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.
To see a basket starfish close inshore is one of those rare happenings that lives always in memory, but it is far otherwise with certain other members of the spiny-skinned tribe of echinoderms—the holothurians, or sea cucumbers. I have never waded far out onto the flats without meeting them. Their large dark forms, shaped much like the vegetable whose name they have been given, stand out clearly against the white sand where they lie sluggishly, sometimes partly buried. The holothurians perform a function in the sea that is roughly comparable to that of earthworms on land, ingesting quantities of sand and mud and passing it through their bodies. Most of them use a crown of blunt tentacles operated by strong muscles to shovel the bottom sediments into their mouths, then extract food particles from this detritus as it passes through their bodies. Perhaps some calcareous materials are dissolved out by the chemistry of the holothurian body.
Because of their abundance and the nature of their activities, the sea cucumbers profoundly influence the distribution of the bottom deposits around the coral reefs and islands. In a single year, it has been estimated, the holothurians in an area less than two miles square may redistribute 1000 tons of bottom substance. And there is evidence also concerning their work on sea bottoms lying at abyssal depths. The carpeting sediments, which accumulate slowly but unceasingly, lie in orderly layers from which geologists can read many chapters of the past history of the earth. But sometimes the layers are curiously disturbed. Bits of volcanic ash shard originating, for example, from some ancient eruptions of Vesuvius, may in some places lie, not in a thin layer representing and dating the eruption, but widely scattered through the overlying layers of other sediments. Geologists regard this as the work of deep-sea holothurians. And other evidence from deep dredgings and bottom samplings suggests the existence of herds of holothurians on the sea floor at great depths, working over a bottom area, then moving or in a vast migration directed, not by seasonal change, but by the scarcity of food in those deep and lightless regions.
Except in those parts of the world where they are sought as human food (they are the "trepang," or beche-de-mer, of Oriental markets) the sea cucumbers have few known enemies, yet they possess a strange defense mechanism that they employ when strongly disturbed. Then the holothurian may contract strongly and hurl out the greater part of its internal organs through a rupture in the body wall. Sometimes this action is suicidal, but often the creature continues to live and grows a new set of organs.
Dr. Ross Nigrelli and his associates of the New York Zoological Society have recently discovered that the large West Indian sea cucumber (also found about the Florida Keys) produces one of the most powerful of all known animal poisons, presumably as a chemical means of defense. Laboratory experiments showed that even small doses of the poison affect all kinds of animals, from protozoa to mammals. Fish confined in a tank with the cucumber always die when the act of evisceration occurs. The study of this natural toxin reveals the hazardous existence of many small creatures that live in association with another. The sea cucumber attracts a number of such animal associates or commensals. This particular species very often has a small pearl fish, Fierasfer, living within the shelter of the cloacal cavity, which the respiratory activities of the cucumber keep supplied with well-oxygenated water. But the well-being, and indeed the very life of the small Fierasfer seem to be constantly endangered, for the commensal fish is actually living beside a vat of deadly poison that may at any moment be ruptured. Apparently the fish has not developed an immunity to the poison of the holothurian, for Dr. Nigrelli found that if the cucumber was disturbed, its tenant Fierasfer would drift out in a moribund condition, even if actual evisceration did not take place.
Dark patches like the shadows of clouds are scattered over the inshore shallows of the reef flats. Each is a dense growth of sea grass pushing up flat blades through the sand, forming a drowned island of shelter and security for many animals. About the Keys these grass patches consist largely of stands of turtle grass, with which manatee grass and shoal grass may be intermingled. All belong to the highest group of plants—the seed plants—and so are different from the algae or seaweeds. The algae are the earth's oldest plants, and they have always belonged to the sea or the fresh waters. But the seed plants originated on land only within the past 60 million years or so and those now living in the sea are descend
ed from ancestors who returned to it from the land—how or why it is hard to say. Now they live where the salt sea covers them and rises above them. They open their flowers under the water; their pollen is water-borne; their seeds mature and fall and are carried away by the tide. Thrusting down their roots into the sand and the shifting coral debris, the sea grasses achieve a firmer attachment than the rootless algae do; where they grow thickly they help to secure the offshore sands against the currents, as on land the dune grasses hold the dry sands against the winds.
In the islands of turtle grass many animals find food and shelter. The giant starfish, Oreaster, lives here. So do the large pink or queen conch, the fighting conch, the tulip band shell, the helmet shells and the cask shells. A strange, armor-encased fish, the cowfish, swims just above the bottom, parting grass blades to which pipefish and sea horses cling. Baby octopuses hide among the roots and when pursued dive down deep into the yielding sand and disappear from view. Down in that grass-root under-turf many other small beings, of diverse kinds, live deep within the shadowed coolness, to come out only when night and darkness hide them.
