the NCBI trace archive without preprocessing.

Significant hits to each Saccoglossus protein were extracted from the echinoderm nucleotide sequence databases and placed into species-specific groups. The nucleotide sequences in each species group were then assembled in isolation using CAP3 [23]. Assembled sequences thus produced were searched against metazoan protein sequences from the NCBI NR protein sequence database (blastp) and translated against the best match using the estwise program from the genewise package [24].

Each of the resulting assembled protein sequences was then searched against the combined proteomes of Saccoglossus, Branchiostoma and Strongylocentrotus using blastp, and only those for which the best hits were consistent with the members of the initial Skow, Bflo and Spur orthologous trio were retained.

Sets of proteins for each Skow, Bflo and Spur trio were then aligned with all predicted orthologous echinoderm and hemichordate proteins using PROBCONS [25]. A phylogeny was constructed for each protein alignment using P hy ML [26], using the WAG model [27], estimating the proportion of invariable sites, estimating the gamma distribution parameter allowing four gamma rate categories (WAG + IG4) and optimizing the tree topology and branch lengths. Each phylogeny was then rooted with the Branchiostoma sequence (the chordate outgroup). Only those alignments that then yielded a monophyletic cluster of echinoderms were retained (i.e. no echinoderm sequences were allowed to branch within the hemichordates or between hemichordates and the root). This test will have the effect of excluding gene sets that include paralogous echinoderm sequences. Paralogous sequences would cluster outside a set of orthologous genes in a correct phylogeny. Rooting with the Branchiostoma orthologue will cause any paralogous echinoderm sequences to branch between this root and hemichordate (the presence of which is inevitable given the composition of our seed orthologue trios). Branching order of echinoderm classes within the echinoderm monophyletic group was not constrained during this process. Tree re-rooting and tests for monophyletic groups were performed using custom perl and python scripts. Final alignments were also screened for RNA contamination against all RNA sequences from the Rfam database [28], using tblastn [22].

(d) Additional filters on final alignment

Only proteins for which at least four out of five echinoderm classes were represented were included in the concatenated alignment (echinoids were invariably present owing to the presence of a S. purpuratus protein in the seed orthologue trios). We finally processed the alignment by visual inspection to remove unreliably aligned sections. The final concatenated alignment contained 219 genes.

(e) Phylogenetic analyses

P hylo B ayes analyses were performed with the computationally intensive site-heterogeneous CAT + GTR + Γ mixture model taking approximately one month per analysis. This model accounts for across-site heterogeneities in the amino acid replacement process and is implemented in an MCMC framework by the program P hylo B ayes v. 3.3d [29]. Two independent runs were performed with a total length of 6000 cycles, saved every 10 cycles. To construct the tree, the first 2000 points were discarded as burn-in, and the topology and posterior consensus support was computed on the 4000 remaining trees. We also checked for convergence of the two independent runs following burn-in of 2000 using bpcomp, which in each case showed a ‘maxdiff’ (maximum discrepancy) below 0.1.

In order to assess the reliability of trees, we calculated phylogenies on 50 bootstrap replicates (i.e. sampling with replacement to create an alignment the same size as the original) and 100 jack-knife replicates (i.e. sampling without replacement, alignments 25% original length), in both cases using P hylo B ayes with the CAT + GTR + Γ model. Chains were run for 1000 cycles, and consensus trees calculated using bpcomp, with 200 cycles discarded as burn-in.

(f) Cross-validation

We performed statistical comparisons of the CAT + GTR + Γ, the WAG + Γ and GTR + Γ models using cross-validation tests as described in [30]. Ten replicates were run: 9/10 of the positions randomly drawn from the alignment were used as the learning set and the remaining 1/10 as the test set. For each model, MCMC were run for 1100 cycles, 100 being discarded as burn-in.

(g) Phylogenetic signal dissection: site rate ranking

Recapitulating the approach of Pisani et al. [14], the number of changes at each site was calculated over a maximum-parsimony tree constructed with unresolved relationships between echinoderm classes. The number of changes for each character over the tree was normalized to correct for positions with missing data, by dividing by the number of changes by the number of taxa with a character at that position. The positions in the alignment were then divided into one sub-alignment containing heterogeneously evolving sites (comprised all invariant positions plus the most frequently substituting quarter of the variable sites) and a second sub-alignment containing the more homogeneously evolving positions comprised the remaining 75% of the more slowly evolving variable sites. These two datasets were analysed separately and the results compared.

(h) Phylogenetic signal dissection: gene rate ranking

Using an alignment for each gene, a maximum-likelihood tree was calculated using P hy ML [26] and the total length of that tree (in estimated substitutions per position across all branches) was divided by the number of taxa on the tree to give an estimate of the rate of evolution for each gene. Genes were concatenated in order of their evolutionary rates and the resulting dataset divided into four approximately equal-sized quartiles containing genes ranging from the slowest to the fastest. Each of these four datasets was analysed separately and the results compared.

