7   Echinoderms  ...any of a variety of invertebrate marine animals belonging to the phylum Echinodermata, characterized by a hard, spiny covering or skin.

  More than 21 classes have been identified, but only about 6,000 species of six classes are known to still exist. The six are:

   Crinoidea (  feather stars and  sea lilies), 

 Asteroidea (starfishes),

  Ophiuroidea (  brittle stars and  basket stars), 

 Echinoidea (  sea urchins,  cake urchins, and  heart urchins),

 Concentricycloidea (sea daisies), and 

 Holothurioidea (sea cucumbers). Echinoderms are widely distributed in all the oceans, occurring in marine environments ranging from the intertidal zone to the deepest oceanic trenches.

  The most apparent characteristics of echinoderms are the calcite skeleton and the five-rayed, or pentamerous, radial symmetry of the adult body form. The skeletal structure may be a  test, a hollow structure formed of solid plates, as found in sea urchins, or it may be composed of many separate smooth or spiny ossicles held together by muscles or ligaments, which is the case in the starfishes.

  Most echinoderms have numerous small appendages, called  tube feet, that are contained in grooves on the animals' tentacles. Movement of the tube feet is controlled by a hydraulic, or water-vascular, system. Water is taken in and passes into five major canals that channel it into branches that carry it to the tube feet. Local muscle action in contracting or releasing the canals functions to extend or retract the tube feet. The tube feet themselves may be variously modified to serve in locomotion, respiration, tunneling, sensory perception, feeding, and grasping.

  Reproduction among echinoderms is either sexual or asexual. In sexual reproduction, eggs are fertilized in the water by sperm and either pass through a larval stage before transforming into juvenile echinoderms or develop directly into juveniles. One kind of indirect development involves fission of the body and regeneration of the missing parts. Starfishes, for example, can regrow an entire organism from one arm if a small portion of the central disk remains attached.

  Most echinoderms feed on microscopic detritus or suspended matter (either alive or dead), but many urchins and starfishes graze on plant life. Some starfishes are carnivores and eat mainly mollusks. A few of the echinoderm species are economically important. Some sea urchin roes and the warty sea cucumbers of the genus  Holothuria are used for food in certain countries (see bęche-de-mer). Various species of starfishes, such as  Asterias vulgaris and  A. forbesi, that prey on clams and oysters are pests in commercial shellfish beds.

 In most species the sexes are separate; i.e., there are males and females. Although reproduction is usually sexual, involving fertilization of eggs by spermatozoa, several species of sea cucumbers, starfishes, and brittle stars can also reproduce asexually.

 Ecology.         Habitats. 

  Echinoderms are exclusively marine animals, with only a few species tolerating even brackish water. Among the exceptions are a few tropical holothurians that can withstand partial drying if stranded on a beach by a receding tide. Most echinoderms cannot tolerate marked changes in salinity, temperature, and light intensity and tend to move away from areas where these factors are not optimal. The behaviour of a large proportion of shallow-water species is regulated by  light; i.e., individuals remain concealed during the day and emerge from concealment at night for active feeding. Echinoderms are found in the warmest and coldest of the world's seas; those species that can tolerate a broad temperature range usually also have a broad geographic range. The horizontal or vertical distribution of many species is also governed by water temperature. The influence of pressure upon echinoderms has not yet been thoroughly investigated.

  Echinoderms occupy a variety of habitats. Along a rocky shore, starfishes and sea urchins may cling to rocks beneath which sea cucumbers and brittle stars are concealed. Some sea urchins have special adaptations for coping with surf pounding against rocks (e.g., particularly strong skeletons and well-developed tube feet for attachment). In sandy areas starfishes, brittle stars, irregular sea urchins, and sea cucumbers may bury themselves or move on the surface. Large populations of all living groups of echinoderms can be found in mud and ooze offshore. In some marine areas, echinoderms are the dominant organism; in the deepest ocean trenches, for example, holothurians may constitute more than 90 percent by weight of the living organisms. Perhaps the most unusual habitat is exploited by sea daisies and a small family of asteroids; these animals occur only on pieces of waterlogged wood on the deep-sea floor.

