- Key People:
- Alexander Agassiz
Asteroids and echinoids, which use spines and tube feet in locomotion, may move forward with any area of the body and reverse direction without turning around. The feet may be used either as levers, by means of which the echinoderm steps along a surface, or as attachment mechanisms that pull the animal. Sea daisies presumably move in the same way. Ophiuroids tend to move by thrashing the arms in one of several possible methods, including a rowing motion in which strokes are taken by two pairs of extended arms; the fifth arm either is extended forward in the direction in which the animal is traveling or trails behind.
Holothurians (sea cucumbers) generally lead with the mouth, or oral, end, movement being carried out by both the tube feet and contraction and expansion of the body; sluglike movement is common. Holothurians of the family Synaptidae are able to pull themselves across a surface using their sticky tentacles as anchors.
Stalked crinoids (sea lilies), so called because they have stems, generally are firmly fixed to a surface by structures at the ends of the stalks called holdfasts. Some fossil and living forms release themselves to move to new attachment areas. The unstalked crinoids (feather stars) generally swim by thrashing their numerous arms up and down in a coordinated way; for example, in a 10-armed species, when arms 1, 3, 5, 7, and 9 are raised upward, arms 2, 4, 6, 8, and 10 are forcibly pushed downward; then the former group of arms thrashes downward as the latter is raised. Feather stars that do not swim pull themselves across a surface using their arms.
Swimming is known to occur in crinoids, ophiuroids, and holothurians. Some holothurians, formerly regarded as strictly bottom-living forms, are capable of efficient swimming; others, with gelatinous or flattened bodies and reduced calcareous skeletons, spend most of their lives swimming in deep water.
Righting response
Among echinoderms a normal position may be with the mouth either facing a surface, as in asteroids, ophiuroids, concentricycloids, and echinoids, or facing away from it, as in crinoids and holothurians. When overturned, echinoderms exhibit a righting response. Starfishes show this response most effectively, using the tube feet and the arms to perform a slow, graceful somersault that restores their normal position. Sea urchins roll themselves over by a concerted action of their tube feet and spines. The flat sand dollar can turn itself over only by burrowing into the sand until its position is vertical, then toppling over. In more agile groups such as holothurians, crinoids, and ophiuroids, righting is performed with relative ease.
Burrowing
Many echinoderms burrow in rock or soft sediments. Crinoids do not burrow because their feeding apparatus must be kept clear of sediment. Some urchins use the combined abrasive actions of their spines and teeth to burrow several inches into rock, usually in areas of severe wave and tidal action. The so-called irregular echinoids excavate soft sediments to various depths; most sand dollars burrow just below the surface, and some heart urchins may be found at depths of 38 cm (15 inches) or more. Holothurians use tentacles and contraction of the body wall in burrowing that generally is related to feeding. Several asteroid species bury themselves in sandy or muddy areas. The characteristic position of several ophiuroid groups involves burying the body into a surface and leaving only the tips of the arms projecting for food gathering.
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.
Aggregation
Echinoderms tend to aggregate in large numbers and evidently also did so in the past; fossil beds consisting almost exclusively of large numbers of one or a few species are known from as early as the Lower Cambrian. In present-day seas, ophiuroids may cover large areas of the seafloor; vast aggregations of echinoids are also common. Holothurians, crinoids, and some asteroids also often show a tendency to aggregate.
The phenomenon of aggregation apparently is a response to one or more environmental factors, chief of which is availability of food; e.g., large numbers of ophiuroids and crinoids occupy areas in which strong currents carry large amounts of plankton. An ophiuroid raises some arms into the water to capture food, using other arms to hold on to other nearby ophiuroids; in this way, a large aggregation can maintain its position in an environment in which a single ophiuroid or a small clump of them would be swept away. As stated previously, aggregation also enhances possibilities for successful propagation of a species and possibly may afford some protection from predators. Aggregation may be a passive phenomenon resulting from interactions between individuals and the environment as well as a demonstration of true social behaviour, a result of interactions among individuals.
Form and function of external features
General features
Echinoderms have a skeleton composed of numerous plates of mineral calcium carbonate (calcite). Part of the body cavity, or coelom, is a water-vascular system, consisting of fluid-filled vessels that are pushed out from the body surface as tube feet, papillae, and other structures that are used in locomotion, feeding, respiration, and sensory perception. The conspicuous five-rayed, or pentamerous, radial symmetry of living echinoderms tends to obliterate their fundamental bilateral symmetry.
Symmetry and body form
Many of the earliest echinoderms either lacked symmetry or were bilaterally symmetrical. Bilateral symmetry occurs in all living groups and is especially marked in the larval stages. A tendency toward radial symmetry (the arrangement of body parts as rays) developed early in echinoderm evolution and eventually became superimposed upon the fundamental bilateral symmetry, often obliterating it. Radial pentamerous symmetry is conspicuous among all groups of living echinoderms. Although the reasons for the success of radial symmetry are not yet completely understood, it has been suggested that a pentamerous arrangement of skeletal parts strengthens an animal’s skeleton more than would, for example, a three-rayed symmetry.
Pentamerous structure is evident in the arrangement of the tube feet, which usually radiate from the mouth in five bands. Many of the major organ systems, including the water-vascular system, muscles, hemal system (a series of fluid-filled spaces of indeterminate function), and parts of the nervous system are also pentamerous. The skeleton follows a pentamerous pattern, except in holothurians, where it is usually reduced to microscopic ossicles (bones).
