The phylum contains about 7,000 living species, making it the second-largest grouping of deuterostomes, after the chordates; they are also the largest phylum that has no freshwater or terrestrial representatives.
The word derives from the Greek εχινοδέρματα (echinodermata), plural of εχινόδερμα (echinoderma), "spiny skin" and that from εχινός (echinos), "sea-urchin", originally "hedgehog + δέρμα (derma), "skin.
The Echinoderms are important both biologically and geologically: biologically because few other groupings are so abundant in the biotic desert of the deep sea, as well as the shallower oceans, and geologically as their ossified skeletons are major contributors to many limestone formations, and can provide valuable clues as to the geological environment. Further, it is held by some that the radiation of echinoderms was responsible for the Mesozoic revolution of marine life.
Two main subdivisions of Echinoderms are traditionally recognised: the more familiar, motile Eleutherozoa, which encompasses the Asteroidea (starfish), Ophiuroidea (brittle stars), Echinoidea (sea urchins and sand dollars) and Holothuroidea (sea cucumbers); and the sessile Pelmatazoa, which consists of the crinoids. Some crinoids, the feather stars, have secondarily re-evolved a free-living lifestyle.
A fifth class of Eleuetherozoa consisting of just two species, the Concentricycloidea (sea daisies), were recently merged into the Asteroidea. The fossil record contains a host of other classes which do not appear to fall into any extant crown group.
All echinoderms are exhibited fivefold radial symmetry in portions of their body at some stage of life, even if they have secondary bilateral symmetry. Many crinoids and some starfish exhibit symmetry in multiples of the basic five, with starfish such as Helicoilaster spp. known to possess up to 50 arms, and the sea-lily Comanthina schlegelii boasting 200.
In spite of their potentially misleading name and sometimes foreboding appearance, the echinoderms do not possess an external skeleton. Rather, a thin outermost skin covers a mesodermal endoskeleton made of tiny calcified plates and spines, which forms a rigid support contained within tissues of the organism. Some groups, such as the sea urchins, also possess calcareous spines that serve to protect the organism from predation and colonisation by encrusting organisms; the sea cucumbers secondarily use these spines for locomotion. These spines too are covered by a thin layer of epidermis.
The calcite grown by the organisms is diagnostically rich in the element magnesium; they may consist of 3 to 15 % magnesium oxide. The abundance of this small element property confers them a higher skeletal density, and the chemical properties of magnesium encourage it to form stronger bonds — making for a stronger, more resistant skeleton. The feeding apparatus of the echinoderms is particularly enriched in magnesium; the rock-grazing lifestyle of the sea urchins makes their mandibles especially prone to wear, thus the extra strength provides a significant advantage, outweighing the metabolic costs involved in concentrating the magnesium.
Despite the robustness of the individual skeletal modules, complete echinoderm skeletons are rare in the fossil record. This is because they quickly disarticulate once the encompassing skin rots away, and in the absence of tissue there is nothing to hold the plates together. The modular construction is a result of the growth system employed by echinoderms, which adds new segments at the centre of the radial limbs, pushing the existing plates outwards in the fashion of a conveyor belt. The spines of sea urchins are most readily lost, as they are not even attached to the main skeleton in life. Each spine can be moved individually and is thus only loosely attached in life; a walk above a rocky shore will often reveal a large number of spineless but otherwise complete sea urchin skeletons.
Skeletal elements are also deployed in some specialised ways; as well as the famous feeding organ of the sea urchins, the "Aristotle's lantern", crinoids' stalks and the supportive "lime ring" of sea cucumbers consist of specialised calcite plates.
The epidermis itself consists of cells responsible for the support and maintenance of the skeleton, as well as pigment cells, mechanoreceptor cells, which detect motion on the animal's surface, and sometimes gland cells which secrete sticky fluids or even toxins.
