Echinoderms

The Phylum Echinodermata consists of at least 6,500 living species in six named classes, which include sea stars, sea urchins, and related animals. The beauty, appeal, and importance of living and fossil echinoderms have been recognized since ancient times. They have ranked among the most conspicuous marine animals since the Cambrian and are distributed today from the intertidal zone to the deepest ocean trenches, often in immense numbers; they are of considerable ecological significance. Echinoderms are most diverse in the tropics, but they are found in greatest profusion in productive temperate and cold-water ecosystems. They range in size from basket stars (Gorgonocephalus) 1 m in diameter and sea cucumbers (Synapta) 3 m in length to sea urchins (Echinocyamus) smaller than 1 cm. Perhaps the largest echinoderm and the most spectacular of all invertebrate fossils is a sea lily, Seirocrinus subangularis, with stems more than 20 m in length; it is found in massive groups attached to logs in Lower Jurassic shale.

The term Echinodermata was derived from the Greek for spiny (echinos) and skin (derma), and it refers to the prominent, flesh-covered skeletal structures of many species. The echin-oderm skeleton is an internal system, unique in consisting of highly porous, calcium carbonate ossicles ("little bones"), each of which has the optical properties of a single calcite crystal. Another echinoderm characteristic and key to the group's success is the "mutable collagenous tissue" (MCT), which connects the ossicles, reinforces the integument, and enables echinoderms to transform nearly instantaneously from a rocky rigidity to puddinglike pliancy—and to just as quickly reverse the process. MCT is also capable of irreversible disintegration, giving echinoderms the ability to voluntarily sever appendages and to reproduce by fission.

The water vascular system (WVS), another exclusive echinoderm attribute, consists of a fluid-filled, ring-shaped canal that sends out five radial canals. Pairs of tube feet, which operate hydraulically, issue at intervals from the canals, protrude through the body wall, and serve in sensory reception, locomotion, feeding, and respiration. The radial canals and tube feet, accompanied by other coelomic systems, the nervous system, and specialized skeletal structures compose the ambulacra (Gr.: "walk, path"), which demarcate the radial, pen-tamerous (Gr.: "five-part") symmetry that characterizes the echinoderm body plan.

Among the traits that typify echinoderms is their ability for regeneration. It is widely believed that any piece cut from a sea star can reconstitute an entire individual, but in fact at least a portion of the disk is required for regrowth. All echinoderms have the capacity to regenerate damaged structures, and many can voluntarily sever their appendages in self-defense and then regrow them. The most extreme manifestation of regeneration occurs in echinoderms that reproduce asexually by dividing into parts and regenerating. That is also a faculty of some echinoderm larvae, which can clone themselves. In general, however, echinoderms reproduce sexually and produce planktonic larvae that undergo a dramatic metamorphosis, or they brood embryos and release crawl-away juveniles. The bilateral structure of echinoderm larvae is evidence that the pentamerous symmetry of the adult evolved secondarily.

Phylogenetic Relationships, Origin, and Fossil History

The ancestry of the extinct echinoderm lineages, and of the extant classes, has long been debated. At present, crinoids are generally regarded as a sister group to the other living echinoderms. Among the other classes, echi-noids and holothuroids (=Echinozoa) are consistently viewed as sister groups. The connections between the asteroids, concentricycloids, and ophiuroids, however, and their precise relationship to the Echinozoa, are uncertain. Ideas are in flux regarding the relationship between echinoderms and other phyla, but it is clear that echinoderms, hemichordates (acorn worms), and chordates (tunicates, lancelets, and vertebrates) are the sole members of the animal branch Deuterostomia. Molecular systematic studies and other lines of evidence indicate that the echinoderms and hemichordates are each other's closest relatives.

The echinoderms probably evolved in the

Proterozoic, and the earliest purported representatives belong to the Ediacara Fauna. The first undisputed echinoderm fossils, however, are from the Early Cambrian, some 520 million years ago. These include small, squat heli-coplacoids and edrioasteroids that lived anchored in sediment. By the end of the Cambrian they were displaced, probably by vagile organisms that destabilized the substrate, including such echinoderms as solutes and sty-lophorans, and by sessile, stalked, arm-bearing eocrinoids. Many of these ancient forms lacked pentamerous symmetry, and in appearance they were quite unlike modern echinoderms.

The radiation of the species and higher groups of echinoderms accelerated during the Ordovician Period, 505 to 440 million years ago, yielding a Paleozoic fauna with as many as twenty distinctive classes that was dominated by suspension-feeding rhombiferans and crinoids. These flower-shaped animals lived attached to the ocean floor in congregations that must have resembled underwater gardens. The classes surviving to the present day, including crinoids, holothuroids, asteroids, ophiuroids, and echinoids, were definitely in place during the Ordovician. The long stalks of Middle Ordovician crinoids put them in reach of plankton inaccessible to smaller organisms. Ophiocistioids and echinoids of that period were the first potentially herbivorous echinoderms. The appearance of inferred asteroid carnivores in the Ordovician and infaunal deposit-feeding holothurians later in the Paleozoic further presaged the mobility and trophic diversity characterizing modern echinoderms.