(i) Partition by gene using M r B ayes v. 3.2

We partitioned the dataset into individual genes and, by unlinking the partitions, we allowed each gene to find its own best fitting empirical model from among the 10 possible in M r B ayes (poisson, jones, dayhoff, mtrev, mtmam, wag, rtrev, cprev, vt or blosum). We ran two runs of four chains each for a total of 500 000 generations well beyond convergence of the two runs. We additionally unlinked the rate multiplier associated with each gene partition allowing the genes to share the same topology and branch length but to be scaled independently and also use individually best fitting empirical model as before.

3. Results

(a) Data assembly

We gathered a set of 3872 Saccoglossus, Branchiostoma and Strongylocentrotus orthologous trios and used these to search our echinoderm dataset, producing 3537 alignments including additional echinoderm data. Of these, 2630 passed tests for echinoderm monophyly—that is, the echinoderm sequences formed a subtree that included all echinoderm sequences and none of the outgroup species.

Several species that were present in the NCBI est_others sequence database, and thus captured by our automated procedures, had particularly low representation in the final alignment and were removed from subsequent analysis, namely Patiria miniata (asteroid), Heliocidaris erythrogramma (echinoid), Pseudocentrotus depressus (echinoid) and Strongylocentrotus intermedius (echinoid). We also excluded the hemichordate Rhabdopleura compacta, again owing to low coverage. Although incidental to the question of echinoderm phylogeny, this meant we were unable to address the issue of enteropneust monophyly (but see [31] for a convincing conclusion based on miRNAs).

In total, 219 genes were found to have representatives from at least four echinoderm classes. Fifteen taxa were included in the final alignment of 80 666 columns. No hits to the Rfam database were found in the sequences included in these alignments. After filtering for confidently aligned regions, 45 818 positions remained. The full dataset is described in the electronic supplementary material, table S1.

Of the individual gene trees for genes represented in the final alignment, 31 contained monophyletic Cryptosyringida and 49 contained monophyletic Asterozoa. Requiring both crinoid and holothurian sequences to be present (which a priori we would expect to produce more reliable results) enriched Asterozoa (10 trees) relative to Cryptosyringida (three trees).

(b) Echinoderm phylogeny

Analysis using P hylo B ayes [29] on the full dataset of 45 818 reliably aligned amino acids from 15 taxa produced the tree shown in figure 2. As widely recognized, the crinoids diverge first and form a sistergroup to the four other classes—the Eleutherozoa. Within the Eleutherozoa, the Asterozoa clade (asteroids + ophiuroids) and the Echinozoa (echinoids + holothurians) are recovered as sister clades. Within the echinoids, the cidaroid E. tribuloides is the deepest branching and within the hemichordates, the ptychoderids clustered together, as expected. All clades have a posterior probability of 1. This topology is supported by analyses using both the widely employed site-homogeneous WAG + Γ and GTR + Γ models (gamma with five rate categories) and the more complex site-heterogeneous CAT + GTR model (with the default Dirichlet process for estimating rates across sites [32]). We verified by cross-validation that the CAT + GTR + Γ model had a better fit to our dataset than the GTR + Γ (ΔlnL 228.36 ± 23.5159) and that the GTR + Γ had a better fit than WAG + Γ (ΔlnL 290.48 ± 17.613).

Figure 2. Full tree from complete dataset (see text for description).

We performed 50 bootstrap replicates using the CAT + GTR + Γ model: 90% of replicates supported Asterozoa. Of the remainder, 8% placed ophiuroids as sister to holothurians, with only 2% supporting Cryptosyringida. We also performed 100 jack-knife replicates using just 25% of the full dataset: the results with such a reduced dataset were naturally less robust nevertheless 57% of trees support the asterozoan topology. As with the bootstrap samples, the next most common result in the jack-knife set (25%) contained the unlikely grouping of holothurians and ophiuroids. Cryptosyringids sensu stricto ((H,E),O) were observed in only 8% of trees, with a further 5% containing monophyletic cryptosyringids although with ophiuroids and holothurians as sister groups ((H,O),E). In summary, both bootstrap and jack-knife replication provide support for asterozoans. The cryptosyringids were the third best supported topology with both approaches, and less well supported than a grouping of holothurians and ophiuroids, which, to the best of our knowledge, has no serious support from other sources and can likely be discounted as a long-branch artefact.

The Asterozoa–Echinozoa topology has been supported by the majority of molecular analyses of this problem. By contrast, the recent work by Pisani et al. addressing the possibility of systematic error due to unequal rates of evolution and saturation using an approach they call phylogenetic signal dissection supports, instead, the cryptosyringid clade (Echinozoa + Ophiuroidea). This result was supported in the Pisani et al. analysis only when using the portion of the alignment remaining after the constant and the most variable sites had been removed. We set out to replicate this approach with our larger and more taxonomically complete dataset. We used two approaches to test the effects of removing faster evolving and less reliable data: first, the site-based method as described, which we term site rate ranking (SRR) second, we classified each gene in our alignment into one of four quartiles according to its rate of evolution and analysed each of these quartiles in turn—we call this gene rate ranking (GRR). The GRR method has the advantage that it does not depend on pre-specifying any topology, whereas the SRR approach has the advantage that it treats each site independently.