The varied and often vivid colours of the echinoderms are produced by the action of the skin pigment cells. These may be light sensitive, and as a result many species change appearance completely as night falls. The reaction can happen very quickly — the sea urchin Centrostephanus discolours longispinus changes from jet black to grey-brown in just 50 minutes when exposed to light. The colours are produced by a variable combination of coloured pigments, such as the dark Melanin, red Carotinoids, and Carotinproteins, which can be blue, green or violet.
They have a simple radial nervous system that consists of a modified nerve net — interconnected neurons with no central brain (although some do possess ganglia.) Nerves radiate from central rings around the mouth into each arm; the branches of these nerves coordinate the movements of the organism.
The gonads of the organisms occupy the entire body cavities of sea urchins and sea cucumbers; the less voluminous crinoids, brittle stars and starfish having two gonads per arm. Whilst the primitive condition is considered to be one genital aperture, many organisms have multiple holes through which eggs or sperm may be released.
In some species of feather star, the embryos develop in special breeding bags, where the eggs are held until sperm released by a male happen to find them and fertilize the contents. This can also be found among sea urchins and sea cucumbers, where exhibit care for their young can occur, for instance in a few species of sand dollars who carry their young between the pricks of their oral side, and heart urchins possess breeding chambers. With brittle stars, special chambers can be developed near the stomach bags, in which the development of the young takes place. Species of sea cucumbers with specialized care for their offspring may also nurse the young in body cavities or on their surfaces. In rare cases, direct development without passing through a bilateral larval stage can occur in some starfish and brittle stars. Another strategy that has evolved in some starfish and brittle stars is the ability to reproduce asexually by dividing in two halves while they are small juveniles, while turning to sexual reproduction when they have reached sexual maturity. These species have six arms.
The development of an echinoderm begins with a bilaterally symmetrical embryo, with a coeloblastula developing first. Gastrulation marks the opening of the "second mouth" that places them within the deuterostomes, and the mesoderm, which will host the skeleton, migrates inwards. The secondary body cavity, the coelom, forms by the partitioning of three body cavities.
Upon metamorphosis, each taxon produces a distinct larvum, the left hand side of which develops into the adult organism, the right hand side eventually being absorbed; the left hand side typically becomes the oral plate.
F.M. Balfour and D.I. Williamson hold that no echinoderms acquired larvae until after the classes of the phylum were established, i.e. after the Ordovician. Some modern brittle stars and heart urchins have no larvae, and they develop as protostomes. H.B. Fell and Williamson (2003) argue that the original echinoderms were radial protostomes and the bilateral larvae were later additions.
The larvæ of many echinoderms, especially starfish and sea urchins, are pelagic, and with the aid of ocean currents can swim great distances, reinforcing the global distribution of the phylum.
Crinoids employ a large net-like structure to sieve water as it is swept by currents, and to adsorb any particles of matter sinking from the ocean overhead. Once a particle touches the arms of the creature, the tube feet act to swish it to the central mouth of the crinoid, where it is ingested, nutrients removed, and the remains egested through its anus to the underlying water column.
Many sea urchins graze on the surfaces of rocks, scraping off the thin layer of algae covering the surfaces. Other toothless breeds devour smaller organisms, which they may catch with their tube feet, whole. Sand dollars may perform suspension feeding.
Sea cucumbers may be suspension feeders, sucking vast quantities of sea water through their guts and absorbing any useful matter. Others use their feeding apparatus to actively capture food from the sea floor. Yet others deploy their feeding apparatus as a net, in which smaller organisms become ensnared.
Whilst some starfish are detritovores, extracting the organic material from mud, and others mimic the crinoids' filter feeding, most are active hunters, attacking other starfish or shellfish. The latter are seized and held by the tube feet; starfish then stiffen their legs, expanding the shell. The starfish can use catch connective tissue to lock their arms in place and maintain a force on the prey whilst exerting minimal effort; the unfortunate victim must expend energy resisting the force with its adductor muscle. When the abductor tires, the starfish can insert its stomach through the opening and release gastric juices, digesting the prey alive.
