Phylum Echinodermata: Spiny-Skinned Sea Animals

Phylum Echinodermata is the group of animals we call sea stars, sea urchins, brittle stars, sea cucumbers and feather stars, the spiny-skinned animals of the sea. An echinoderm is an animal that walks on hundreds of tiny hydraulic tube feet, wears its skeleton inside its skin, and can regrow a lost arm or even its whole gut. No echinoderm has ever lived in fresh water or on land: every one of the roughly 7,000 living species is marine, from tide pools to the deep ocean floor.

Sea stars, urchins and sea cucumbers look so different from one another that a first-time observer could mistake them for three separate phyla, yet they share one unmistakable body plan. Adults are built around five, or a multiple of five, radiating from a central disc, a pattern called pentaradial symmetry. Their larvae, however, are bilateral, swimming on two sides like other animal larvae, and only during metamorphosis does the five-part adult body take shape. That switch from bilateral larva to pentaradial adult is one of the most striking transformations in the animal kingdom, and it is the key to understanding the whole phylum.

The word Echinodermata comes from the Greek echinos, meaning spiny or hedgehog, and derma, meaning skin: the spiny-skinned ones. The spines belong to a skeleton of chalky plates, called ossicles, that lies inside the skin rather than outside it, and the whole animal is powered by a water vascular system, a set of canals filled with seawater that works its tube feet like the hydraulics of a machine. Echinoderms are also deuterostomes, which makes them, surprisingly, closer cousins of chordates, the group that includes fish, birds and humans, than of the worms and molluscs they share the seabed with. This article explains their body plan, their five classes, their reproduction and regeneration, and their ecological roles, in plain language and at full depth.

Phylum Echinodermata at a Glance

Echinoderms are triploblastic, coelomate animals with an enterocoelous coelom, a calcareous endoskeleton of ossicles, and a water vascular system that is unique in the animal kingdom. They have no head, no brain, no heart and no excretory organs, yet they are among the most successful marine animals on Earth. Their success rests on three inventions: a hydraulic water vascular system, a skeleton that can stiffen or soften on demand, and a capacity for regeneration that few animals can match.

The phylum contains about 7,000 described living species and roughly 13,000 known fossil species, a record that stretches back to the Cambrian period more than 500 million years ago. Living echinoderms fall into five classes: the sea stars (Asteroidea), the brittle stars (Ophiuroidea), the sea urchins and sand dollars (Echinoidea), the sea lilies and feather stars (Crinoidea), and the sea cucumbers (Holothuroidea). Brittle stars are the largest class, with about 2,100 species, followed by sea stars with about 1,900, sea cucumbers with about 1,700, sea urchins with about 1,000, and crinoids with about 700.

Echinoderms live at every ocean depth and in every sea, from the Arctic to the Antarctic, from the intertidal zone where the tide leaves them exposed twice a day to the abyssal plains six kilometres down. They are benthic animals, living on or in the seabed, with only a few crinoids and holothuroids able to swim weakly for short distances. Their ecological roles are large: sea stars are keystone predators, sea urchins are grazers that can reshape whole kelp forests, sea cucumbers recycle the sediment of the ocean floor, and crinoids filter the water column. Understanding echinoderms therefore means understanding how a large part of the seabed works.

The Name and the Spiny Skin

The name Echinodermata was given to the group because the skin of most of its members is armed with spines. The spines are not separate structures stuck onto the skin: they are extensions of the internal skeleton, the ossicles, that push outward through the epidermis. In a sea urchin the spines are long, sharp and movable, worked by small muscles at their bases, and they give the animal its famous ball of needles. In a sea star the spines are short and blunt, often arranged in rows along the arms, and in a sea cucumber they are reduced to tiny microscopic plates buried in the soft body wall, so the animal feels leathery rather than spiny.

Between the spines, many echinoderms carry a second defensive tool: the pedicellariae. These are tiny pincer-like organs, each with two or three jaws worked by muscles, that sit on flexible stalks on the body surface. Pedicellariae snap shut on anything that settles on the animal, from settling larvae to small parasites, keeping the surface clean, and in some sea urchins they carry venom and serve as a genuine defence. A sea urchin bristling with spines and pedicellariae is a difficult meal, which is one reason urchins can graze openly on the seabed with few predators brave enough to tackle them.

The skin itself is a thin epidermis over a dermis that holds the ossicles. The ossicles are made of calcite, a form of calcium carbonate rich in magnesium, laid down as a porous, sponge-like mesh called stereom. Because the skeleton is inside the skin, it is an endoskeleton, although it is very different from the endoskeleton of a vertebrate: echinoderm ossicles are separate plates joined by connective tissue and muscle, not a single rigid frame. That connective tissue is itself remarkable. It is a mutable collagenous tissue, sometimes called catch connective tissue, whose stiffness the animal can change in seconds under nervous control. A sea star can make its arms rigid to lever open a mussel and then soft again to crawl away, and a sea cucumber can stiffen its body wall when disturbed and soften it to squeeze into a crevice.

Exclusively Marine: A Phylum That Never Left the Sea

Every echinoderm is marine, and no species has ever colonised fresh water or land. The reason is physiological: echinoderms are osmoconformers, which means the salt concentration of their body fluids follows the salt concentration of the seawater around them. They have no kidneys or other organs for regulating salt and water, and no waterproof skin. In fresh water their tissues would swell and their water vascular system would fail, and on land they would dry out within minutes. Even in the sea they are sensitive to changes in salinity, and most species live in full-strength seawater, although a few hardy sea stars and brittle stars tolerate the brackish water of estuaries.

