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Revision as of 21:36, 13 February 2013

Echinoderm
Temporal range: Cambrian–recent
Haeckel's diagrams of Asteroidea specimens
Scientific classification
Kingdom:
Animalia
Subkingdom:
Eumetazoa
Superphylum:
Deuterostomia
Phylum:
Echinodermata

Klein, 1734
Subphyla & Classes
Homostelea †
Homoiostelea †
Stylophora †
Ctenocystoidea † Robison & Sprinkle, 1969
Crinoidea
Paracrinoidea † Regnéll, 1945
Cystoidea † von Buch, 1846
Ophiuroidea
Asteroidea
Echinoidea
Holothuroidea
Ophiocistioidea †
Helicoplacoidea †
?Arkarua †
Edrioasteroidea †
Blastoidea †
Eocrinoidea †Jaekel, 1899

† = Extinct

Echinoderms (Phylum Echinodermata) are a phylum of marine animals. The adults are recognized easily by their (usually five-point) radial symmetry, and include such well-known animals as starfish, sea urchins, sand dollars, and sea cucumbers. Echinoderms are found at every ocean depth, from the intertidal zone to the abyssal zone. The phylum contains about 7000 living species,[1] making it the second-largest grouping of deuterostomes (a superphylum), after the chordates (which include the vertebrates, such as humans, sharks and frogs). Echinoderms are also the largest phylum that has no freshwater or terrestrial (land-based) representatives.

Aside from the hard-to-classify Arkarua (a Precambrian animal with Echinoderm-like pentamerous radial symmetry), the first definitive members of the phylum appeared near the start of the Cambrian period. The word "echinoderm" is made up from Greek ἐχινόδερμα (echinóderma), "spiny skin", cf. ἐχῖνος (echínos), "hedgehog; sea-urchin" and δέρμα (dérma), "skin", echinodérmata being the Greek plural form.

The echinoderms are important both biologically and geologically. Biologically, there are few other groupings so abundant in the biotic desert of the deep sea, as well as shallower oceans. The more notably distinct trait, which most echinoderms have, is their remarkable powers of regeneration of tissue, organs, limbs, and of asexual reproduction, and in some cases, complete regeneration from a single limb. Geologically, the value of echinoderms is in their ossified skeletons, which are major contributors to many limestone formations, and can provide valuable clues as to the geological environment. Further, it is held by some scientists that the radiation of echinoderms was responsible for the Mesozoic revolution of marine life.

Taxonomy

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 Pelmatozoa, which consists of the crinoids and of the extinct blastoids and Paracrinoids. Some crinoids, such as the feather stars, are also highly motile and able to move themselves around.

A brittle star, Ophionereis reticulata

A fifth class of Eleutherozoa consisting of just three species, the Concentricycloidea (sea daisies), were recently[2] merged into the Asteroidea. The fossil record contains a host of other classes which do not appear to fall into any extant crown group.

Anatomy and physiology

Echinoderms evolved from animals with bilateral symmetry. Although adult echinoderms possess pentaradial, or five-sided, symmetry, echinoderm larvae are ciliated, free-swimming organisms that organize in bilateral symmetry which 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.[3]

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 seastars exhibit symmetry in multiples of the basic five, with seastars such as Labidiaster annulatus known to possess up to fifty arms, and the sea-lily Comaster schlegelii having two hundred.[4]

Skin and skeleton

Echinoderms have a mesodermal skeleton composed of calcareous plates or ossicles. Each one of these, even the articulating spine of a sea urchin, is composed mineralogically of a crystal of calcite. If solid, these would form a heavy skeleton, so they have a sponge-like porous structure known as stereom.[5] Ossicles may be fused together, as in the test of sea urchins, or may articulate with each other as in the arms of sea stars, brittle stars and crinoids. The ossicles may be flat plates or bear external projections in the form of spines, granules or warts and they are supported by a tough epidermis (skin). Skeletal elements are also deployed in some specialized ways, such as the "Aristotle's lantern" mouthparts of sea urchins used for grinding, the supportive stalks of crinoids and the structural "lime ring" of sea cucumbers.[3]