Echinoderms provide a key ecological role in ecosystems. For example, the grazing of sea urchins reduces the rate of colonization of bare rock; the burrowing of sand dollars and sea cucumbers depleted the sea floor of nutrients and encouraged deeper penetration of the sea floor, increasing the depth to which oxygenation occurs and allowing a more complex ecological tiering to develop. Starfish and brittle stars prevent the growth of algal mats on coral reefs, which would obstruct the filter-feeding constituent organisms. Some sea urchins can bore into solid rock; this bioerosion can destabilise rock faces and release nutrients into the ocean.
The echinoderms are also the staple diet of many organisms, most notably the otter; conversely, many sea cucumbers provide a habitat for parasites, including crabs, worms and snails. The extinction of large quantities of echinoderms appears to have caused a subsequent overrunning of ecosystems by seaweed, or the destruction of an entire reef.
The first universally accepted echinoderms appear in the Cambrian period around . Echinoderms left behind an extensive fossil record. Despite this, there are numerous conflicting hypotheses on their phylogeny. Based on their bilateral larvae, many zoologists argue that echinoderm ancestors were bilateral and that their coelom had three pairs of spaces (trimeric).
Some have proposed that radial symmetry arose in a free-moving echinoderm ancestor and that sessile groups were derived several times independently from free-moving ancestors. Unfortunately, this view does not address the significance of radial symmetry as an adaptation for a sessile existence.
The more traditional view is that the first echinoderms were sessile, became radial as an adaptation to that existence, and then gave rise to free-moving groups. This view perceives the evolution of endoskeletal plates with stereom structure and of external ciliary grooves for feeding as early echinoderm developments.
The extinct members of Class Homalozoa, commonly referred to as carpoids, had stereom ossicles but were not radially symmetrical, and the status of their water-vascular system is not known. Further, extinct members of the Class Helicoplacoidea possessed three, true ambulacral grooves, and their mouth was on the side of their body.
Attachment to a substratum would have selected for radial symmetry and may have marked the origin of the Class Crinoidea. Members of Crinoidea, along with the extinct members of Class Cystoidea, were primitively attached to a substratum by an aboral stalk. An ancestor that became free-moving might have given rise to Asteroidea, Ophiuroidia, Holothuroidea, and Echinoidea.
The economic impact of Echinoderms is primarily local. Around 50,000 tons of sea urchins are captured each year, the gonads of which are consumed particularly in Japan, Peru and in France. The taste is described as soft and melting, like a mix of seafood and fruit. The quality depends on the color, which can range from light yellow to bright orange.
Sea cucumbers are also considered a delicacy in some countries of south east Asia; particularly popular are the pineapple roller Thelenota ananas (susuhan) and the red Halodeima edulis. They are well known as bêche de mer or Trepang in China and Indonesia. The sea cucumbers are dried, and the potentially poisonous entrails removed. The strong poisons of the sea cucumbers are often psychoactive, but their effects are not well studied. It does appear that some sea cucumber toxins restrain the growth rate of tumour cells, which has sparked interest from cancer researchers.
The calcareous tests or shells of echinoderms are used as a source of lime by farmers in areas where limestone is unavailable; indeed 4,000 tons of the animals are used annually for this purpose. This trade is often carried out in conjunction with shellfish farmers, for whom the starfish pose a major irritation by eating their stocks.
Echinoderms, like chordates, are deuterostomes and are therefore thought to be the most closely related of the major phyla to the chordates, being a sister group to chordates plus hemichordates. (Some believe that acorn worms are more closely related to echinoderms than chordates.) Because of a controversial interpretation of Homalozoa, a minority of classifiers place the echinoderms into the Chordata). Williamson (2003) disputes the links to hemichordates and chordates. They are based on larvae, which (Williamson claims) were later additions to life-histories. And pteropod hemichordates have larvae resembling trochophores, which would link them with annelids and molluscs. The phylogeny below is based on Smith (2005). It should be noted that this topology is not unilaterally agreed upon. Asteroids and Ophiuroids are frequently supported as sister groups using fossil evidence and molecular data.