Using either of these approaches with our dataset, we find no support for the cryptosyringid clade. Electronic supplementary material, figure S2, shows the results using the site-based SRR method. With the supposedly optimal portion of the alignment (constant and most variable sites removed), we still find strong support for the Asterozoa–Echinozoa topology (electronic supplementary material, figure S2a). The support is slightly lower than when using the whole alignment (support for Asterozoa pp = 0.94), presumably because the alignment is shorter (16 678 sites compared to 46 913 for the complete alignment) and hence less informative. Using the suboptimal sites (constant and variable), we recover an improbable topology with Holothuroidea and Ophiuroidea as sister taxa (both are long branches) and the relative positions of echinoids, asteroids and the holothurian/ophiuroid group are unresolved (electronic supplementary material, figure S2b).

Earlier support for cryptosyringids came from a dataset that did not include holothurians [14] and so we repeated the SRR method with this class excluded. We recovered identical relative positions for each of the remaining classes and so this difference in taxonomic representation does not explain the difference between our results and those of Pisani et al. [14] (results not shown).

Figure 3 shows the results from the gene-based GRR approach. The slowest quartile (most reliable) shows the strongest support for Asterozoa (pp = 1, figure 3a). The second and third quartiles still support this clade but with diminishing support (second quartile pp = 0.83, figure 3b third quartile pp = 0.71, figure 3c) The fourth quartile (fastest evolving genes and likely least reliable) places the ophiuroids as sister group to the Echinozoa + Asteroidea, presumably due to LBA between the relatively long-branched ophiuroids and the outgroups (figure 3d). All other aspects of the tree (including the Echinozoa clade) do not change as the datasets get faster and all other clades receive the maximum support (pp = 1) in analyses of all four quartets. The difference in rate is very evident when comparing these trees (shortest (figure 3a) to longest (figure 3d)), which are all drawn to the same scale.

Figure 3. Results from our data stratified by overall length of gene trees (gene rate ranking), from (a), the slowest evolving quartile, to (d) the fastest evolving quartile.

We conducted two experiments using analyses performed with M r B ayes in which we partitioned the dataset by genes and allowed genes to adopt firstly gene-specific empirical models of amino acid substitution and secondly both gene-specific empirical models and a gene-specific rate. In both experiments, the topology was the same as in figure 2 with posterior probability of 1.0 for all nodes.

4. Discussion

(a) Echinoderm phylogeny

All of our analyses unequivocally support a clade of Ophiuroidea plus Asteroidea: Asterozoa. Despite using two methods to stratify our data by evolutionary rate, we found no support for the alternative Cryptosyringida hypothesis of a sister group relationship of Ophiuroidea and Echinoidea + Holothuroidea. Our dataset is considerably larger than others that have been brought to bear on this question, includes at least two members of each of the relevant classes and deep sampling of the closest outgroup (the crinoid). A priori, we might expect such a dataset to produce a more reliable result, and we have not been able to identify confounding factors. Our trees do reveal very short branches leading to the asterozoan and echinozoan clades, however. These divergences appear to have taken place in a compressed time interval, a fact perhaps underscored by the absence of any miRNAs that may have helped resolve the relative positioning of these taxa [14]. Further molecular testing of our topology might come from even deeper taxon sampling or new types of data. The order of the echinoid Hox genes, for instance, at least as observed in S. purpuratus, is derived relative to that seen in hemichordates, and apparently coupled with this, the HOX4 orthologue has been lost [33,34]. HOX4 orthologues have, however, been identified in crinoids [35] and asteroids [36]. Their status in ophiuroids is uncertain (the Stegophiura sladeni sequence referred to as SS-11 in the previous study [37] is not convincing), but our phylogeny would be problematic if ophiuroids and echinoids shared a complex molecular character state [38], such as a Hox cluster rearrangement and gene loss, to the exclusion of asteroids. We are not aware of any other complex molecular character states that might be informative for this phylogenetic question: the systematic exploration of their existence, presence and absence must await complete genome sequences of representatives of other echinoderm classes.

(b) Asterozoa and the origins of the larval skeleton

The most recent molecular study to address echinoderm phylogeny [14] stated that their result (i.e. supporting cryptosyringids) ‘confirms that that the morphologically similar pluteus larval stages of echinoids and ophiuroids are indeed homologous rather than convergent’. It is not clear, however, that the larval skeleton can be used as a phylogenetic character to distinguish between the alternative phylogenetic trees without making assumptions about the relative ease of gain or loss events (electronic supplementary material, figure S3). Under the scenario of a single origin for the larval skeletons (the evidence for which we review briefly below), the cryptosyringid hypothesis is indeed more parsimonious than the asterozoan, requiring a single loss of the skeleton in holothurians (and concomitant reversal to a auricularia/bipinnaria type of larva), with the asterozoan hypothesis requiring an additional loss/reversal in asteroids (electronic supplementary material, figures S3a and S3c). If a dual origin of the larval skeleton is favoured, the two phylogenetic scenarios are indistinguishable on parsimony grounds (so far as it relates to the skeleton) (electronic supplementary material, figures S3b and S3d).