Fossil crinoids from Iowa

Despite the robustness of the individual skeletal modules complete skeletons of starfish, brittle stars and crinoids are rare in the fossil record. This is because they quickly disarticulate (disconnect from each other) 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 and lengthening the arms. Sea urchins on the other hand are often well preserved in chalk beds or limestone. During fossilization, the cavities in the stereom are filled in with calcite that is in crystalline continuity with the surrounding material. On fracturing such rock, distinctive cleavage patterns can be seen and sometimes even the intricate internal and external structure of the test.[6]

Echinoderms exhibit a wide range of colours.

The epidermis 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 echinoderms are produced by the action of skin pigment cells. These are produced by a variable combination of coloured pigments, such as the dark melanin, red carotinoids, and carotin proteins, which can be blue, green, or violet. These may be light-sensitive, and as a result many echinoderms change appearance completely as night falls. The reaction can happen quickly — the sea urchin Centrostephanus longispinus changes from jet black to grey-brown in just fifty minutes when exposed to light.[7]

One characteristic of most echinoderms is a special kind of tissue known as "catch connective tissue". This collagenous material can change its mechanical properties in a few seconds or minutes through nervous control rather than by muscular means. This tissue enables a starfish to change from moving flexibly around the seabed to becoming rigid while prying open a bivalve mollusc or preventing itself from being extracted from a crevice. Similarly, sea urchins can lock their normally mobile spines rigidly as a defensive mechanism when attacked.[8]

The water vascular system

Main article: water vascular system

Echinoderms possess a unique water vascular system. This is a network of fluid-filled canals derived from the coelom (body cavity) that function in gas exchange, feeding, sensory reception and locomotion. This system varies between different classes of echinoderm but typically opens to the exterior through a sieve-like madreporite on the aboral (upper) surface of the animal. The madreporite is linked to a slender duct, the stone canal, which extends to a ring canal that encircles the mouth or oesophagus. From this, radial canals extend along the arms of asteroids and adjoin the test in the the ambulacral areas of echinoids. Short lateral canals branch off the radial canals, each one ending in an ampulla. Part of the ampulla can protrude through a pore (or a pair of pores in sea urchins) to the exterior and is known as a podium or tube foot. The water vascular system assists with the distribution of nutrients throughout the animal's body and is most obviously expressed in the tube feet which can be extended or contracted by the redistribution of fluid between the foot and the internal sac.[9]

The organisation of the system is somewhat different in ophiuroids where the medreporite may be on the oral surface and the podia lack suckers.[10] In holothuroids, the podia may be reduced or absent and the madreporite opens into the body cavity so that the circulating liquid is coelomic fluid rather than sea water.[11] The arrangements in crinoids is similar to asteroids but the tube feet lack suckers and are used to pass food particles captured by the arms towards the central mouth. In the asteroids, the same wafting motion is employed to move the animal across the ground.[12] 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 eaten. Some burrowing sea stars extend their elongated dorsal tube feet to the surface of the sand or mud above and use them to absorb oxygen from the water column.[13]

Other organs

Echinoderms possess a simple digestive system which varies according to the animal's diet. Starfish are mostly carnivorous and have a mouth, oesophagus, two-part stomach, intestine and rectum, with the anus located in the centre of the aboral body surface. In many species, the large cardiac stomach can be everted and digest food outside the body. In other species, whole food items such as molluscs may be ingested.[14] Brittle stars have a blind gut with no intestine or anus. They have varying diets and expel food waste through their mouth.[15] Sea urchins are herbivores and use their specialised mouthparts to graze, tear and chew algae and sometimes other animal or vegetable material. They have an oesophagus, a large stomach and a rectum with the anus at the apex of the test.[16] Sea cucumbers are mostly detritivores, sorting through the sediment with their buccal tentacles which are modified tube feet. Sand and mud accompanies their food through their simple gut which has a long coiled intestine and a capacious cloaca.[17] Crinoids are passive suspension feeders, catching plankton with their outstretched arms. Boluses of mucus-trapped food are passed to the mouth which is linked to the anus by a loop consisting of a short oesophagus and longer intestine.[18]