Within the sea, echinoderms occupy almost every habitat. Sea stars and urchins are common on rocky shores, where the tide exposes them to air, sun and hungry birds, and their tube feet and protective spines let them cling and survive until the tide returns. Sand dollars and heart urchins burrow in sand, sea cucumbers plough through soft mud on the deep seafloor, and stalked sea lilies anchor themselves to hard ground in deep water, waving their feathery arms in the current. Crinoids were far more common in the Palaeozoic seas, when vast meadows of stalked sea lilies covered the seabed, and their fossilised stalks, the little discs called St Cuthbert's beads, are still collected on beaches today.

The exclusively marine life of echinoderms has a deep evolutionary meaning. The phylum evolved in the sea and never left it, and its whole body plan, the water vascular system, the calcareous skeleton and the tube feet, is a solution to life in seawater. The skeleton itself depends on seawater: calcite is easy to precipitate from seawater, and the animal draws its building material directly from the water around it. That dependence is also a vulnerability today, because as the oceans absorb carbon dioxide and grow more acidic, calcite becomes harder to lay down, and the larvae and skeletons of echinoderms are among the first to show the strain.

Two Symmetries in One Life: Bilateral Larvae, Pentaradial Adults

The most surprising fact about echinoderms is that the five-part symmetry of the adult is secondary. The larva is bilateral, with a clear left and right, a mouth and an anus, and a band of cilia that sweeps food into its mouth as it swims in the plankton. The adult symmetry is not present at birth: it appears during metamorphosis, when the larva settles to the seabed and rebuilds its body around five radii. Echinoderms are therefore a living demonstration that radial symmetry can evolve from bilateral ancestors, and that the adult body plan of a phylum need not match the body plan of its own young.

Each class has its own larval form, and the names of the larvae are part of every zoology course. Sea stars produce a bipinnaria larva, a transparent, ciliated swimmer with two bands of cilia, which in many species develops into a brachiolaria larva with three short arms used for settling. Brittle stars and sea urchins produce pluteus larvae, delicate, transparent forms held up by long, slender arms stiffened with calcareous rods; the brittle-star larva is called an ophiopluteus and the sea-urchin larva an echinopluteus. Sea cucumbers produce an auricularia larva, shaped like a human ear, which becomes a barrel-shaped doliolaria larva with rings of cilia. Crinoids produce a doliolaria larva directly, a small ciliated barrel that swims briefly before settling and growing a stalk.

Metamorphosis is radical. The larval tissues are largely broken down and the adult body grows from a small cluster of cells called the imaginal rudiment, which is already laid out in five parts. The larval mouth and anus are lost and new ones form in new positions, the larval skeleton dissolves, and the water vascular system appears. In sea stars the adult grows out of one side of the larva, so the finished animal is, in a real sense, built sideways onto its own young. Once settled, the young echinoderm is a miniature adult, and it grows by adding new ossicles and lengthening its arms, not by changing its basic pattern.

Not every adult is perfectly five-part. Some sea stars have many arms, the sunflower star can have up to 24, and some species habitually have six or seven. Heart urchins and sand dollars are secondarily bilateral, elongated in the direction of movement so that they can burrow efficiently, and sea cucumbers are secondarily bilateral too, stretched out along an oral-aboral axis so that they lie on their sides and crawl like worms. In each of these cases the five-part pattern is still visible in the internal anatomy, in the five ambulacral grooves or the five rows of tube feet, even when the outside of the animal has lost it. The five-part symmetry is therefore a deep feature of the phylum, and its exceptions prove the rule.

The Calcareous Endoskeleton: Ossicles, Spines and Plates

The echinoderm skeleton is an endoskeleton, but it is built on a different principle from the skeleton of a vertebrate. Instead of a few large bones, it consists of many small plates and rods, the ossicles, each one a single crystal of magnesium-rich calcite laid down by cells of the dermis. The ossicles are joined by ligaments of mutable collagenous tissue and by muscles, so the skeleton is a flexible framework that can be stiffened or relaxed at will. In a sea star the ossicles form a mesh in the body wall, with larger plates along the arms, and the animal can lock the mesh rigid to prise open a mussel. In a sea urchin the ossicles are fused into a rigid test, a hollow ball of interlocking plates pierced by rows of tiny holes through which the tube feet emerge.

The shape and arrangement of the ossicles define each class. Sea stars have a central disc from which five or more arms radiate, each arm carrying a groove on its underside, the ambulacral groove, along which the tube feet are arranged in rows, and a central nerve cord and water vascular canal run the length of each arm. Brittle stars have a sharply defined central disc and five long, slender, highly flexible arms that are clearly separate from the disc and are used for rowing across the seabed. Sea urchins have no arms at all: the test is a rigid sphere or flattened disc, with five pairs of rows of tube feet, the ambulacra, alternating with five interambulacra, and a mouth on the underside equipped with a complex jaw apparatus. Sea cucumbers have reduced the ossicles to microscopic plates, spicules and wheels embedded in a thick, muscular body wall, so the animal is soft and worm-like, with five rows of tube feet running along its length. Crinoids have a cup-shaped calyx of plates on top of a stalk of stacked disc-like ossicles, with five feathery arms branching from the calyx.