Although the larval skeleton seems intuitively to be a complex trait, in the sense that it might be regarded as unlikely to have evolved twice, all echinoderm classes share an adult skeleton, and the evolution of the larval skeleton may be the result of a relatively simple redeployment of this, making the possibility of convergence more likely. Gao & Davidson [39], comparing larval and adult skeletogenesis, have identified the genes specifically required for embryonic activation of the skeletogenesis network in the echinoid S. purpuratus, showing that this is only a small component of the overall skeletogenic GRN. It is worth noting that the key gene in the embryonic skeleotogenic GRN, Pmar1, involved in the establishment of the double negative gate that de-represses skeletogenesis [5] currently appears to be an echinoid autapomorphy. We could not find evidence of Pmar1 orthologue presence in any taxa in our echinoderm EST database (or non-echinoderm databases) except for S. purpuratus. Although Vaughn et al. [40] mention an unpublished ophiuroid Pmar1 orthologue, their inability to find Pmar1 transcripts expressed in any stage of brittlestar development supported their conclusion that ‘embryonic skeleton formation in sea urchins and brittle stars represents convergent evolution by independent co-optation of a shared pathway used in adult skeleton formation’ [40, p. 14].

The suggestion that the echinoid larval skeleton may be a derived state is further supported by McCauley et al.'s [41] analysis of holothurian embryogenesis. Based on an analysis of the embryonic expression patterns of mesodermal regulatory genes in Parastichopus parvimensis, they suggest ‘that the ancestor of the sea cucumber + sea urchin clade may have had a mesoderm with a regulatory state similar to that of extant sea cucumbers and may have developed only a rudimentary larval skeleton’ [41, p. 5], and argue that, as such, holothurians are unlikely to have lost a complex larval skeleton. If we accept the sister group relationship of echinoderms and holothurians, this in turn suggests that the larval skeleton was not a plesiomorphy of these two taxa and thus unlikely to be a plesiomorphy of any possible clade including the ophiuroids. Other authors have argued on morphological grounds that the two kinds of plutei are convergent forms [42,43], but as the defining feature of the pluteus is its skeleton, most desirable would be a comparison of the fine details of the developmental genetics responsible for initiating production of the larval skeletons, which we would expect to more exactly overlap in homologous plutei. We emphasize that such evidence would be useful for ruling out the scenario of a single origin for the pluteus within cryptosyringids (electronic supplementary material, figure S3c), not for distinguishing between the dual origin scenarios—however, our molecular evidence clearly favours the Asterozoa (i.e. electronic supplementary material, figure S3b).

(c) Adult morphology

The cryptosyringids owe their name to the fact that their radial water vascular system and radial nerve become covered during development, a trait that was viewed as a synapomorphy of the group [44]. In later work, however, Smith and co-workers [7] pointed out that the developmental mechanism by which this occurs differs within the echinoids, suggesting that its presence in some instances may be due to convergence, and as such, it would not be a reliable phylogenetic marker. To the best of our knowledge, there has been no work on the molecular mechanisms leading to covered radial nerves and water vascular system in ophiuroids and echinoids. Under the Asterozoa hypothesis, we would predict differing molecular mechanisms, suggesting parallel evolution of the trait in echinoids and ophiuroids, unless the trait were secondarily absent from asteroids, in which case the fossil record may be the best source of evidence.

If our inference of monophyletic Asterozoa is correct, it means that the hypothesis of a star-like common ancestor of Eleutherozoa can neither be rejected nor supported. This in turn has implications for the interpretation of the fossil record: in particular, based on our analysis, there is no reason to rule out a non-stellate ancestor for the echinoid + holothurian stem group.

Echinodermata: Systematics

I n traditional taxonomy, there are five classes of living echinoderms: Crinoidea (sea lilies), Asteroidea (starfish), Ophiuroidea (brittle stars or snake stars), Echinoidea (sea urchins and sand dollars), and Holothuroidea (sea cucumbers). An unusual echinoderm, Xyloplax , was found in 1986, on sunken wood in the deep sea. It may represent a sixth class, the Concentricycloidea (Baker et al., 1986), but many zoologists now consider Xyloplax to be an aberrant asteroid.

Fossil echinoderms were much more diverse in form than living ones. Many Paleozoic forms, now extinct, had very unusual morphologies. There is still debate over how they fit into the scheme of echinoderm phylogeny in fact, some Cambrian echinoderm-like fossils in the Homalozoa have been suspected of being chordates. The cladogram you see here is a composite of molecular and morphological data. Like all cladograms, it is subject to revision as new data comes in.

Baker, A. N., Rowe, F. W. E., and Clark, H. E. S. 1986. A new class of Echinodermata from New Zealand. Nature 321: 862-864.

David, B. & R. Mooi. 1998. Major events in the evolution of echinoderms viewed by the light of embryology. Pp. 21-28 in Echinoderms: San Francisco.

Raff, R. A., Field, K. G., Ghiselin, M. T., Lae, D. J., Olsen, G. J., Pace, N. R., Parks, A. L., Parr, B. A., and Raff, E. C. 1988. Molecular analysis of distant phylogenetic relationships in echinoderms. In: Paul, C. R. C. and Smith, A. B. (eds.) Echinoderm Physiology and Evolutionary Biology. Clarendon Press, Oxford.

