echinoderm
- Key People:
- Alexander Agassiz
echinoderm, any of a variety of invertebrate marine animals belonging to the phylum Echinodermata, characterized by a hard, spiny covering or skin. Beginning with the dawn of the Cambrian Period (542 million to 488 million years ago), echinoderms have a rich fossil history and are well represented by many bizarre groups, most of which are now extinct. Living representatives include the classes Crinoidea (sea lilies and feather stars), Echinoidea (sea urchins), Holothuroidea (sea cucumbers), Asteroidea (starfishes, or sea stars), Ophiuroidea (basket stars and serpent stars, or brittle stars), and Concentricycloidea (sea daisies; discovered in the 1980s).
Echinoderms have been recognized since ancient times; echinoids, for example, were used extensively by Greeks and Romans for medicinal purposes and as food. During the Middle Ages, fossil echinoids and parts of fossil crinoids were objects of superstition. In the early part of the 19th century, Echinodermata was recognized as a distinct group of animals and was occasionally associated with the cnidarians and selected other phyla in a division of the animal kingdom known as the Radiata; the concept of a superphylum called Radiata is no longer valid.
Echinoderms are separated into 21 classes, based mainly on differences in skeletal structures. The number of extant species exceeds 6,500, and approximately 13,000 fossil species have been described.
General features
Size range and diversity of structure
Although most echinoderms are of small size, ranging up to 10 cm (four inches) in length or diameter, some reach relatively large sizes; e.g., some sea cucumbers are as long as two metres (about 6.6 feet), and a few starfishes have a diameter of up to one metre. Among the largest echinoderms were some extinct (fossil) crinoids (sea lilies), whose stems exceeded 20 metres (66 feet) in length.
Echinoderms exhibit a great diversity of body forms, especially among the extinct groups. Although all living echinoderms have a pentamerous (five-part) radial symmetry, an internal skeleton, and a water-vascular system derived from the coelom (central cavity), their general appearance ranges from that of the stemmed, flowerlike sea lilies, to the wormlike, burrowing sea cucumbers, to the heavily armoured intertidal starfish or sea urchin. The general shape of the echinoderm may be that of a star with arms extended from a central disk or with branched and feathery arms extended from a body often attached to a stalk, or it may be round to cylindrical. Plates of the internal skeleton may articulate with each other (as in sea stars) or be sutured together to form a rigid test (sea urchins). Projections from the skeleton, sometimes resembling spikes, which are typical of echinoderms, give the phylum its name (from Greek echinos, “spiny,” and derma, “skin”). The surface of holothurians, however, is merely warty.
Echinoderms also exhibit especially brilliant colours such as reds, oranges, greens, and purples. Many tropical species are dark brown to black, but lighter colours, particularly yellows, are common among species not normally exposed to strong sunlight.
Distribution and abundance
Diverse echinoderm faunas consisting of many individuals and many species are found in all marine waters of the world except the Arctic, where few species occur. Echinoids, including globular spiny urchins and flattened sand dollars, and asteroids are commonly found along the seashore. Although many species are restricted to specific temperate regions, Arctic, Antarctic, and tropical forms often are widely distributed; many species associated with coral reefs, for example, range across the entire Indian and Pacific oceans. Many of the echinoderms of Antarctica are distributed around the continent; those with a floating (planktonic) larval stage may be widely distributed, carried great distances by ocean currents. Some species, particularly those in Antarctic and deep-sea regions, have achieved a wide distribution without benefit of a floating larval stage. They may have done so by migration of adults across the seafloor or, in the case of shallow-water species, by passive transport across oceans in rafts of seaweed. Echinoderms tend to have a fairly limited depth range; species occurring in near-shore environments do not normally reach depths greater than 100 metres (328 feet). Some deep-sea species may be found over a considerable range of depths, often from 1,000 metres (3,280 feet) to more than 5,000 metres (16,400 feet). One sea cucumber species has a known range of 37–5,205 metres (121–17,077 feet). Only sea cucumbers reach ocean depths of 10,000 metres (32,800 feet) and more.
Importance
Role in nature
Echinoderms are efficient scavengers of decaying matter on the seafloor, and they prey upon a variety of small organisms, thereby helping to regulate their numbers. When present in large numbers, sea urchins can devastate sea-grass beds in the tropics, adversely affecting the organisms dwelling within. Sea urchins that burrow into rocks and along a shore can accelerate the erosion of shorelines. Other tropical species of sea urchins, however, control the growth of seaweeds in coral reefs, thereby permitting the corals to flourish. Removal of the sea urchins results in the overgrowth of seaweeds and the devastation of the coral reef habitat. Echinoderms can alter the structure of seafloor sediments in a variety of ways. Many sea cucumbers feed by swallowing large quantities of sediment, extracting organic matter as the sediment passes through the intestine, and ejecting the remainder. Large populations of sea cucumbers in an area can turn over vast quantities of surface sediments and can greatly alter the physical and chemical composition of the sediments. Burrowing starfish, sand dollars, and heart urchins disturb surface and subsurface sediments, sometimes to depths of 30 cm (12 inches) or more. In addition, echinoderms produce vast numbers of larvae that provide food for other planktonic organisms.
Relation to human life
Some of the larger species of tropical sea cucumbers, known commercially as trepang or bêche-de-mer, are dried and used in soups, particularly in Asia. Raw or cooked mature sex organs, or gonads, of sea urchins are regarded as a delicacy in some parts of the world, including parts of Europe, the Mediterranean region, Japan, and Chile. Some tropical holothurians produce a toxin, known as holothurin, which is lethal to many kinds of animals; Pacific islanders kill fish by poisoning waters with holothurian body tissues that release the toxin. Holothurin does not appear to harm human beings; in fact, the toxin has been found to reduce the rate of growth of certain types of tumours and thus may have medical significance. The eggs and spermatozoa of echinoderms, particularly those of sea urchins and starfishes, are easily obtained and have been used to conduct research in developmental biology. Indeed, echinoids have been collected in such large numbers that they have become rare or have disappeared altogether from the vicinity of several marine biologic laboratories.
Starfishes that prey upon commercially usable mollusks, such as oysters, have caused extensive destruction of oyster beds. Sea urchins along the California coast have interfered with the regrowth of commercial species of seaweed by eating the young plants before they could become firmly established. The crown-of-thorns starfish, which feeds on living polyps of reef corals, has caused extensive short-term damage to coral reefs in some parts of the Pacific and Indian oceans.
Reproduction and life cycle
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.