The ossicles also carry the spines, the pedicellariae and, in many species, the small bumps and knobs that give each species its texture. Because ossicles are made of calcite, they fossilise well, and the fossil record of echinoderms is one of the richest in the animal kingdom: entire fossil crinoid meadows, complete fossil sea urchins and the isolated ossicles of animals that lived hundreds of millions of years ago. The skeleton is also the reason echinoderms are so sensitive to ocean acidification: as seawater pH falls, the calcite of the ossicles dissolves more easily, and the larvae, which must build their first skeletons from seawater, are the most vulnerable of all.

The Water Vascular System: A Hydraulic Body Plan

The water vascular system is the defining feature of the phylum, and no other animal has anything like it. It is a system of canals filled with seawater, derived from the coelom, that powers the tube feet. Seawater enters through the madreporite, a perforated, sieve-like plate on the upper, aboral surface of the animal, and flows down the stone canal, so called because its walls are stiffened with calcareous rings, into a ring canal that encircles the mouth. From the ring canal, five radial canals run out along the ambulacral grooves, one into each arm or ambulacrum, and from each radial canal, side branches lead to the tube feet. Each tube foot is a hollow, muscular tube ending in a sucker, and at its base sits a small muscular sac, the ampulla. When the ampulla contracts, it forces water into the tube foot, which extends; when the muscles of the tube foot contract, water is forced back into the ampulla and the foot shortens. By coordinating hundreds of tube feet in waves, the animal walks, climbs, clings and handles food.

The madreporite is not in the same place in every class. In sea stars, brittle stars and sea urchins it sits on the aboral surface, a pale, porous plate that is easy to see. In sea cucumbers it is internal, opening into the coelom instead of to the outside, so the water vascular system is filled from the body cavity rather than from the sea. In crinoids the madreporite is also modified, with many small pores opening into the cup. Whatever its position, the madreporite acts as both an inlet and a filter, keeping sand and debris out of the canals while letting seawater in to replace what is lost.

The tube feet, or podia, do far more than walk. Their walls are thin and permeable, so they are the animal's main surface for gas exchange: oxygen diffuses in and carbon dioxide diffuses out across them. They are also sensory, carrying chemical and touch receptors that let the animal taste the surface it walks on, and they are the animal's hands: a sea star uses its tube feet to grip a mussel shell, to pull it open, and to hold it while the stomach is everted into it; a sea urchin uses them to cling to rock in heavy surf; a sea cucumber uses them to anchor itself while it feeds. Some tube feet end in suckers, some in pointed tips used for burrowing, and in feather stars the tube feet are modified into small, food-catching pinnules along the arms. One hydraulic system thus serves locomotion, feeding, breathing and sensing, and it is the reason echinoderms can do so much with so little in the way of muscles, nerves and organs.

The water vascular system works together with the rest of the coelom. Echinoderms have a large body cavity, the perivisceral coelom, which houses the gut and gonads, and the water vascular system is a separate, specialised part of the coelom, connected to it only through the madreporite and the stone canal. The body cavity also contains the haemal system, a reduced set of vessels without a heart, in which a contractile vessel moves fluid sluggishly around the gut. There is no blood and no heart in the vertebrate sense, and the circulation of nutrients depends largely on the movement of coelomic fluid and the beating of cilia. Excretion is equally simple: there are no kidneys, and ammonia and other wastes diffuse out across the thin walls of the tube feet, the skin gills called papulae, and in sea cucumbers the respiratory trees, so the animal's waste simply leaks away into the sea.

Feeding and Digestion: From Everted Stomachs to Deposit Feeding

Each class of echinoderm feeds in its own way, and the differences show how one body plan can be adapted to many diets. Sea stars are the most dramatic feeders. Many species prey on bivalve molluscs, and they open a mussel or clam by a slow contest of strength: the sea star wraps its arms around the shell, attaches hundreds of tube feet to the two valves, and pulls steadily for hours. A mussel can resist a sudden pull, but it cannot resist a steady one, and eventually its adductor muscle tires and the shell gapes a fraction of a millimetre. That is enough. The sea star everts its stomach, pushing the cardiac stomach out of its mouth and into the shell, where it secretes digestive enzymes that liquefy the mollusc's soft tissues, and then draws the stomach back in, carrying the digested meal with it. Other sea stars are scavengers, detritus feeders or predators of worms, small crustaceans and other echinoderms, and a few, like the crown-of-thorns starfish, feed on coral polyps, sometimes in outbreaks that can strip a reef.

Brittle stars feed more quietly. Most are deposit feeders or suspension feeders: they extend one or two arms into the water or across the sediment, trap particles on the mucus of their tube feet, and pass the food along the arm to the mouth. Their arms are jointed and flexible, worked by muscles between the vertebral ossicles, and they can also sweep food into the mouth with a rowing motion. Some brittle stars are scavengers and a few are active predators of small animals, but none of them everts a stomach, and their gut is a simple sac without an anus, so undigested material is ejected through the mouth.

Sea urchins are grazers, and their feeding apparatus is the most complex mouth in the phylum. The mouth, on the underside of the test, holds Aristotle's lantern, a five-sided jaw apparatus made of five calcareous teeth, each carried on a pyramid-shaped ossicle and worked by a set of muscles. The name goes back to Aristotle, who described the structure and thought it looked like a lantern. With it, an urchin scrapes algae and encrusting organisms from rock, bites through kelp, and even bores into soft rock. Regular urchins, the round ones, graze on the surface; irregular urchins, the sand dollars and heart urchins, have lost the lantern and burrow through sediment, feeding on organic particles as they go. A dense population of urchins can graze a kelp forest down to bare rock, creating what ecologists call an urchin barren, a landscape of pink coralline algae and little else.