Sprinkle, J. 1992. Radiation of Echinodermata. In: Lipps, J. H. and Signor, P. W. (eds.) Origin and Early Evolution of the Metazoa. Plenum Press, New York.

Phylum Echinodermata: Characters and Classification | Animals

All existing echinoderms are marine. They generally live at sea bottom borne are pelagic (free swimming in open water) and a few are sessile (attached to the substratum).

It varies considerably. The body is star-shaped, spherical or cylindri­cal. It is un-segmented. The body lacks head.

3. Spines and Pedicellariae:

Many echinoderms bear spines and pincer-like pedicellariae. The spines are protective in function. The pedicellariae keep the body surface clear of debris and minute organisms.

The symmetry is bilateral in larvae but pentamerous radial in adults i. e., body parts are arranged in fives or multiples of five.

Epidermis is single layered and ciliated. In many echinoderms there is endoskeleton of calcareous plates in the dermis which are mesodermal in origin.

There is a true enterocoelic coelom.

7. Ambulacral System (= Water Vascular System):

Presence of ambulacral system is the characteristic feature of phylum echinodermata. A perforated plate called madreporite is present in this system. The pores of the madreporite allow water into the system Tube teet of this system help in locomotion, capture of food and respiration. Water vascular system is of coelomic origin.

It is usually complete. Brittle stars have incomplete digestive tract.

9. Haemal and Perihaemal Systems:

Instead of blood vascular system, there are present haemal and perihaemal systems which are of coelome ongin.Thus the so called circulatory system is open type and includes haemal and perihaemal systems. The so blood is often without a respiratory pigment. There is no heart.

10. Respiratory Organs:

Gaseous exchange occurs by dermal branchae or papulae in star fishes peristominal gills in sea urchins, genital bursae in brittle stars, and cloac respiratory ‘trees in holothnrians. Exchange of gases also takes place through tnbe feet.

Specialized excretory organs are absent. Nitrogenous wastes are diffused out via gills. Ammonia is chief excretory matter.

It consists of a nerve ring and radial nerve cords. Brain as such is absent.

They are poorly developed.

14. Sexes and Fertilization:

Except a few individuals, the sexes are separate. There is no sexual dimorphism. Fertilization is usually external.

15. Asexual Reproduction:

Some forms reproduce asexually by self-division.

16. Autotomy and Regeneration:

Phenomena of autotomy and regeneration are often well marked in echinoderms.

The development is indirect and includes a ciliated, bilaterally sym­metrical larva that undergoes metamorphosis to change into the radially symmetrical adult. Different larval forms are found which are mentioned in the classes of Echinodermata.

(i) Presence of spines and pedicellariae.

(ii) Ambulacral system (water vascular system),

(iv) Mesodermal endoskeleton of calcareous plates,

(v) Bilateral symmetry in the larva and radial symmetry in the adult.

Degenerate Characters:

(iii) Incomplete digestive tract in some forms,

(iv) Reduced circulatory system,

(v) Absence of excretory system.

Resemblance with Chordates:

(i) Radial and indeterminate cleavage,

(ii) Gastrulation by invagination,

(iii) Mouth derived as an ectodermal invagination,

(iv) Adult anus derived from embryonic blastopore,

(v) Mesodermal endoskeleton.

(vii) Both are deuterostomes.

From these resemblances, it is clearly proved that the Echinoderms are nearer to the Chordates than any other group. It also indicates that the chordates have been evolved from Echinoderm-like ancestors.

Classification of Phylum Echinodermata:

Phylum Echinodermata is divided into five classes.

Class 1. Asteroidea (Gk. aster- star, eidos- form):

Body is star-like. Five arms are usually present which are not sharply marked off from the central disc. Larval forms are Bipinnaria and Brachiolaria. Examples: Asterias (Star fish), Pentacews (Star fish), Astrvpecten (Star fish).

Class 2. Ophiuroidea (Gk. Ophis- snake, Oura- tail, eidos- form):

Body is star-like. Arms are sharply marked off from the central disc. Ambulacral grooves”Sre absent. Pedicellariae are absent. Larval form is Ophiopluteus.

Ophiothrix (brittle star), Ophioderma (brittle star), Ophiocoma (brittle star), Ophiura (brittle star).

Class 3. Echinoidea (Gk. echinos- hedgehog, eidos- form):

Body is globular or dislike. Biting and chewing apparatus with teeth called Aristotle’s Lantern is present. Ambu­lacral grooves are absent. Larval forms are Platens and Echinopluteus.

Echinus (sea-urchin), Clypeaster (cake urchin), Echinarachinus (sand dollar), Echinocardium (heart urchin).

Class 4. Holothuroidea (Gk. Holothurion- sea cucumber, eidos- form):

Body is elon­gated and cylindrical. Oral end has mouth surrounded by tentacles. Ambulacral grooves are absent. Spines and pedicellaria are absent. Larval forms are Auricularia and Doliolaria.

Holothuria (sea cucumber), Cucumaria (sea cucumber).

