The world of the reef flats is inhabited by echinoderms of every sort: starfishes, brittle stars, sea urchins, sand dollars, and holothurians all are at home on the coral rock, in the shifting coral sands, among the gorgonian sea gardens and the grass-carpeted bottoms. All are important in the economy of the marine world—as links in the living chains by which materials are taken from the sea, passed from one to another, returned to the sea, borrowed again. Some are important also in the geologic processes of earth building and earth destruction—the processes by which rock is worn away and ground to sand, by which the sediments that carpet the sea floor are accumulated, shifted, sorted, and distributed. And at death their hard skeletons contribute calcium for the needs of other animals or for the building of the reefs.
Out on the reefs the long-spined black sea urchin excavates cavities along the base of the coral wall; each sinks into its depression and turns its spines outward, so that a swimmer moving along the reef sees forests of black quills. This urchin also wanders in over the reef flats, where it nestles close to the base of a loggerhead sponge, or sometimes, apparently finding no need of concealment, rests in open, sand-floored areas.
A full-grown black urchin may have a body or test nearly 4 inches in diameter, with spines 12 to 15 inches long. This is one of the comparatively few shore animals that are poisonous to the touch, and the effect of contact with one of the slender, hollow spines is said to be like that of a hornet sting, or may even be more serious for a child or an especially susceptible adult. Apparently the mucous coating of the spines bears the irritant or poison.
This urchin is extraordinary in the degree of its awareness of the surroundings. A hand extended over it will cause all the spines to swivel about on their mountings, pointing menacingly at the intruding object. If the hand is moved from side to side the spines swing about, following it. According to Professor Norman Millott of the University College of the West Indies, nerve receptors scattered widely over the body receive the message conveyed by a change in the intensity of light, responding most sharply to suddenly decreased light as a shadowy portent of danger. To this extent, then, the urchin may actually “see” moving objects passing nearby.
Linked in some mysterious way with one of the great rhythms of nature, this sea urchin spawns at the time of the full moon. The eggs and sperm are shed into the water once in each lunar month during the summer season, on the nights of strongest moonlight. Whatever the stimulus to which all the individuals of the species respond, it assures that prodigal and simultaneous release of reproductive cells that nature often demands for the perpetuation of a species.
Off some of the Keys, in shallow water, lives the so-called slate-pencil urchin, named for its short stout spines. This is an urchin of solitary habit, single individuals sheltering under or among the reef rocks near the low-tide level. It seems a sluggish creature of dull perceptions, unaware of the presence of an intruder, and making no effort to cling by means of its tube feet when it is picked up. It belongs to the only family of modern echinoderms that also existed in Paleozoic time; the recent members of the group show little change from the form of ancestors that lived hundreds of millions of years ago.
Another urchin with short and slender spines and color variations ranging from deep violet to green, rose, or white, sometimes occurs abundantly on sandy bottoms carpeted with turtle grass, camouflaging itself with bits of grass and shell and coral fragments held in its tube feet. Like many other urchins, it performs a geologic function. Nibbling away at shells and coral rock with its white teeth, it chips off fragments that are then passed through the grinding mill of its digestive tract; these organic fragments, trimmed, ground, and polished within the urchins, contribute to the sands of tropical beaches.
And the tribes of the starfish and the brittle stars are everywhere represented on these coral flats. The great sea star, Oreaster, stout and powerful of body, perhaps lives more abundantly a little offshore, where whole constellations of them gather on the white sand. But solitary specimens wander inshore, seeking especially the grassy areas.
A small reddish-brown starfish, Linkia, has the strange habit of breaking off an arm, which then grows a cluster of four new arms that are temporarily in a “comet” form. Sometimes the animal breaks across the central disc; regeneration may result in six- or seven-rayed animals. These divisions seem to be a method of reproduction practiced by the young, for adult animals cease to fragment and produce eggs.
About the bases of gorgonians, under and inside of sponges, under movable rocks and down in little, eroded caverns in the coral rock live the brittle stars. With their long and flexible arms, each composed of a series of “vertebrae” shaped like hourglasses, they are capable of sinuous and graceful motion. Sometimes they stand on the tips of two arms and sway in the motion of the water currents, bending the other arms in movements as graceful as those of a ballet dancer. They creep over the substratum by throwing two of their arms forward and pulling up the body or disc and the remaining arms. The brittle stars feed on minute mollusks and worms and other small animals. In turn, they are eaten by many fish and other predators, and sometimes fall victims to certain parasites. A small green alga may live in the skin of the brittle star; there it dissolves the calcareous plates, so that the arms may break apart. Or a curious little degenerate copepod may live as a parasite within the gonads, destroying them and rendering the animal sterile.
My first meeting with a live West Indian basket star was something I shall never forget. I was wading off Ohio Key in water little more than knee deep when I found it among some seaweeds, gently drifting on the tide. Its upper surface was the color of a young fawn, with lighter shades beneath. The searching, exploring, testing branchlets at the tips of the arms reminded me of the delicate tendrils by which a growing vine seeks out places to which it may attach itself. For many minutes I stood beside it, lost to all but its extraordinary and somehow fragile beauty. I had no wish to “collect” it; to disturb such a being would have seemed a desecration. Finally the rising tide and the need to visit other parts of the flat before they became too deeply flooded drove me on, and when I returned the basket star had disappeared.
The basket starfish or basket fish is related to the brittle stars and serpent stars but displays remarkable differences of structure: each of the five arms diverges into branching V’s, which branch again, and then again and again until a maze of curling tendrils forms the periphery of the animal. Indulging their taste for the dramatic, early naturalists named the basket stars for those monsters of Greek mythology, the Gorgons, who wore snakes in place of hair and whose hideous aspect was supposed to turn men to stone; so the family comprising these bizarre echinoderms is known as the Gorgonocephalidae. To some imaginations their appearance may be “snaky-locked,” but the effect is one of beauty, grace, and elegance.
All the way from the Arctic to the West Indies basket stars of one species or another live in coastal waters, and many go down to lightless sea bottoms nearly a mile beneath the surface. They may walk about over the ocean floor, moving delicately on the tips of their arms. As Alexander Agassiz long ago described it, the animal stands “as it were on tiptoe, so that the ramifications of the arms form a kind of trellis-work all around it, reaching to the ground, while the disk forms a roof.” Or again they may cling to gorgonians or other fixed sea growths and reach out into the water. The branching arms serve as a finemeshed net to ensnare small sea creatures. On some grounds the basket stars are not only abundant but associate in herds of many individuals as though for a common purpose. Then the arms of neighboring animals become entwined in a continuous living net to capture all the small fry of the sea who venture, or are helplessly carried, within reach of the millions of grasping tendrils.