Crinoids are suspension feeders. A feather star sits on the seabed or clings to a rock and spreads its feathery arms into the current, and the tube feet along the arms, modified into pinnules, catch plankton and detritus and pass them along food grooves to the mouth at the centre of the cup. Stalked sea lilies do the same from the top of a stalk, sometimes a metre tall, filtering water that flows past. Sea cucumbers are mostly deposit feeders: they shovel sediment into their mouths with feeding tentacles, which are modified tube feet around the mouth, and extract the organic matter, passing the cleaned sand out the other end. A single sea cucumber can process many kilograms of sediment in a year, and in doing so it aerates the seabed and recycles nutrients. A few sea cucumbers are suspension feeders, holding their branched tentacles in the water like a crinoid. Across the five classes, then, the same tube feet and the same mouth have been turned to predation, grazing, suspension feeding and deposit feeding, and that versatility is a large part of the phylum's success.

Nervous System: No Brain, No Problem

Echinoderms have no brain and no centralised nervous system, yet they coordinate complex behaviour: a sea star can aim its arms, an urchin can move toward shade, and a brittle star can flee a predator in a chosen direction. The nervous system is a decentralised network of two parts. The ectoneural system lies just under the epidermis and handles sensory input and the motor control of the tube feet and body wall, and the hyponeural system lies deeper and controls the muscles of the gut and the water vascular system. Both are organised as a nerve ring around the mouth, from which five radial nerve cords run out along the ambulacra, one into each arm or ambulacrum, and from these, branches reach the tube feet, spines and skin.

The sense organs are simple but effective. Sea stars carry a simple eyespot at the tip of each arm, a cluster of pigmented cells that can detect light and shade and, in some species, form a crude image, enough to steer the animal toward a reef or away from open sand. The tube feet are rich in chemical receptors, so a sea star can smell its prey, and the skin carries touch receptors that detect contact and water movement. Sea urchins have no eyespots but are sensitive to light across their whole body, and some species can distinguish light from dark well enough to move toward shelter. Brittle stars are famously sensitive to light and to touch, and a shadow passing over a brittle star is enough to send it rowing away. Sea cucumbers have sensory tentacles around the mouth and are sensitive to touch and to chemicals in the water, and crinoids respond to touch and to changes in current by folding their arms.

The absence of a brain does not mean the absence of coordination. A sea star that has lost an arm can still walk, and each arm can act semi-independently, yet the arms cooperate when the animal rights itself after being overturned, a classic test of echinoderm coordination in which the arms work in sequence to roll the animal over. Experiments have shown that the nerve ring is not essential for this: a sea star with its nerve ring removed can still right itself, because the radial nerve cords and the local reflexes of each arm are enough. The nervous system of an echinoderm is therefore a model of distributed control, a network in which intelligence is spread across the body rather than concentrated in a head, and it works because the animal's behaviour, walking, gripping, feeding and righting, is built from local reflexes that add up to a coherent whole.

Reproduction and Life Cycle: Larvae and Metamorphosis

Most echinoderms are dioecious, with separate males and females, and they reproduce by external fertilisation. The gonads are simple sacs, five of them in sea stars, urchins and brittle stars, one in each arm or interambulacrum, and a single gonad in most sea cucumbers, and they discharge eggs and sperm into the water in spawning events that are often synchronised by season, temperature and the phase of the moon. A single female sea star can release millions of eggs in a season, and a single female sea urchin millions more, and the odds for any one egg are small: most are eaten or fail to find food, and only a tiny fraction survive the weeks of planktonic life to settle and metamorphose.

The planktonic larva is the dispersal stage, and it is what carries the species to new shores. The larvae drift with the currents, feeding on phytoplankton, and after days or weeks they settle on the seabed, attach with a specialised adhesive structure, and metamorphose into the adult form. Settlement is not random: larvae respond to chemical cues from the seabed, from the adults of their own species, and from the biofilms that coat rock and shell, and they choose surfaces that will suit the adult. Once settled, the young echinoderm grows rapidly, adding ossicles and lengthening its arms, and most species reach maturity in one to three years, although deep-sea species can grow slowly for decades.

Asexual reproduction also occurs. Some sea stars and brittle stars reproduce by fission, splitting the central disc in two, with each half regenerating the missing arms, and some sea cucumbers can divide transversely. The most famous asexual feat belongs to the sea star genus Linckia, in which a single severed arm, if it carries even a small fragment of the central disc, can regenerate a whole new animal, a process that has been called comet formation because the regenerating arm trails a small new body like the tail of a comet. Sexual reproduction, however, remains the rule, and the combination of enormous egg numbers, a planktonic larval stage and a remarkable power of regeneration gives echinoderms a resilience that has carried them through half a billion years of ocean history.

Classification: The Five Classes of Echinodermata

Living echinoderms are divided into five classes, and the differences between them are among the clearest in the animal kingdom. The table below summarises each class, its common name, its body plan, its feeding and its familiar examples.