Class 5. Crinoidea (Gk. Crinon- lily, eidos- form):

Body has a central disc which is attached to the substratum. Arms are branched. Spines and pedicellariae and madreporite are absent. Larval form is Doliolaria. They are commonly called feather stars or sea lilies.

Antedon (feather star). Sea lilies.

It possesses great power of regeneration and shows autotomy. At the terminal end of each ambulacral groove lies a bright red eye. The aboral surface bears many stout spines distributed irregularly. In between the spines, there are present soft dermal branchiae.

They act as respiratory and excretory organs. In between two arms near the anus, there is present a perforated circular plate, the madreporite. There are present microscopic pincer-like structures known as pedicellariae. They also act as organs of offence.

Brittle stars also swim like snake with their arms. Anus is absent.

It moves with the help of spines. The sea urchin has a masti­catory apparatus, called Aristotle’s Lantern because of its resemblance to ancient Greek ship lantern. It is formed by five strong and sharp teeth.

Cucumaria (Sea cucumber):

The sea cucumbers respire by respiratory trees in the cloaca. For this, water is drawn in and expelled through the anus alternately. The mouth is anterior and is surrounded by tentacles.

It has great power of autotomy and regeneration. The body comprises a cup shaped central disc and five slender arms. Each arm is bifurcated, bearing a row of pinnules on each side. It is attached to the substratum.

Easily Study The Characteristics and Classes of Echinoderms

Echinoderms, as their name indicates (echino = spiny, derma = skin), are creatures with spines that stick out from an endoskeleton. Their endoskeleton is made of calcareous plaques that, in addition to spines, contain pedicellaria, small pincers used to clean the body and to help capture prey. They also contain a hydrovascular system known as the ambulacral system. Adult echinoderms have pentaradial symmetry the radial symmetry in these animals is secondary, as it is present only in adults.

4. How can the endoskeleton of echinoderms be compared to similar structures among vertebrates, arthropods and molluscs?

The skeleton of echinoderm is internal that is, it is an endoskeleton. It is made of calcium carbonate (calcareous).

Vertebrates also have an internal skeleton made of bones and cartilage. Arthropods have an external shell made of chitin, a chitinous exoskeleton. Some molluscs have a calcareous shell that functions as an exoskeleton.

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Echinoderm Physiology

5. What system allows echinoderms to move around and attach to certain substances?

The system that allows echinoderms to move and to attach to substrates is called the ambulacral system. In these animals, water enters through a structure called the madreporite, passes through channels and reaches the ambulacral feet along the undersurface of the body. In the ambulacral region in contact with the substrate, there are tube feet which empty and fill with water, thus acting as suckers.

6. What type of digestive system echinoderms contain?

Echinoderms contain a complete digestive system, with a mouth and anus.

7. Do sea urchins have teeth?

Sea urchins have a teeth-like structure attached to the mouth and made of five teeth connected to ossicles and muscle fibers. This structure, known as Aristotle’s lantern, is use to scratch food, mainly algae, from marine rocks.

8. What characteristic of echinoderm embryos makes this phylum evolutionarily resemble chordates?

Echinoderms and chordates are deuterostomes, meaning that, during their embryonic development, the blastopore turns into their anus. All other animals with complete digestive system are protostomes, meaning that their blastopore turns into their mouth.

The blastopore is the first opening of the digestive tract to appear during embryonic development.

Phylum Echinodermata Review - Image Diversity: blastopore

9. Do echinoderms have respiratory and circulatory systems?

In echinoderms, respiratory and circulatory systems are not well-defined (with the exception of the holothurian group). The ambulacral hydrovascular system carries out the tasks of these systems.

10. Do echinoderms have an excretory system? How is excretion carried out in these animals?

Echinoderms do not have an excretory system. Their excretions are eliminated by diffusion.

11. How can the symmetry and the nervous system be described in echinoderms?

Adult echinoderms, along with cnidarians, present radial symmetry, meaning that their body structures are distributed around a central point. However, the radial symmetry in echinoderms is secondary radial symmetry, since their larval stage has bilateral symmetry and the radial pattern appears only in adult specimens (there are a few adult echinoderms with lateral symmetry). All other animals have lateral symmetry with the exception of poriferans (they have no defined symmetry).

Echinoderms do not present cephalization. They have a diffuse network of nerves and neurons made of a neural ring around the mouth and radial nerves that split off into branches to follow the pentaradial structure of the body.

Reproduction in Echinoderms

12. Do echinoderms use internal or external fertilization? Are they divided into separate sexes?

Fertilization among echinoderms is external, as gametes are released into the water, where fertillization occurs.

The majority of echinoderms are dioecious, containing both males and females.

13. Do echinoderms have a larval stage?

In echinoderms, embryonic development is indirect, with ciliated larvae.

Echinoderm Classes

14. Into what classes is the phylum Echinodermata divided?

The five classes of echinoderms are: asteroids (starfish), ophiuroids, crinoids, holothuroids (sea cucumbers) and echinoids (sea urchins and sand dollars).

Summary of Echinoderms

15. The main features of echinoderms. How can echinoderms be described according to examples of representative species, basic morphology, type of symmetry, germ layers and coelom, digestive system, respiratory system, circulatory system, excretory system, nervous system and types of reproduction?