Echinoderms have a haemal system, and often also a perihaemal system. Both are derived from the coelom and form an open and reduced circulatory system. This usually consists of a central ring and five radial vessels. There is no true heart and the blood often lacks any respiratory pigment. Gaseous exchange occurs via dermal branchae or papulae in starfish, genital bursae in brittle stars, peristominal gills in sea urchins and cloacal trees in sea cucumbers. Exchange of gases also takes place through the tube feet. Echinoderms lack specialized excretory (waste disposal) organs and so nitrogenous waste, chiefly in the form of ammonia, diffuses out through the respiratory surfaces.[9]

Echinoderms have a simple radial nervous system that consists of a modified nerve net consisting of interconnecting neurons with no central brain, although some do possess ganglia. Nerves radiate from central rings around the mouth into each arm or along the body wall; the branches of these nerves coordinate the movements of the organism and the synchronisation of the tube feet. Starfish have sensory cells in the epithelium and have simple eyespots and touch-sensitive tentacle-like tube feet at the tips of their arms. Sea urchins have no particular sense organs but do have statocysts that assist in gravitational orientation, and they have sensory cells in their epidermis, particularly in the tube feet, spines and pedicellariae. Brittle stars, crinoids and sea cucumbers in general do not have sensory organs but some burrowing sea cucumbers of the order Apodida have a single statocyst adjoining each radial nerve and some have an eyespot at the base of each tentacle.[19]

The gonads occupy much of the body cavities of sea urchins and sea cucumbers, while the less voluminous crinoids, brittle stars and starfish have two gonads in each arm. While the primitive condition is considered to be the possession of one genital aperture, many organisms have multiple gonopores through which eggs or sperm may be released.[19]

Regeneration

Many echinoderms have remarkable powers of regeneration. Many species routinely autotomize and regenerate arms and viscera. Sea cucumbers often discharge parts of their internal organs if they perceive themselves to be threatened. The discharged organs and tissues are regenerated over the course of several months. Sea urchins are constantly replacing spines lost through damage. Sea stars and sea lilies readily lose and regenerate their arms. In most cases, a single severed arm cannot grow into a new starfish in the absence of at least part of the disc.[20][21][22][23] However, in a few species a single arm can survive and develop into a complete individual[21][22][23] and in some species, the arms are intentionally detached for the purpose of asexual reproduction. During periods when they have lost their digestive tracts, sea cucumbers live off stored nutrients and absorb dissolved organic matter directly from the water.[24]

The regeneration of lost parts involves both epimorphosis and morphallaxis. In epimorphosis stem cells - either from a reserve pool or those produced by dedifferentiation- form a blastema and generate new tissues. Morphallactic regeneration involves the movement and remodelling of existing tissues to replace lost parts. Direct transdifferentiation of one type of tissue to another during tissue replacement is also observed.[25]

The robust larval regeneration is responsible for many of them being popular model organisms in developmental biology.

Reproduction

Sexual reproduction

Echinoderms become sexually mature after approximately two to three years, depending on the species and the environmental conditions. They are nearly all gonochoric, though a few species are hermaphroditic. The eggs and sperm cells are typically released into open water, where fertilisation takes place. The release of sperm and eggs is synchronised in some species, usually with regard to the lunar cycle. In other species, individuals may aggregate during the reproductive season, thereby increasing the likelihood of successful fertilisation. Internal fertilisation has currently been observed in three species of sea star, three brittle stars and a deep water sea cucumber. Even at abyssal depths, where no light penetrates, synchronisation of reproductive activity in echinoderms is surprisingly frequent.[26]