  Echinoderms frequently use other animals as homes; thousands of brittle stars, for example, may live in some tropical sponges. Sea cucumbers may attach themselves to the spines of sluggish Antarctic echinoids, and one sea cucumber attaches itself to the skin of a deep-sea fish. On the other hand, echinoderms are also hosts to a wide variety of organisms. Various crustaceans and barnacles, for example, cause the formation of galls, or tumourlike growths, in the skeletons of sea urchins, and crinoids are hosts of specialized parasitic worms. Commensal worms, which do no damage, are associated with most groups; an interesting case of  commensalism is the association between various tropical sea cucumbers and the slender pearlfish, which often is found in the rectum of the holothurian, head protruding through its anus. Pinnotherid crabs may be found in the rectum of echinoids and holothurians in Peru and Chile, and highly modified parasitic gastropod mollusks are frequently found in the body cavities of holothurians. A conspicuous parasitic sponge grows on two species of Antarctic ophiuroids.  Predation and defense.

 

  Although echinoderm populations do not generally suffer from heavy predation by other animals, ophiuroids form a significant part of the diet of various fishes and some asteroids. Echinoids are frequently eaten by sharks, bony fishes, spider crabs, and gastropod mollusks; crows, herring gulls, and eider ducks may either peck their tests (internal skeletons) or drop them repeatedly until they break; and mammals, including the Arctic fox, sea otters, and humans, eat them in considerable numbers. Asteroids are eaten by other asteroids, mollusks, and crustaceans. Some holothurians are eaten by fishes and by humans. Crinoids appear to have no consistent predators.

  Echinoderms can protect themselves from predation in a variety of ways, most of which are passive. The presence of a firm skeleton often deters predators; echinoids, for example, have a formidable array of spines and, in some cases, highly poisonous stinging pincerlike organs (pedicellariae), some of which may cause intense pain and fever in humans. Some asteroids use chemical secretions to stimulate violent escape responses in other animals, particularly predatory mollusks. Some holothurians eject from the anus a sticky mass of white threads, known as cuvierian tubules, which may entangle or distract predators; others produce  holothurin, a toxin lethal to many would-be predators.

 

  bęche-de-mer,  plural BĘCHE-DE-MER, or BĘCHES-DE-MER,  also called TREPANG, boiled, dried, and smoked flesh of  sea cucumbers (phylum Echinodermata) used to make soups. Most bęche-de-mer comes from the southwestern Pacific, where the animals (any of a dozen species of the genera Holothuria, Stichopus, and Thelonota) are obtained on coral reefs. Bęche-de-mer is consumed chiefly in China.  Bęche-de-mer, or Beach-la-Mar, is a pidgin English term used in New Guinea and nearby islands, where the trepang trade has long been important. The term  Bęche-de-Mer has also come to designate the pidgin English language spoken in these regions.

   Sea cucumber,   any of the 1,100 species of marine invertebrates constituting the class Holothuroidea (or Holothurioidea) of the phylum Echinodermata. The soft, cylindrical body, 2 to 200 cm (about 3/4 inch to 6 1/2 feet) long and 1 to 20 cm (0.4-8 inches) thick, usually is a dull, dark colour and often warty, thus resembling a cucumber. The internal skeleton is reduced to numerous distinctively shaped, tiny bits in the skin. Most species have five rows of tube feet extending from mouth to anus. The anal opening is used for both respiration and discharging wastes. The 10 or more retractile tentacles surrounding the mouth are used for food-taking (mud containing nutrients or small aquatic animals) or burrowing. Locomotion is sluglike.  Many sea cucumbers can expel their internal organs from the anus and grow new ones; this may be a device for escape from a predator, or it may occur for physiological reasons. Some species also expel sticky filaments that ensnare or confuse an enemy. Cucumbers shelter  pearlfish (Carapus species) in the anal cavity, with the head of the fish extruding. A number of sea cucumbers exude a toxin that is lethal to small animals but not to humans; South Sea islanders place sea cucumber juices in water to kill or stupefy fish.

 Sea cucumbers are found in all oceans, mostly in shallow water but sometimes at great depths. They are best represented in the Indian Ocean and the western Pacific. The 80 to 100 species of large, warty cucumbers of the genus  Holothuria are abundant on coral reefs.

 

Sea urchin.... any of about 700 living species of echinoid marine invertebrates (phylum Echinodermata) with a globular body and a radial arrangement of organs, shown by five bands of pores running from mouth to anus over the test (internal skeleton). The pores accommodate tube feet, which are slender, extensible, and often sucker-tipped. From nodules on the test arise long, movable spines and pedicellariae (pincerlike organs); these structures may have poison glands. The mouth, on the underside of the body, has a complex dental apparatus called  Aristotle's lantern, which also may be venomous. Teeth are extruded to scrape algae and other food from rocks, and some urchins can excavate hiding places in coral or rock--even in steel. Sea urchins live on the ocean floor, usually on hard surfaces, and use the tube feet or spines to move about.