Examples of representative species: sea cucumbers, sea urchins, starfish. Basic morphology: calcareous endoskeleton with spines, ambulacral system. Type of symmetry: secondary radial. Germ layers and coelom: triploblastics, coelomates. Digestive system: complete, deuterostomes. Respiratory system: nonexistent. Circulatory system: nonexistent. Excretory system: nonexistent. Nervous system: simple, nerve network without ganglia or cephalization. Type of reproduction: sexual, with a larval stage.

Problem: The _______________ comprise the largest class of echinoderms in terms of numbers.a. brittle starsb. sand dollarsc. sea urchinsd. sea liliese. sea stars

The _______________ comprise the largest class of echinoderms in terms of numbers.

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Introduction: What are Echinoderms?

Echinoderms are a group of marine animals consisting of well known organisms such as the starfish, sea cucumber and the sand dollar. The phylum Echinodermata consists of about 7000 living species and the phylum is divided into five smaller classes. Echinodermata is Greek for “spiny skinned.” This is clearly seen on echinoderms such as the brittle star and the sea urchin. The most well-known echinoderms are the species of five-armed sea stars. However, other sea stars species have been found to have up to 40 arms (National Geographic). Many species of echinoderms also have unique features in their bodies which allow them to regenerate a lost limb, spine, or even intestine if it is lost, for example, to predation (Mashanov, 2014). Some echinoderms can regenerate a whole new body from a severed arm (National Geographic). This process has important consequences for scientist studying regeneration in vertebrates, like humans (Mashanov, 2014). Echinoderms are very important in both the environment and to people as well. Sometimes these effects by the echinoderms can be positive or negative. Without echinoderms, many areas of the ocean would be greatly affected and therefore, echinoderms are an important animal phylum to learn about.

It is estimated that there are up to 13,000 extinct species of echinoderms and that the very first echinoderm was alive in the Lower Cambrian period. This period of time would range from 490-540 million years ago. The oldest fossil available is called Arkarua. This species was small, round and disc-like with five grooves extending from the center (Echinoderm Fossils). The first echinoderm was thought to be very simple (Knott, 2004). The organism was motile and bilateral in symmetry. Bilateral symmetry means the organism can be cut right down the middle and be split into two equal halves. The echinoderm ancestry later developed radial symmetry as it was thought to be more advantageous to the species. The bilateral symmetry can still be seen in the larvae of echinoderms but once they reach adulthood, they develop radial symmetry. The first picture below shows an echinoderm larvae and the bilateral symmetry is clearly shown. The concept of radial symmetry is clearly illustrated in starfish including the Horned starfish (Protoreaster nodosus), shown below. Species of starfish, like the common starfish, have five radially symmetrical projections projecting from a central disk. These feet have symmetrical outer and inner structures (Zubi, 2013).

Bilateral Symmetry in Starfish Larvae

This picture represents the bilateral symmetry of the echinoderm larvae. The red line dissects down the middle and divides the larvae into two equal halves. Throughout development the bilateral symmetry is lost and becomes radial symmetry.

Radial Symmetry in an adult Starfish

This picture clearly shows the radial symmetry of starfish. Specifically this starfish has pentaradial symmetry.

The extant echinoderms are divided into five clades including the Sea Lilies (Crinoidea), Starfish (Asteroidea), Brittle Stars (Ophiuroidea), Sea Urchins (Echinoidea), and Sea Cucumbers (Holothuroidea). Out of these it is clear that they form a monophyletic group, however there is doubt as to their phylogenetic relationship within the tree itself. This debate is based on whether Brittle Stars (Ophiuroidea) and Starfish (Asteroidea) form a sister clade, i.e. they are each others closest relative, or not (Wray, 1999). Today there are only really two well supported hypotheses those are as follows:

1. Asterozoan Hypothesis: In this hypothesis it is believed that Brittle Stars and Starfish form a sister clade, and just like in the Cryptosyringid hypothesis Sea Urchins and Sea Cucumbers form another sister clade and Sea Lilies is the most basal group. This hypothesis is based off of molecular phylogenetic studies which help to show that even though Brittle Stars has a pluteus-type larva which is the larval form of both Sea Urchins and Sea Cucumbers this could just be a result of convergent evolution or that Starfish reverted to an older form of larval form (Telford, 2014).

2. Cryptosyringid Hypothesis: Similar to the previous hypothesis, Sea Lilies is the most basal group, however in this hypothesis Brittle Stars and Starfish do not form a sister clade. This hypothesis has support in the development of the organism so that Brittle Stars are sister to Sea Urchins and Sea Cucumbers. This is because they all share a common larval state during early development which could imply that Brittle Stars are more closely related to the sister group containing Sea Urchins and Sea Cucumbers than Starfish (Telford, 2014).

Now that their placement among themselves is better understood, where do Echinoderms in general fit in with other animals and other organisms? Echinoderms fit in the superphylum deuterostomes of which composes animals who during development the anus forms first unlike the protostomes which have mouth first development. Humans also fall into this superphylum whereas snails and insects develop mouth first. they are within the supergroup unikonts which is also composed of many animals.