Class	Common name	Body plan	Feeding	Examples
Asteroidea	Sea stars	Central disc with five or more arms; ambulacral grooves on the underside; anus present	Predators and scavengers; many evert the stomach onto prey	Common starfish Asterias rubens, crown-of-thorns Acanthaster planci, sunflower star Pycnopodia helianthoides
Ophiuroidea	Brittle stars	Sharp central disc with five long, slender, flexible arms; no anus; gut a simple sac	Deposit and suspension feeders; some scavengers and predators	Common brittle star Ophiothrix fragilis, basket stars, serpent stars
Echinoidea	Sea urchins, sand dollars	No arms; rigid test of fused plates; Aristotle's lantern in regular urchins	Grazers on algae; sand dollars and heart urchins are burrowing deposit feeders	Edible urchin Paracentrotus lividus, long-spined urchin Diadema, sand dollar Echinarachnius parma
Crinoidea	Sea lilies, feather stars	Cup-shaped calyx with five feathery arms; stalked or free-swimming; mouth and anus on the upper surface	Suspension feeders; pinnules catch plankton	Stalked sea lily Metacrinus, feather star Antedon
Holothuroidea	Sea cucumbers	Soft, elongated, worm-like body; ossicles reduced to microscopic plates; five rows of tube feet	Deposit feeders on sediment; some suspension feeders	Edible sea cucumber Holothuria edulis, Stichopus, the swimming Enypniastes

The five classes are not equally old, and the fossil record shows that the crinoids are the most ancient living group, with stalked sea lilies dominating the seas of the Palaeozoic era, while the asteroids, ophiuroids, echinoids and holothuroids diversified later, especially after the great extinction at the end of the Permian period that removed many of the older echinoderm groups. The extinct classes are numerous: the blastoids, the cystoids, the edrioasteroids and the helicoplacoids are all echinoderms that left no living descendants, and their fossils are among the most beautiful in palaeontology. The living five classes are therefore the survivors of a much larger radiation, and the diversity of the phylum today is only a part of what it once was.

The larvae of the five classes are as distinctive as the adults, and they are the reason the phylum can be recognised in the plankton. The table below lists the larval form of each class.

Class	Larva	Notes
Asteroidea	Bipinnaria, then brachiolaria	Two ciliated bands; the brachiolaria has three adhesive arms for settling
Ophiuroidea	Ophiopluteus	Slender arms stiffened by calcareous rods; the skeleton is reduced compared with the echinopluteus
Echinoidea	Echinopluteus	Long arms supported by an elaborate calcareous skeleton; one of the most elegant larvae in the plankton
Crinoidea	Doliolaria	Small barrel-shaped larva with rings of cilia; swims only briefly before settling
Holothuroidea	Auricularia, then doliolaria	The auricularia is ear-shaped; it transforms into a doliolaria before settling

Regeneration and Autotomy: The Echinoderm Superpower

Regeneration is the most famous echinoderm talent, and it is also one of the most studied, because it shows what animal tissues can do when they are allowed to rebuild. A sea star that loses an arm to a predator regrows it, and the new arm is complete, with its own skeleton, tube feet, nerve cord and gonad. The process begins with wound healing, then a mass of undifferentiated cells, a blastema, forms at the cut surface, and the missing structures grow back over weeks or months. The rate depends on temperature and food, and a large arm can take a year to replace fully. Some species go further: in the genus Linckia, a single arm with a fragment of the central disc can regenerate the entire animal, and in a few species an arm without any disc fragment can do the same, although this is rare.

Autotomy, the deliberate shedding of a body part, is the companion of regeneration. A brittle star that is seized by a predator can detach an arm at a specialised breakage plane, where the ossicles are weakly joined and the muscles and nerves are arranged to snap cleanly, and the detached arm continues to writhe, distracting the predator while the animal escapes. The arm is then regrown. Sea cucumbers practise a more extreme form of self-defence: evisceration. When threatened, many species contract violently and expel their gut, respiratory trees and sometimes their gonads through the anus or through a rupture in the body wall, leaving the predator with a tangle of sticky, sometimes toxic, organs while the animal crawls away. The expelled organs are then regenerated over several weeks. A few sea cucumbers can also expel sticky white threads, the Cuvierian tubules, which swell in seawater and entangle an attacker.

The biology behind this regeneration is being studied for its medical promise. Echinoderm regeneration depends on cells that retain the ability to dedifferentiate and rebuild, on a mutable connective tissue that can be remodelled, and on a nervous system that can regrow its cords. Sea cucumbers can regenerate their entire digestive tract, sea stars can regenerate their radial nerve cords, and brittle stars can regenerate their arms repeatedly throughout life. The genes and signals that control this rebuilding are shared, in broad outline, with the genes that control wound healing in vertebrates, which is one reason echinoderm regeneration is studied not only by zoologists but also by researchers interested in regenerative medicine.

Ecological Roles: Keystone Predators and Reef Engineers

Echinoderms shape the ecosystems they live in, and the clearest example is the ochre sea star, Pisaster ochraceus, on the rocky shores of the Pacific coast of North America. In the 1960s, the ecologist Robert Paine removed Pisaster from a stretch of shore and watched what happened: the mussels that the starfish had kept in check spread and crowded out the barnacles, limpets and algae, and the diversity of the shore collapsed. When the starfish returned, diversity recovered. Paine coined the term keystone species for a predator whose effect on its community is far larger than its numbers would suggest, and the ochre sea star became the textbook example. Sea stars are keystone predators in many places, holding the balance between competing prey species and keeping any one of them from taking over.

Sea urchins are the other great engineers of the seabed, but their effect is often the opposite of the sea star's. Where urchins are abundant, they graze algae so heavily that kelp forests disappear and the seabed becomes an urchin barren, a pavement of encrusting pink coralline algae with little food or shelter for other animals. The balance between kelp forest and urchin barren is held by the urchins' own predators, sea otters, lobsters and large fish, and when those predators are removed by hunting or fishing, the urchins multiply and the forest falls. The recovery of sea otters along the Pacific coast, and the return of kelp forests in their wake, is one of the best-documented trophic cascades in ecology, and it runs through the sea urchin.