The number of echinoderm classes gradually declined during the waning of the Paleozoic, although great epicontinental seas of the Carboniferous Period supported vast populations of crinoids and blastoids. The Permo-Triassic extinction marking the transition to the Mesozoic Era 245 million years ago resulted in the demise of 90 percent of marine species including the Paleozoic crinoids and the other stalked echinoderms. Nevertheless, the Triassic marked the establishment of the lineage of articulates that gave rise to all living crinoids, and heralded the ascendancy of stalkless crinoids and other free-living echinoderms.

During the Mesozoic, echinoids replaced crinoids as the dominant groups of echino-derms. Remarkably, by the Permian echinoids had dwindled to just six known species, but the limited number of survivors seemingly saved the entire class from extinction. Similar bottlenecks affected the other echinoderm classes. Echinoid generic diversity peaked in the early Cenozoic and overshadowed the diversity of the other echinoderm classes, but at present the numbers of ophiuroid, asteroid, and holothuroid species exceed the species of echi-noids. Although the current generic diversity among living echinoderms equals or greatly surpasses the diversity at any other point during the Phanerozoic, the disparity is probably an artifact of preservation in the fossil record.

Class Asteroidea (Sea Stars or Starfish)

There are 1,800 species of sea stars, which are stellate to spherical animals with a central disk and usually five, but as many as fifty, arms. The arms and disk enclose a spacious body cavity occupied by a complex digestive tract and reproductive organs. The ventral surface of the arms has an open ambulacral groove that can close to protect the tube feet.

Asteroids have a well-earned reputation as carnivores, although certain species consume sediment or plants. Carnivorous asteroids are by no means all detrimental, but asteriid species, sometimes numbering nearly 150,000 per hectare, have caused enormous damage to commercial shellfisheries. Their arms and tube feet are capable of exerting sufficient force to open bivalves, and a gap of only 0.1 mm is suf

Red starfish (Fromia indica). There are 1,800 species of starfish. Starfish usually have five arms, but they can have as many as fifty. (Amos Nachoum/Corbis)

ficient for the asteroid to insert its stomach and digest its victim. Outbreaks of Acanthaster planci, the crown-of-thorns sea star (COT), have decimated Indo-Pacific reefs, which require decades to recover. Individual COT can release a remarkable 60 million eggs per year. There is evidence that their success has been enhanced by the anthropogenic release of nutrients and destruction of Acanthaster's predators. Conflicting information, however, shows that numbers of COT have been comparable or greater over the last 8,000 years.

Concentricycloidea (Daisy Stars)

There are but one genus and only two species of daisy stars, which were discovered in waterlogged wood recovered from depths of 1,000 to 2,000 m. They are flat, round, small animals, no more than 12 mm in diameter, with quite distinctive features. Their water vascular system is configured in a double ring, and the tube feet are arrayed in single file on the edge of the body. They are sexually dimorphic, and males have copulatory organs and produce spermatozoa that are morphologically unlike those of other echinoderms. The introduction of the concentricycloids as a new class of echino-derms in 1986 has generated much controversy. Evidence including DNA sequencing has been marshaled to show that they are structurally modified asteroids, but additional attention to the systematics and the biology of these animals is still well warranted.

Class Crinoidea (Feather Stars and Sea Lilies)

There are 100 species of sea lilies, which are restricted to depths exceeding 100 m, and 600

species of unstalked feather stars. The globular crinoid body carries an upward-directed mouth and anus and downward-directed attachment structures in the form of the grasping, jointed cirri of feather stars and the long, segmented stalk of sea lilies. Crinoid arms, which have delicate, alternating side branches, are usually 10 and up to 250 in number. Although a stalk is absent in adult feather stars, one develops after metamorphosis in the juvenile and is discarded after the arms and cirri develop.

Crinoids are current-loving, and veritable suspension-feeding machines. They actively orient their arms to capture small animals, plants, and nutritive particles. Food is captured by the tube feet, bound in mucus, and carried to the mouth along the ciliated ambulacra. Feather stars and some sea lilies can crawl, and certain feather stars can swim; since they have muscles only on the oral side of their arms, however, they depend on the elastic properties of MCT ligament to generate downward power-strokes. The visceral mass and the arms of crinoids can regenerate, and even the stalks of some "decapitated" sea lilies can regrow most of the body and arms.