Some echinoderms brood their eggs. This is especially common in cold water species where planktonic larvae might not be able to find sufficient food. These retained eggs are usually few in number and are supplied with large yolks to nourish the developing embryos. In starfish, the female may carry the eggs in special pouches, under her arms, under her arched body or even in her cardiac stomach.[27] Many brittle stars are hermaphrodites. Egg brooding is quite common and usually takes place in special chambers on their oral surfaces, but sometimes thee ovary or coelom is used.[28] In these starfish and brittle stars, direct development without passing through a bilateral larval stage usually takes place.[27] A few sea urchins and one species of sand dollar carry their eggs in cavities, or near their anus, holding them in place with their spines.[29] Some sea cucumbers use their buccal tentacles to transfer their eggs to their underside or back where they are retained. In a very small number of species, the eggs are retained in the coelom where they develop viviparously, later emerging through ruptures in the body wall.[30] In some species of crinoid, the embryos develop in special breeding bags, where the eggs are held until sperm released by a male happens to find them.[31]

Asexual reproduction

One species of seastar, Ophidiaster granifer, reproduces asexually by parthenogenesis.[32] In certain other asterozoans, the adults reproduce asexually for a while before they mature after which time they reproduce sexually. In most of these species, asexual reproduction is by transverse fission with the disc splitting in two. Regrowth of both the lost disc area and the missing arms occur[23][33] so that an individual may have arms of varying lengths. Though in most species at least part of the disc is needed for complete regeneration, in a few species of sea stars, a single severed arm can grow into a complete individual over a period of several months.[21][22][23] In at least some of these species, they actively use this as a method of asexual reproduction.[21][34] A fracture develops on the lower surface of the arm and the arm pulls itself free from the body which holds onto the substrate during the process.[34] During the period of regrowth, they have a few tiny arms and one large arm, thus often being referred to as "comets".[22][34]

Asexual reproduction by transverse fission has also been observed in adult sea cucumbers. Holothuria parvula uses this method frequently, an individual splitting into two a little in front of the mid point. The two halves each regenerate their missing organs over a period of several months but the missing genital organs are often very slow to develop.[35]

The larvae of some echinoderm species are capable of asexual reproduction. This has long been known to occur among starfish and brittle stars but has been more recently observed in a sea cucumber, a sand dollar and a sea urchin. These species belong to four of the major classes of echinoderms except crinozoans (as of 2011).[36] Asexual reproduction in the planktonic larvae occurs through numerous modes. They may autotomise parts that develop into secondary larvae, grow buds or undergo paratomy. The parts that are autotomised or the buds may develop directly into fully formed larvae or may develop through a gastrula or even a blastula stage. The parts that develop into the new larvae vary from the preoral hood (a mound like structure above the mouth), the side body wall, the postero-lateral arms or their rear ends.[36][37][38]

The process of cloning is a cost borne by the larva both in resources as well as in development time. Larvae have been observed to undergo this process when food is plentiful[39] or temperature conditions are optimal.[38] It has also been suggested that cloning may occur to make use of the tissues that are normally lost during metamorphosis.[40] Recent research has shown that the larvae of some sand dollars clone themselves when they detect predators (by sensing dissolved fish mucus).[38][40] Asexual reproduction produces many smaller larvae that escape planktivorous fish better.[41]

Larval development

An echinopluteus larva with larval 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 echinoderms 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. The larvae are mostly planktonic but in some species the eggs are retained inside the female and in some, the larvae are also brooded by the female.[42]

The larvae of echininoderms pass through a number of stages and these have specific names derived from the taxonomic names of the adults or from their appearance. For example a sea urchin has an 'echinopluteus' larva while a brittle star has an 'ophiopluteus' larva. A starfish has a 'bipinnaria' larva but this later develops into a multi-armed 'brachiolaria' larva. A sea cucumber larva is an 'auricularia' while a crinoid one is a 'vitellaria'. All these larvae are bilaterally symmetrical and have bands of cilia with which they swim and some, usually known as 'pluteus' larvae, have arms. When fully developed they settle on the seabed to undergo metamorphosis and the larval arms and gut degenerate. The left hand side of the larva develops into the oral surface of the juvenile while the right side becomes the aboral surface. At this stage the bilateral symmetry is lost and radial symmetry develops.[42][43]