Sea cucumbers engineer the seabed from below. As deposit feeders they pass huge quantities of sediment through their guts, and in doing so they mix the sediment, release trapped nutrients and keep the surface of the seabed oxygenated, a process called bioturbation. On coral reefs, sea cucumbers also play a chemical role: their digestion of sediment releases calcium carbonate and raises the alkalinity of the surrounding water, which helps corals to build their skeletons. Where sea cucumbers have been overfished for the bêche-de-mer trade, the sediment becomes compacted and poor in oxygen, and the reef suffers. Crinoids, meanwhile, filter the water column, and their faeces fall to the seabed as a rain of organic particles that feeds the animals below. Together, the five classes move energy and material between the water, the sediment and the reef, and the seabed without echinoderms would be a poorer and less stable place.

Deuterostomes: Echinoderms and the Chordate Connection

Echinoderms belong to the deuterostomes, one of the two great branches of the bilaterian animals, and this placement is the reason they matter far beyond the seabed. In a deuterostome, the first opening of the embryo, the blastopore, becomes the anus, and the mouth forms secondarily, whereas in the protostomes, the worms, molluscs and arthropods, the blastopore becomes the mouth. Deuterostome cleavage is radial and indeterminate, which means the early cells are not yet committed to a fate, and the coelom forms by enterocoely, as outpocketings of the gut, rather than by splitting of the mesoderm. These are developmental characters, visible only under the microscope, but they are among the most reliable guides to animal relationships, and they place echinoderms in the same great group as the chordates, the animals with a notochord, which include fish, amphibians, reptiles, birds, mammals and humans.

The closest living relatives of the echinoderms are the hemichordates, the acorn worms and the pterobranchs, and together the echinoderms and hemichordates form a group called the Ambulacraria. The chordates are the sister group of the Ambulacraria, so the echinoderms are, in evolutionary terms, the cousins of every vertebrate. The relationship is confirmed by molecular evidence: the genomes of sea urchins and humans share many of the same developmental genes, including the Hox genes that lay out the body plan, and the sea urchin genome, sequenced in 2006, has become a model for studying the genetic toolkit that all deuterostomes share. A sea urchin is therefore not only a fascinating animal in its own right but also a window into the early history of our own lineage, a living record of the developmental system from which the vertebrate body plan evolved.

The relationship also explains a puzzle: why do echinoderms, which are so close to chordates, look so unlike them? The answer is that the pentaradial symmetry of the adult is a secondary specialisation, laid over a bilateral, deuterostome body plan that is still visible in the larva and in the early embryo. The echinoderm lineage took the deuterostome toolkit and built a sessile or slow-moving, spiny, hydraulic animal out of it, while the chordate lineage took the same toolkit and built a swimming, bilaterally symmetrical animal with a notochord and a dorsal nerve cord. The two lineages are a reminder that evolution works with what it has, and that the same developmental genes can produce a sea star and a human being.

Echinoderms and People

People have eaten echinoderms for thousands of years. Sea urchin roe, called uni in Japan, is a prized food, eaten raw as sushi or in sauces, and the gonads of several species are harvested commercially in Japan, the Mediterranean, Chile and the Pacific. Sea cucumbers are eaten across Asia as bêche-de-mer or trepang, dried and then rehydrated for soups and stews, and the trade in dried sea cucumbers is worth hundreds of millions of dollars a year, which has led to severe overfishing in many places. Sea stars are rarely eaten, although some species are used in traditional medicine, and the most famous human use of a sea star is as a laboratory animal, because the eggs of sea stars and sea urchins are large, transparent and easy to fertilise, and they have taught embryologists more about fertilisation and early development than almost any other animal.

Echinoderms also cause problems. The crown-of-thorns starfish, Acanthaster planci, feeds on coral, and periodic outbreaks, in which thousands of starfish appear on a reef, can destroy large areas of living coral in a matter of months. The outbreaks are natural, but they are made worse by nutrient pollution, which feeds the planktonic larvae, and by the removal of the starfish's predators, and they are one of the greatest threats to the Great Barrier Reef. Sea urchins can be pests too: when their predators are removed, their grazing destroys kelp forests and the fisheries that depend on them, and their spines cause painful injuries to swimmers and divers, with some species carrying venom. Sea cucumbers, when handled roughly, can eviscerate and release toxins, and a few species are poisonous if eaten.

The future of echinoderms is tied to the future of the oceans. Ocean acidification threatens their calcite skeletons and their larvae, warming waters shift their distributions and trigger disease outbreaks, including the sea star wasting disease that has devastated starfish populations along the Pacific coast of North America, and overfishing removes sea cucumbers and urchins faster than they can reproduce. At the same time, echinoderms are protected in marine reserves, studied as models of regeneration and development, and valued as indicators of ocean health: a reef with a healthy, diverse echinoderm fauna is a reef that is working. Their fate is a measure of the state of the sea.

Exam Tips: Echinodermata in FSc, BS and MDCAT Revision

For students revising animal diversity, Echinodermata is a high-yield phylum because it is compact, distinctive and heavily examined. The points that recur in FSc, BS and MDCAT questions are: the meaning of the name, the exclusively marine habitat, the water vascular system and its parts, the five classes with one example each, the change from bilateral larva to pentaradial adult, the calcareous endoskeleton of ossicles, the absence of a brain, heart and excretory organs, and the deuterostome character. The commonest trap is the symmetry question: the larva is bilateral and the adult is pentaradial, and the two must not be confused. The second trap is the skeleton: it is an endoskeleton, not an exoskeleton, because the ossicles lie inside the skin. The third trap is the madreporite: it is the inlet of the water vascular system, not an excretory organ.