The above figure represents the phylogenetic tree of the Echinodermata back to the supergroup Unikonts (Keeling, 2009). The associated divergence dates, or estimated time periods a group split from a common ancestor, are included above in millions of years (MYA) (Hedges, 2006).

The oldest echinoderms found to date are from the Cambrian period. This period was about 540 million years ago. Some fossils have been found that may be an ancient echinoderm, but there is no definite proof at the moment. The ancient phyla of echinoderms was divided into classes based on body geometry, type of plating, body symmetry and the absence or presence of appendages. Three basic body plans emerged during the Cambrian echinoderms (Scripps Institution of Oceanography, 2011).

  • Ctenocystoids: with or without appendages, tessellate plate type and a lateralized and symmetrical/asymmetrical body plan.
  • Helicoplacoidea: no appendages, imbricate type plates, ellipsoidal shaped body and helical symmetry.
  • Edrioasteroid: no appendages, tessellate and imbricate plate type, disc shaped body and pentaradial symmetry.

From the middle of the Cambrian period to the mid to late Ordovician period, the class diversification of the echinoderms occurred twice. According to the fossil record, the diversification decreased at the end of the Cambrian period but this may be due to the lack of artifact preservation. No diversification is more significant than the time known as the Great Ordovician Biodiversification Event (GOBE). The class level during this period was as high as 21. From the Cambrian period to the Ordovician period, eleven new classes originated. Since this peak of diversification, the amount of class diversity gradually decreased. Eventually the amount of classes decreased to eight. With the Blastoids, Ophiocistiods and Isorophid edrioasteroids going extinct in the Permian period, there were only five classes that survived the Mesozoic. These five classes are the same classes that are around today, including, Starfish (Asteroidia), Sea Lilies (Crinoidea), Sea Urchins and Sand Dollars (Echinoidia), Sea Cucumbers (Holothuroidea), and Brittle Stars (Ophiuroidea)(Fossil record of Echinoderms).

Key evolutionary innovations:

Echinoderms developed many key evolutionary characteristics that define all species within the phylum, making them one of the most unique animal phyla. Four major synapomorphies are identifiable within all species of the Echinoderms that distinguish all members of the phylum. A synapomorphy are traits or characters recognized specifically with that species.

Radial Symmetry: Unlike chordates, like humans or sharks, echinoderms possess a radially symmetrical body plan. In almost all situations involving echinoderms, the species exhibits pentamerous radial symmetry (pentaradial), or five sided radial symmetry. What this means is that observed head on, an observer will be able to distinguish five separate, interconnected segments that are all similar in shape, appearance, and anatomy (Morris, 2009). The best group of animals to show this radial symmetry are the starfish.

Water Vascular System: In Echinoderms, the water vascular system is their key to everyday living. It provides Echinoderms with many functions, including gas exchange, locomotion, feeding, and respiration. The system allows sea-water to be facilitated through an external pore located on the upper portion of the organism called a madreporite, which acts as like a filtered water pump to bring in and excrete water. This system also provides Echinoderms their locomotion through specialized tube feet. Tube feet provide locomotion for most Echinoderms by expanding and retracting from an individual when water is pushed into or syphoned out of these structures, allowing them to move within their environment to hunt for food and locate shelter. These tube feet also provide Echinoderms with their primary sensory perception as they possess numerous nerve endings, giving them a “view” of their surrounding environment (Class Notes, Knott, 2014). One species which takes advantage of tube feet locomotion is the pincushion sea urchin (Lytechinus variegatus). They posses many tube feet which provide them with sensory information about their environment and assist with locomotion. Below is a video of the starfish using its tubed feet to walk along the tank.

Sea Urchin Tubed Feet

This video shows how the Sea Urchin uses its tubed feet to attach to the wall of an aquarium. They suction cup onto the glass for attachment and movement.

Mesodermal Skeleton: Echinoderm’s skeleton is unique to the animal kingdom. It is made up of many tiny plates or spines called ossicles, which are comprised of calcium carbonate. In a typical animal, this would lead to the organism having a heavy skeleton, but in the case of Echinoderms, they remain light through a sponge like material called stereom. Instead of having a rigid skeleton, the stereom is porous, being comprised of a network of calcium crystals that give an echinoderm its shape and rigidity without carrying extra mass (Manton, 2014). Below is a photo of an exposed skeleton of the common starfish (Asterias rubens).

This is a photograph of an exposed skeleton of a starfish, as indicated by the arrow. The network of porous ossicles is evident in this structure.

Mutable Collagenous Tissue: Echinoderms possess special type of tissue that in effect can very rapidly change from a rigid state to a free moving, or loose, state using its nervous system. These tissues are key to connecting ossicles together as ligaments made up of primarily collagen. This allows Echinoderms to achieve a wide variety of body positions with very minimal, to no muscular effort, and then instantly lock into place. This provides a unique feeding advantage as well, as in the case of sea stars where they can envelop a selected prey species in a loose tissue state, and then incapacitate them by quickly changing to a rigid state (Knott, 2004).

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