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| A brittle star resting on a brain coral|
† = extinct
Echinoderms (Phylum Echinodermata, from the Greek for spiny skin) are a phylum of marine animals found at all ocean depths. The phylum appeared near the start of the Cambrian period, and contains about 7,000 living species, making it the second largest grouping of deuterostomes, after the chordates; they are the largest phylum without freshwater or terrestrial representatives.
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 urchin and sand dollar) and Holothuroidea (sea cucumbers); and the sessile Pelmatazoa, which consist of the crinoids. Some crinoids, the feather stars, have secondarily re-evolved a free-living lifestyle. A fifth class of Eleutherozoa consisting of just two species, the Concentricycloidea (sea daisies), were recently[How to reference and link to summary or text] merged into the Asteroidea. The fossil record contains a host of other classes which do not appear to fall into any extant crown group.
Echinoderms evolved from animals with bilateral symmetry; although echinoderms themselves posses radial symmetry. Echinoderms' larvae are ciliated, free-swimming organisms that organize in a bilaterally symmetric fashion that makes them look like embryonic chordates. Later, the left side of the body grows at the expense of the right side, which is eventually absorbed. The left side then grows in a pentaradially symmetric fashion, in which the body is arranged in five parts around a central axis. All echinoderms exhibit 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 Heli coil aster spp. known to possess up to 50 arms, and the sea-lily Comanthina schlegelii boasting 200.
Describing a symmetrical organismEdit
With a symmetrical animal, the traditional designation of a front and back, posterior and anterior becomes troublesome - what does one term the front? To get around this, a different terminology is used with the Echinoderms. The mouth and anus of the developing adult migrate simultaneously from the front and back of the larvum to opposite ends of the organism, allowing the description of an "oral" (containing the mouth) and opposite "aboral" side. Grazing organisms such as sea urchins tend to have their mouth on the substrate upon which they are feeding, and their anus on their "top" surface; burrowers such as sea cucumbers have their mouth at their front, and the anus behind their direction of travel. Hence the sea-cucumbers appear to have evolved from a sea-urchin-like organism which gradually tipped on its side and lengthened.
The individual limbs can be named on the basis of a break in the symmetry provided by the "filter plate". This lies beside the anus on the aboral plate, between two of the radial arms; the radial limbs are designated letters A to E in a clockwise direction from this marker.
Skin and skeletonEdit
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.
The water vascular systemEdit
Echinoderms possess a water vascular system, a network of fluid-filled canals that function in gas exchange, feeding, and secondarily in locomotion. This system may have allowed them to function without the gill slits found in other Deuterostomes. The system comprises a central ring, the hydrocoel, and radial ambulacra stretching along each limb of the organism. As well as assisting with the distribution of nutrients through the animal, the system is most obviously expressed in the "tube-feet" of most echinoderms. These are extensions of the water vascular system which poke through holes in the skeleton and can be extended or contracted by the redistribution of fluid between the foot and an internal sac. In the crinoids, these tube feet waft food particles captured on the radial limbs towards the central mouth; in the asteroids, the same wafting motion is employed to move the animal across the ground. Sea urchins use their feet to prevent the larvae of encrusting organisms from settling on their surfaces; potential settlers are moved to the urchin's mouth and devoured. Some burrowing sea poke their tube feet through the surface of the sand or mud above them into the water column and use them to attain oxygen from the water column.
Whilst echinoderms possess a complete digestive tube (tubular gut), it is very simple, often simply leading directly from mouth to anus. It can generally be divided into a throat, stomach, intestine and rectum. They also possess an open and reduced circulatory system — consisting of a central ring and five radial vessels, but no heart.
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 animal.
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.
Echinoderms become sexually mature after approximately two to three years, depending on the species and the environmental conditions. The eggs and sperm cells are released into open water, where fertilization takes place. The release of sperm and eggs is co-ordinated temporally in some species, and spatially in others. Internal fertilisation has currently been observed in three species of starfish, three brittle stars and a deep water sea cucumber.