 The largest urchin (known from a single specimen) is Sperostoma giganteum of deep waters off Japan. Hatpin urchins, such as Centrostephanus longispinus of the Mediterranean and eastern Atlantic, Diadema (formerly Centrechinus) setosum of the Indo-Pacific, and D. antillarum of Florida and the West Indies, have toxic spines up to 30 centimetres (12 inches) long. The slate-pencil urchin (Heterocentrotus mammillatus) of the Indo-Pacific has 12-cm spines that may be 1 cm thick--stout enough to be used for writing. Lytechinus variegatus, a pale-greenish urchin of the southeastern coast of the United States and the Caribbean, and the large, short-spined Psammechinus (sometimes Echinus) miliaris of Iceland, Europe, and western Africa use their tube feet to hold up bits of seaweed or shell as a shield against sunlight in shallow water.

 The small, reddish or purplish urchins of the genus Arbacia, such as A. punctulata, the common urchin from Cape Cod to the West Indies, are familiar subjects in embryology; a female may release several million eggs at a time. In the West Indies, sea eggs--the ovaries of Tripneustes ventricosus--are eaten raw or fried; in the Mediterranean region, frutta di mare is the egg mass of Paracentrotus lividus (the best known rock borer) and other Paracentrotus species; and, on the U.S. Pacific coast, the eggs of the giant purple (or red) urchin (Strongylocentrotus franciscanus) are similarly considered a delicacy. The slightly smaller S. purpuratus, of the same region, is known to excavate holes in steel pilings.

 

Cake Urchins... any of the echinoid marine invertebrates of the order  Clypeastroida (phylum Echinodermata), in which the body is flattened. The surface is covered with short spines (often furlike) and inconspicuous pedicellariae (pincerlike organs). In many species the hollow, slightly elongated test (internal skeleton), which accommodates the water-vascular system, is symmetrically notched on the edge or has narrow perforations (lunules). The upper surface of the test has a radial, flowerlike arrangement of five porous spaces, called petaloids; the pores permit the extension of tube feet modified for respiration. Tube feet on the underside of the body are used for taking food and may also assist the spines in locomotion.

 Found worldwide in shallow seas, cake urchins burrow in sand and feed on organic particles wafted to the mouth on water currents. They are in turn the prey of other echinoderms and mollusks. 

 Cake urchins in which the body is a coinlike, thin-edged disk are called  sand dollars, a name sometimes given to cake urchins generally. The test of the common sand dollar (Echinarachnius parma) is often washed up on beaches of North America and Japan.  Species with lunules are generally called keyhole urchins. The largest and thinnest cake urchin is the yellow or purple sea pancake (Echinodiscus auritus) of the East African coast.

Sand dollar, any of the invertebrate marine animals of the order  Clypeastroida (class Echinoidea, phylum Echinodermata) that has a flat, disk-shaped body. It is a  species of cake urchin, intermediate between sea urchins and heart urchins. The sand dollar is particularly well adapted for burrowing in sandy substrates. Very small spines used for digging and crawling cover the entire surface of its body and are appressed backwards toward the posterior anus. The mouth is located in the centre of the body's underside. The upper surface exhibits pentaradiate symmetry, with a pattern of five "petals" spreading out from the centre. Some species found stranded on the shores of North America have five or six slots, or lunules, through the test (external skeleton). Most sand dollars measure from 5 to 10 cm (2 to 4 inches) in diameter. Species of comparable size occur in shallow coastal waters throughout the rest of the world, except in Europe and Antarctica.

  Heart urchin,         any echinoid marine invertebrate of the order Spatangoidea (phylum Echinodermata), in which the body is usually oval or heart-shaped. The test (internal skeleton) is rather fragile with four porous spaces, or petaloids. The body is covered with fine, usually short spines.  Heart urchins live in burrows lined with mucus. Long tentacles (modified tube feet) reach out over the sand to pick up small particles of food; other tube feet have respiratory and sensory (not locomotive) functions. Movement is carried out by means of the spines.  The common heart urchin (Echinocardium cordatum) occurs in all oceans. Spatangus purpureus is common on the coasts of western Europe, the Mediterranean, and western Africa.