A useful way to revise the classes is to learn one example and one feeding habit for each: sea star, predator with an everted stomach; brittle star, deposit feeder with flexible arms; sea urchin, grazer with Aristotle's lantern; feather star, suspension feeder with pinnules; sea cucumber, deposit feeder that eats sediment. For the larvae, learn the five names and match each to its class: bipinnaria for sea stars, ophiopluteus for brittle stars, echinopluteus for sea urchins, doliolaria for crinoids, auricularia for sea cucumbers. For the water vascular system, learn the sequence from the outside in: madreporite, stone canal, ring canal, radial canal, ampulla, tube foot. If you can draw that sequence and explain what each part does, you can answer almost any question on the system.

Key Takeaways

• Phylum Echinodermata means spiny-skinned animals: about 7,000 living species, all marine, including sea stars, brittle stars, sea urchins, sea lilies and sea cucumbers.
• Adults show pentaradial symmetry, but larvae are bilateral, and the adult body is built during metamorphosis from a five-part rudiment.
• The skeleton is an endoskeleton of calcareous ossicles joined by mutable collagenous tissue, and it carries the spines and pedicellariae.
• The water vascular system is unique to the phylum: seawater enters through the madreporite and powers the tube feet, which serve locomotion, feeding, respiration and sensing.
• The five classes are Asteroidea, Ophiuroidea, Echinoidea, Crinoidea and Holothuroidea, each with its own body plan, larva and feeding habit.
• Echinoderms regenerate arms, guts and even whole bodies, and they practise autotomy and evisceration as defences.
• They are deuterostomes, the closest invertebrate relatives of the chordates, and they include keystone predators such as Pisaster ochraceus and reef engineers such as sea cucumbers.

Short Questions

1. Why are echinoderms found only in the sea?

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Echinoderms are found only in the sea because they are osmoconformers with no organs for regulating salt and water. Their body fluids follow the salt concentration of the surrounding water, and they have no kidneys, no waterproof skin and no way to stop water loss. In fresh water their tissues would swell and the water vascular system would fail, and on land they would dry out within minutes. Their calcareous skeleton is also built from materials drawn directly from seawater, so the whole body plan, from the tube feet to the ossicles, is a solution to life in the sea and cannot work anywhere else.

2. How do tube feet help a sea star move?

See answer

Tube feet help a sea star move by hydraulic pressure. Each tube foot is connected to a muscular ampulla, and when the ampulla contracts it forces seawater into the foot, which extends and presses its sucker against the surface. When the muscles of the foot contract, water returns to the ampulla and the foot shortens, pulling the animal forward. Hundreds of tube feet work in coordinated waves along the ambulacral grooves, so the sea star glides slowly but steadily across rock and sand, climbs vertical surfaces, and grips its prey. The same tube feet also serve respiration and sensing, because their thin walls let gases pass and carry chemical receptors.

3. What is the difference between a sea star and a brittle star?

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A sea star and a brittle star differ in the structure of their arms and their gut. In a sea star the arms merge gradually into the central disc, the arms contain extensions of the gut and gonads, and the animal has an anus and everts its stomach to feed. In a brittle star the five arms are sharply set off from the disc, they contain no gut or gonads, they are worked by vertebral ossicles and move by rowing, and the gut is a simple sac without an anus, so waste leaves through the mouth. Brittle stars are also the largest echinoderm class, with about 2,100 species, and they feed mainly by deposit feeding and suspension feeding rather than by active predation.

4. What is Aristotle's lantern and which animals have it?

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Aristotle's lantern is the jaw apparatus of regular sea urchins, a five-sided structure of five calcareous teeth carried on pyramid-shaped ossicles and worked by muscles, sitting in the mouth on the underside of the test. It is named after Aristotle, who described it and thought it resembled a lantern. With it, a sea urchin scrapes algae and encrusting organisms from rock and bites through kelp. Only the regular, round sea urchins have a lantern: the irregular urchins, the sand dollars and heart urchins, have lost it and feed by burrowing through sediment instead.

5. How do sea cucumbers breathe?

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Sea cucumbers breathe mainly through respiratory trees, a pair of branching, tree-like tubes that open into the cloaca at the rear of the animal. The animal draws seawater in through the anus into the respiratory trees, where oxygen diffuses across their thin walls into the coelomic fluid, and then expels the water. They also exchange gases across their tube feet and body wall. This cloacal breathing is unique among echinoderms and it explains why a sea cucumber can survive buried in sediment with only its rear end exposed: it breathes through its anus while it feeds with its mouth.

6. Why are echinoderms called deuterostomes?

See answer

Echinoderms are called deuterostomes because of the way their embryos develop. In a deuterostome the first opening of the embryo, the blastopore, becomes the anus, and the mouth forms later as a second opening, whereas in protostomes the blastopore becomes the mouth. Their cleavage is radial and indeterminate, and their coelom forms by enterocoely, as pouches budding from the gut. These developmental characters place echinoderms in the same great group as the chordates, which is why echinoderms are the closest invertebrate relatives of vertebrates, including humans.

Frequently Asked Questions

What is phylum Echinodermata in simple words?