Distinct growth patterns among the echinoderms provide some basis for separating the phylum into subphyla. Among homalozoans, the pattern is asymmetrical. In crinozoans and blastozoans, bands of tube feet radiate from the mouth, cross the theca (i.e., sheath or calyx), and extend onto the brachioles or arms; in asterozoans the bands of tube feet radiate outward from the mouth onto the arms to produce a star shape; and in the armless echinozoans, the tube feet form five meridians on the spherical or cylindrical body.
The crinoid (sea lily, feather star) mouth is centrally located on the cup-shaped theca, from which arise a variable number of arms resembling fern fronds. Although five is the primitive number of arms, they branch once or several times in most living forms to produce 10 to 200 arms. Crinoids either are supported on a stem (or stalk) attached to the underside of the theca, or they lack stalks, as is the case with most living forms, and attach themselves by means of slender appendages adapted for grasping (prehensile cirri).
Asteroids have a large central disk from which radiate five or more hollow arms containing parts of the major internal organ systems. The underside (oral surface) of the disk contains a centrally located mouth; the underside of each arm contains five or more bands of tube feet in special grooves called ambulacral furrows. The upper (aboral) surface of the disk has a centrally located anus (often absent) and the sieve plate (madreporite) of the water-vascular system (see below Form and function of internal features). Seven-armed starfish species are not unusual, a deep-sea family has six to 20 arms, and one Antarctic genus may have up to 50 arms. Concentricycloids have a discoid body; the dorsal surface is plated and the ventral surface is naked.
Ophiuroids have a small disk from which five arms radiate. The larger internal organs usually are confined to the disk. The centrally located mouth is on the underside of the disk as are the tube feet, which are not arranged in special grooves. Although most ophiuroids have five arms, a few have six or more, and in one group, the basket stars, the arms are branched to form a complex network.
In echinoids the skeleton forms a rounded, or globular, test of solid plates; tube feet, which emerge through holes in the plates, form five conspicuous bands, or ambulacra. Spaces between bands of feet are called interambulacra. Regular echinoids are roughly spherical in shape, with a centrally located mouth at the junction of the five bands containing tube feet (ambulacra); the anus is located on the side of the body opposite the mouth (aboral). Irregular urchins are elongated or flattened in shape, with the anus on the oral or aboral surface of the body. In regular and some irregular echinoids, the mouth is equipped with five teeth operated by a complex system of plates and muscles called Aristotle’s lantern.
Holothurians are elongated, with mouth and anus at opposite ends of the body. The spaces between the tube feet, which are arranged in five rows, or radii, are known as interradii. The tube feet may be more numerous on the underside of the body than elsewhere, scattered over radii and interradii, or absent. Most holothurians are soft-bodied animals because the skeleton is reduced and the skeletal units, called ossicles or spicules, are microscopic in size. Holothurians usually show bilateral symmetry outside, radial symmetry inside.
Skeleton
The skeleton is dermal but nonetheless conspicuous in echinoderms, with the exception of most holothurians, and forms an effective armour. Each skeletal unit (ossicle) usually consists of two parts, a living tissue (stroma) and a complex lattice (stereom) of mineral calcium carbonate, or calcite, which is derived from the stroma. In living echinoderms, certain properties of calcite are not evident in the stereom because of its latticed structure and the presence of soft stroma. In fossils, however, the stroma may be replaced by secondary calcite (i.e., calcite laid down in continuity with the original skeletal calcite), and recognition of fragments of echinoderm skeletons in fossil strata is easier because no other animal group has the same type of skeleton. Each ossicle is formed from granules in the dermal layer that, after secretion from special lime-secreting cells, enlarge, branch, and fuse to build up a three-dimensional network of calcite. Parts of the skeleton enlarge as an animal grows, and resorption and regeneration of the skeleton may occur.
Echinoderms exhibit a variety of skeletal structures. In the echinoids, a hollow test (skeleton) consisting of 10 columns of plates bears large and small spines as well as pincerlike organs (pedicellariae) used in defense and in the removal of unwanted particles from the body. Pedicellariae, also found in the asteroids, are absent from crinoids, ophiuroids, and holothurians. The complex feeding apparatus (Aristotle’s lantern) of echinoids consists of 40 ossicles held together by muscles and collagenous sutures.
Crinoids have a hollow sheath (theca or calyx) composed of two or three whorls, each consisting of five skeletal plates; the stalk and the slender appendages (cirri) of unstalked forms consist of a series of drum-shaped ossicles. The asteroid skeleton is composed of numerous smooth or spine-bearing ossicles of various shapes held together by muscles and ligaments, permitting flexibility. The arms of asteroids are hollow, those of ophiuroids solid, with the central axis of each arm consisting of elongated ossicles called vertebrae. The microscopically sized ossicles of holothurians are highly variable in form, ranging from flat lattice plates with holes to exquisitely symmetrical wheels, and are usually numerous; one tropical species, for example, has more than 26,000,000 ossicles in its body wall. A ring of plates, called the calcareous ring, surrounds the tube leading from the mouth to the stomach (i.e., the esophagus) of holothurians. Although located in a similar position to that of the echinoid Aristotle’s lantern, the calcareous ring functions as a point of insertion for muscles, not as a feeding apparatus.