Distribution and habitat

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. While 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 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 life

Locomotion

Echinoderms primarily use their tube feet to move about but some sea urchins also use their spines. The tube feet typically have a tip shaped like a suction pad in which a vacuum can be created by contraction of muscles. This along with some stickiness provided by the secretion of mucus provides adhesion. Waves of tube feet contractions and relaxations move along the adherent surface and the animal moves slowly along.[44]

Brittle stars are the most agile of the echinoderms, raising their discs and taking strides when moving. The two forward arms grip the substrate with their tube feet, the two side arms "row", the hindermost arm trails and the animal moves in jerks. The arm spines provide traction and when moving among objects, the supple arms can coil around things. A few species creep around on pointed tube feet.[44] Starfish extend their tube feet in the intended direction of travel and grip the substrate by suction, after which the feet are drawn backwards. The movement of multiple tube feet, coordinated in waves, moves the animal forward, but progress is slow.[45] Some burrowing starfish have points rather than suckers on their tube feet and they are able to "glide" across the seabed at a faster rate.[46]

Sea urchins use their tube feet to move around in a similar way to starfish. Some also use their articulated spines to push or lever themselves along or lift their oral surfaces off the substrate. If a sea urchin is overturned, it can extend its tube feet in one ambulacral area far enough to bring them within reach of the substrate and then successively attach feet from the adjoining area until it is righted. Some species bore into rock and they usually do this by grinding away at the surface with their mouthparts.[47]

Sea cucumbers are generally sluggish animals. Many can move on the surface or burrow through sand or mud using peristaltic movements and some have short tube feet on their under surface with which they can creep along in the manner of a starfish. Some species drag themselves along by means of their buccal tentacles while others can expand and contract their body or rhythmically flex it and "swim". Many live in cracks, hollows and burrows and hardly move at all. Some deep water species are pelagic and can float in the water with webbed papillae forming sails or fins.[17]

The majority of crinoids are motile but the sea lilies are sessile and attached to hard substrates by stalks. These stems can bend and the arms can roll and unroll and that is about the limit of the sea lily's movement, although a few species can relocate themselves on the seabed by crawling. The sea feathers are unattached and usually live in crevices, under corals or inside sponges with their arms the only visible part. Some sea feathers emerge at night and perch themselves on nearby eminences to better exploit the food-bearing current. Many species can "walk" across the seabed, raising their body with the help of their arms. Many can also swim with their arms but most are largely sedentary, seldom moving far from their chosen place of concealment.[48]

Feeding

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 are active hunters.

Crinoids employ a large net-like structure to sieve water as it is swept by currents, and to absorb 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.

While some sea stars are detritovores, extracting the organic material from mud, and others mimic the crinoids' filter feeding, most are active hunters, attacking other sea stars or shellfish. The latter are seized and held by the tube feet; sea stars then stiffen their legs, expanding the shell. The sea stars can use connective tissue to lock their arms in place and maintain a force on the prey while exerting minimal effort; the unfortunate victim must expend energy resisting the force with its adductor muscle. When the adductor tires, the sea star can insert its stomach through the opening and release gastric juices, digesting the prey alive.

Avoiding predation

Despite their low nutrition value and the abundance of indigestible calcite, echinoderms are the prey of many organisms, such as crabs, sharks, sea birds and larger starfish. 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. Being stabbed by a sea urchin may result in painful injury.

Ecology

The Ordovician cystoid Echinosphaerites from northeastern Estonia; approximately 5 cm in diameter.

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.