Class Echinoidea (Sea Urchins, Sand Dollars, and Heart Urchins) The 900 species of echinoids have spine-studded, boxlike bodies composed of twenty columns of interlocking plates. Spherical "regular" echinoids have an anus at the apex and the mouth facing the substrate. In bilaterally symmetrical "irregular" echinoids the anus is located posteriorly. All echinoids, except some sediment-eating irregulars, have a mouth structure (Aristotle's lantern) with five teeth that continuously grow to compensate for dental abrasion.

Echinoids are of pivotal importance for embryological studies, since their copious gametes and transparent embryos are perfect subjects for experimentation on fertilization and development. Roughly 50,000 tons of echinoids are harvested each year for their edible roe, a largely unregulated fishery that has depleted or devastated stocks in several countries. In other regions, the grazing activities of enormous populations of echinoids have ravaged communities of algae, creating nearly sterile habitats called urchin barrens. The causes of echinoid population explosions are difficult to pinpoint, but disruption of natural ecosystems has frequently been implicated. Echinoids may succumb to pathogens, as did the Diadema antillarum, whose populations throughout the Caribbean dropped 93 percent in the 1980s. The extraordinary replacement of Caribbean corals by algae since that time points toward the formerly enormous impact of Diadema's herbivory.

Holothuroidea (Sea Cucumbers)

Holothuroids are somewhat cylindrical animals with a horizontal axis of symmetry, and the mouth and anus at the extreme poles. Most of the 1,400 species have a degree of bilateral symmetry, but it is superimposed on a pentamer-ous body plan. Modified tube feet form eight to thirty finely branching tentacles around the mouth, which are specialized either for suspension or deposit feeding. Depending on the species, additional tube feet may be numerous or entirely absent. Holothuroids are typically soft-bodied because their ossicles rarely form a plated test, but rather are of microscopic size and complex shape. Although characteristically sluggish animals, some holothuroids are capable of swimming, and one gelatinous, deep-sea species is completely pelagic.

Holothuroids are the most numerous large animals on the vast deep seafloor, and therefore one of the earth's dominant animals. Their ecological impact is potentially enor mous, since they can process sediment at a rate of more than 50 kg/square meter per year. A Chinese fishery for holothuroids dates back 1,000 years, and trade in sea cucumbers is still centered in Asia. Worldwide catches of about thirty species exceed 20,000 tons per year. Unfortunately, fishing pressure has increased considerably, and in many countries there are indications that holothuroid resources have been overexploited. Study and regulation will be required to sustain the fishery of these slow-growing and vulnerable echinoderms.

Class Ophiuroidea (Brittle Stars and Basket Stars)

The 2,000 species of graceful, agile ophiuroids have a disk containing a simple stomach (absent an anus), reproductive organs, and invaginated sacs used for respiration and reproduction. The jointed arms are sharply demarcated from the disk and nearly solid in construction, lacking an open ambulacral groove.

The basket stars have branching arms, and large individuals may have 10,000 or more barbed, terminal arm branches that swiftly curl around zooplankton prey. Many accounts have suggested that brittle stars lack eyes, can flex their arms only in a horizontal plane, and can move only by waving their arms. Some ophiuroids, however, have optically perfect lenses in their arm skeleton, which focus light on photoreceptors. The majority of ophiuroids can bend their arms in all directions, and one fish-eating species snares its prey in helical coils of its arms. Many ophiuroids crawl, some can swim by rapidly "rowing" their arms, and others advance using just their tube feet. Surprisingly, certain ophi-uroids are capable of casting off and regenerating their disk and viscera. Ophiuroids are important in the diet of many fish and crustaceans, and sheer numbers ensure their significance in the economy of the sea, where brittle star densities of 7,000 individuals per square meter have been recorded.

—Gordon Hendler

See also: Evolutionary Biodiversity; Five Kingdoms of Nature

Bibliography

Clark, Ailsa M. 1977. Starfishes and Related Echinoderms. Neptune City, NJ: T. F. H.; Giese, Arthur C., John S. Pearse, and Vicki B. Pearse. 1991. Reproduction of Marine Invertebrates. Vol. VI: Echinoderms and Lophophorates. Pacific Grove, CA: Boxwood; Harrison, Frederick W., and Fu-Shiang Chia, eds. 1994. Microscopic Anatomy of Invertebrates. Vol. 14: Echin-odermata. New York: Wiley-Liss; Hendler, Gordon, et al. 1995. Sea Stars, Sea Urchins, and Allies: Echinoderms of Florida and the Caribbean. Washington, DC: Smithsonian Institution Press; Hyman, Libbie H. 1955. The Invertebrates: Echinodermata: The Coelomate Bilateria. New York: McGraw-Hill; Lawrence, John. 1987. A Functional Biology of Echinoderms. London: Croon Helm; Waters, Johnny A., and Christopher G. Maples, eds. 1997. "Geobiology of Echinoderms." Paleontological Society Papers 3: 1-355.

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