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 fertilise the contents. This can also be found among sea urchins and sea cucumbers, where exhibit care for their young can occure, 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 brittlestars.[How to reference and link to summary or text] Antoher strategy that has evolved in some starfish and brittlestars is the ability to reproduce asexually by dividing in two halfes while they are small juveniles, while turning to sexual reproduction when they have reached sexual maturity.[verification needed] These species have six arms.[How to reference and link to summary or text]
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.
Many echinoderms have remarkable powers of regeneration: a sea star cut radially into a number of parts will, over the course of several months, regenerate into as many separate, viable sea stars. A section as small as a single arm (with the commensurate central-body mass and neural tissue) will, in ideal circumstances, successfully regenerate in this way. Some echinoderms go so far as to actively detach parts of their bodies if they perceive themselves to be in danger; sea cucumbers often discharge parts of their internal organs, and sea urchins are constantly losing their spines through damage; all parts are replaceable. Some starfish populations can reproduce entirely asexually purely by the shedding of arms for long periods of time.
Distribution and habitatEdit
Echinoderms are globally distributed in almost all depths, latitudes and environments in the ocean. They reach highest diversity in reef environments but are also widespread on shallow shores, around the poles — refugia where crinoids are at their most abundant — and throughout the deep ocean, where bottom-dwelling and burrowing sea cucumbers are common — sometimes accounting for up to 90 % of organisms. Whilst almost all echinoderms are benthic — that is, they live on the sea floor — some sea-lilies can swim at great velocity for brief periods of time, and a few deep-sea sea cucumbers are fully floating. Some crinoids are pseudo-planktonic, attaching themselves to floating logs and debris, although this behaviour was exercised most extensively in the Paleozoic, before competition from such organisms as barnacles restricted the extent of the behaviour. Some sea cucumbers employ a similar strategy, hitching lifts by attaching to the sides of fish. 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.
Mode of lifeEdit
The modes of feeding vary greatly between the constituent taxa. Crinoids and some brittle stars tend to be passive filter-feeders, absorbing suspended particles from passing water; sea urchins are grazers, sea cucumbers deposit feeders, and starfish active hunters.
Crinoids employ a large net-like structure to sieve water as it is swept by 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 abductor muscle. When the abductor tires, the starfish can insert its stomach through the opening and release gastric juices, digesting the prey alive.
Despite their low nutrition value and the abundance of indigestable calcite, many organisms, such as Crabs, sharks, foxes, sea birds and larger starfish, make a living by feeding on echinoderms. Defensive strategies employed include the presence of spines, toxins, which can be inherent or delivered through the tube feet, and the discharge of sticky entangling threads by sea cucumbers.
Echinoderms provide a key ecological role in ecosystems. For example, the grazing of sea urchins reduces the rate of colonisation 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.
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.
- Black, R M (1973). The Elements of Palaeontology, 3rd impression. Cambridge University Press, 340pp + xviii, ISBN 0-521-09615-4. (Chapter 9 deals with Echinoids).
- Clark, A M (1968). Starfishes and their relations, 2nd edition. Trustees of the British Museum (Natural History), 120pp + vi.
- Clarkson, E N K (1993). Invertebrate Palaeontology and Evolution, 3rd edition. Chapman & Hall, 434pp + ix, ISBN 0-412-47990-7. (Chapter 9 covers Echinoderms).
- Nichols, D (1969). Echinoderms, 4th (revised) edition. Hutchinson University Library, 192pp, ISBN 0-09-065994-5. (This is the same Nichols who produced the seminal work on the mode of life of the irregular echinoid, Micraster, in the English chalk).
- Shrock R R & Twenhofel W H (1953). Principles of Invertebrate Paleontology, 2nd edition. McGraw Hill International Series on the Earth Sciences, 816pp + xx, LCC 52-5341. (Chapter 14 covers Echinoderma).
- Williamson D I (2003). "The Origins of Larvae", xviii + 261 pp, ISBN 1-4020-1514-3. Kluwer. Dordrecht. (Chaps 8-12 cover echinoderm larvae).
- The Echinoid Directory from the Natural History Museum.
- Echinodermata from the Tree of Life Web Project.
- Berkley taxonomy on the Echinodermata]]
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