It has also been estimated that they capture and sequester about 0.1 gigatonnes of carbon per year as calcium carbonate, making them important contributors in the global carbon cycle.[49]

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.

Evolution

Fossil crinoid crowns.

The first universally accepted echinoderms appear in the Lower Cambrian period (Paul and Smith 1984). 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[50] structure and of external ciliary grooves for feeding as early echinoderm developments.

The extinct members of paraphyletic 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.

Use by humans

In 2010, 373,000 tonnes of echinoderms were harvested, mainly for consumption. These were mainly sea cucumbers (158,000 tonnes) and sea urchins (73,000 tonnes).[51]

Sea cucumbers are considered a delicacy in some countries of south east Asia; particularly popular are the (Pineapple) roller Thelenota ananas (susuhan) and the red Holothuria edulis. They are known as bêche de mer or trepang in China and Indonesia. The sea cucumbers are boiled for twenty minutes and then dried both naturally and later over a fire which gives them a smoky tang. In China they are used as a basis for gelatinous soups and stews. [52] Both male and female gonads of sea urchins are also consumed particularly in Japan, Peru, Spain and France. The taste is described as soft and melting, like a mixture of seafood and fruit. The quality is assessed by the colour which can range from light yellow to bright orange.[53]

The calcareous tests or shells of echinoderms are used as a source of lime by farmers in areas where limestone is unavailable and some are used in the manufacture of fish meal.[54] Four thousand tons of the animals are used annually for these purposes. This trade is often carried out in conjunction with shellfish farmers, for whom the starfish pose a major threat by eating their stocks.

Sea urchin and sand dollar skeletons are popular collectibles, as are dried starfish. Sea urchins are used in research, particularly as model organisms in developmental biology.[55]

References

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  2. ^ Siera104. "Echinodermata". Retrieved 15 March 2008.{{cite web}}: CS1 maint: numeric names: authors list (link)
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  4. ^ Messing, Charles. "Crown and calyx". Charles Messing's Crinoid Pages. Retrieved 29 July 2012.
  5. ^ Bäeuerlein, Edmund (2007). Handbook of Biomineralization: Biomimetic and bioinspired chemistry'. Wiley-VCH. p. 393. ISBN 3-527-31805-4.
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  7. ^ Weber, W.; Dambach, M. (1974). "Light-sensitivity of isolated pigment cells of the sea urchin Centrostephanus longispinus". Cell and Tissue Research. 148 (3): 437–440. doi:10.1007/BF00224270.{{cite journal}}: CS1 maint: multiple names: authors list (link)
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  13. ^ Ruppert, Fox & Barnes (2004) p. 905
  14. ^ Ruppert, Fox & Barnes (2004) p. 885
  15. ^ Ruppert, Fox & Barnes (2004) p. 891
  16. ^ Ruppert, Fox & Barnes (2004) pp. 902–904
  17. ^ a b Ruppert, Fox & Barnes (2004) p. 912 Cite error: The named reference "Ruppert912" was defined multiple times with different content (see the help page).
  18. ^ Ruppert, Fox & Barnes (2004) p. 920
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Cited texts

  • Dorit, R. L.; Walker, W. F.; Barnes, R. D. (1991). Zoology, International edition. Saunders College Publishing. ISBN 978-0-03-030504-7.
  • Ruppert, Edward E.; Fox, Richard, S.; Barnes, Robert D. (2004). Invertebrate Zoology, 7th edition. Cengage Learning. ISBN 81-315-0104-3.{{cite book}}: CS1 maint: multiple names: authors list (link)

Further reading

  • 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 nickel
  • 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).
  • Paul C.R.C and A.B. Smith (1984). "The early radiation and phylogeny of echinoderms". Biol. Rev. 59 (4): 443–481. doi:10.1111/j.1469-185X.1984.tb00411.x.
  • 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).
  • Smith, A.B. (2006). "The pre-radial history of echinoderms". Geological Journal. 40 (3): 255–280. doi:10.1002/gj.1018.
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