Phylum Echinodermata is the group of spiny-skinned marine animals that includes sea stars, sea urchins, brittle stars, sea cucumbers and feather stars. They have five-part symmetry as adults, a skeleton of chalky plates inside the skin, and a water vascular system of seawater-filled canals that powers their tube feet. All of them live in the sea.

Are starfish really fish?

No, starfish are not fish, which is why zoologists prefer the name sea stars. A fish is a vertebrate with a backbone, gills and fins, while a sea star is an invertebrate echinoderm with no backbone, no gills and no fins. It moves on tube feet, breathes across its skin and tube feet, and belongs to a phylum that is closer to chordates than to any fish.

Why do adult echinoderms have five-part symmetry?

Adult echinoderms have five-part, or pentaradial, symmetry because their adult body grows from a five-part rudiment during metamorphosis. The larva is bilateral, but as it settles and metamorphoses, the adult body is built around five radii. The five-part pattern is therefore secondary, derived from a bilateral ancestor, and it is visible in the five arms of a sea star, the five ambulacra of an urchin and the five rows of tube feet of a sea cucumber.

How does the water vascular system work?

The water vascular system works by hydraulics. Seawater enters through the madreporite, flows down the stone canal into a ring canal around the mouth, and out through five radial canals to the tube feet. Each tube foot has a muscular ampulla at its base: when the ampulla contracts, water is forced into the foot and it extends; when the foot contracts, water returns to the ampulla and the foot shortens. Coordinated waves of tube feet let the animal walk, cling and feed.

Can a sea star regrow a lost arm?

Yes, a sea star can regrow a lost arm, and the new arm is complete, with skeleton, tube feet, nerve cord and gonad. The process takes weeks to months, beginning with wound healing and the formation of a blastema of undifferentiated cells at the cut surface. In the genus Linckia, a single severed arm carrying a fragment of the central disc can even regenerate an entire new animal.

Are echinoderms related to humans?

Yes, in evolutionary terms. Echinoderms are deuterostomes, the same great group that includes chordates, and their closest living relatives, the hemichordates, are the sister group of chordates. That makes echinoderms the closest invertebrate relatives of vertebrates, including humans. The relationship is visible in development, in the fate of the blastopore and the formation of the coelom, and in the many developmental genes that sea urchins share with humans.

Related reading

• Phylum Cnidaria: Corals, Jellyfish, Hydra
• Introduction to Phylum Protozoa
• Phylum Mollusca

MCQs

1. What does the name Echinodermata mean?

• A. Spiny skin
• B. Many feet
• C. Water animals
• D. Star animals

Answer: A

Explanation: Echinodermata comes from the Greek echinos, meaning spiny, and derma, meaning skin, so it means spiny-skinned animals.

2. Which symmetry do adult echinoderms show?

• A. Bilateral
• B. Pentaradial
• C. Spherical
• D. Asymmetrical

Answer: B

Explanation: Adult echinoderms are built around five radii, a pattern called pentaradial symmetry, although their larvae are bilateral.

3. What is the main function of the water vascular system?

• A. Digestion
• B. Reproduction
• C. Locomotion by tube feet
• D. Blood circulation

Answer: C

Explanation: The water vascular system is a hydraulic system of seawater-filled canals that powers the tube feet, which the animal uses to walk, cling and feed.

4. Which class includes the sea stars?

• A. Asteroidea
• B. Ophiuroidea
• C. Echinoidea
• D. Holothuroidea

Answer: A

Explanation: Sea stars belong to class Asteroidea, named for their star-like, asteroid shape.

5. Which is the largest class of echinoderms by number of species?

• A. Asteroidea
• B. Ophiuroidea
• C. Echinoidea
• D. Crinoidea

Answer: B

Explanation: The brittle stars, class Ophiuroidea, are the largest class, with about 2,100 described species.

6. What are the ossicles of echinoderms made of?

• A. Chitin
• B. Silica
• C. Calcium carbonate
• D. Bone

Answer: C

Explanation: The ossicles are plates of calcite, a form of calcium carbonate rich in magnesium, laid down inside the skin as an endoskeleton.

7. Which echinoderm everts its stomach to feed on bivalves?

• A. Sea star
• B. Brittle star
• C. Sea urchin
• D. Sea cucumber

Answer: A

Explanation: A sea star everts its cardiac stomach out of its mouth into a gaping bivalve shell, digests the soft tissues outside its body, and then draws the stomach back in.

8. What is Aristotle's lantern?

• A. A light organ of deep-sea stars
• B. The larva of a sea urchin
• C. The sieve plate of the water vascular system
• D. The jaw apparatus of sea urchins

Answer: D

Explanation: Aristotle's lantern is the five-sided jaw apparatus of regular sea urchins, with five calcareous teeth used to scrape algae and bite kelp.

9. Echinoderms are deuterostomes, which means that in development:

• A. The blastopore becomes the mouth
• B. The blastopore becomes the anus
• C. The coelom forms by splitting of the mesoderm
• D. Cleavage is spiral and determinate

Answer: B

Explanation: In deuterostomes the blastopore becomes the anus and the mouth forms secondarily, whereas in protostomes the blastopore becomes the mouth.

10. Which echinoderm can expel and regenerate its internal organs?

• A. Feather star
• B. Sand dollar
• C. Sea cucumber
• D. Basket star

Answer: C

Explanation: A threatened sea cucumber can eviscerate, expelling its gut and respiratory trees, and then regenerate the lost organs over several weeks.