But by day many of the bolder inhabitants may be seen by one who wades to the grassy patches and peers down through the clarifying glass of a water telescope, or, swimming above the deeper patches, looks down through a face mask. Here one is most apt to find, in life, the large mollusks that are familiar because their dead and empty shells are common on the beach or in shell collections.
Here in the grass is the queen conch, which in earlier days had a place on almost every Victorian mantel or hearth, and even today is displayed by the hundred at every Florida roadside stand selling tourist souvenirs. Through excessive fishing, however, it is becoming rare in the Florida Keys and is now exported from the Bahamas for use in cutting cameos. The weight and massiveness of its shell, the sharp spire and the heavily armored whorls are eloquent of the defenses raised, through the slow interaction of biology and environment, by myriad ancestral generations. Despite the cumbrous shell and the massive body that thrusts itself out to move over the bottom by grotesque leaps and tumblings, the queen conch seems an alert and sentient creature. Perhaps this effect is heightened by the eyes borne on the tips of two long tubular tentacles. The way the eyes are moved and directed leaves little doubt that they receive impressions of the animal's surroundings and transmit them to the nerve centers that serve in place of a brain.
Although its strength and awareness seem to fit the queen conch for a predatory life, it is probably a scavenger that only occasionally turns to live prey. Its enemies must be comparatively few and ineffectual, but the conch has formed one very curious association. A small fish habitually lives within its mantle cavity. There can be little free space when all of the body and foot are drawn into the shell, but somehow there is room for the cardinal fish, an inch-long creature. Whenever danger threatens, it darts into the fleshy cavern deep within the shell of the conch. There it is temporarily imprisoned when the snail pulls back into its shell and closes the sickle-shaped operculum.
To other, smaller beings that find their way into the interior of the shell, the conch reacts less tolerantly. Current-borne eggs of many sea creatures, larvae of marine worms, minute shrimp or even fish, or non-living particles like grains of sand, may swim or drift inside and, lodging on shell or mantle, set up an irritation. To this the conch responds with ancient defenses, acting to wall off the particle so that it can no longer irritate delicate tissues. The glands of the mantle secrete about this nucleus of foreign matter layer after layer of mother-of-pearl—the same lustrous substance that lines the inside of the shell. In this way the conch creates the pink pearls sometimes found within it.
The human swimmer drifting idly above the turtle grass—if he is patient enough and observant enough—may see something of other lives being lived above the coral sand, from which the thin flat blades of grass reach upward and sway to the motion of the water, leaning shoreward on a flooding tide and seaward on the ebb. If, for example, he looks very carefully he may see what he had thought to be a blade of grass (so perfectly did it simulate one by form and color and movement) detach itself from the sand and go swimming through the water. The pipefish—an incredibly long, slender, and bony-ringed creature that seems quite unfishlike—swims between the grasses slowly and with deliberate movement, now with its body held vertically, now leaning horizontally into the water. The slim head with its long, bony snout is thrust with probing motions into clusters of turtle grass leaves or down among the roots, as the fish searches for small food animals. Suddenly there is a quick, inflating motion of the cheek, and a tiny crustacean is sucked in through the tube-like beak, as one would suck a soda through a straw.
The pipefish begins life in a strange manner, being developed, nurtured, and reared beyond the stage of helpless infancy by the male parent, who keeps his young within a protective pouch. During the mating act of the pair, the ova are fertilized and are placed in this pouch by the female; there they develop and hatch, and to this marsupium the young may return again and again in moments of danger, even long after they are able to swim out into the sea at will.
So effective is the camouflage of another inhabitant of the grass—the sea horse—that only the sharpest eye can detect one at rest, its flexible tail gripping a blade of grass and its bony little body leaning out into the currents like a piece of vegetation. The sea horse is completely encased in an armor composed of interlocking bony plates; these take the place of ordinary scales and seem to be a sort of evolutionary harking back to the time when fish depended on heavy armor to protect them from their enemies. The edges of the plates, where they join and interlock, are produced into ridges, knobs, and spines to form the characteristic surface pattern.
Sea horses often live in vegetation that is floating rather than rooted; such individuals may then become part of that steady northward drift bearing plants, associated animals, and the larvae of countless sea forms into the open Atlantic and eastward toward Europe, or into the Sargasso Sea. Such sea-horse voyagers in the Gulf Stream sometimes are carried ashore on the southern Atlantic coast along with bits of wind- and current-borne sargassum weed to which they cling.
In some of the turtle-grass jungles all of the smaller inhabitants seem to borrow a protective color from their surroundings. I have dragged a small dredge in such a place and found, entangled in the handfuls of grass that came up, dozens of small animals of different species, all an amazing, bright green hue. There were green spider crabs with extremely long, jointed legs. There were small shrimp, also grass-green. Perhaps the most fantastic touch was contributed by several baby cowfish. Like their elders, whose remains one often finds in the debris of the high-tide line, these little cowfish were encased in bony boxes that held head and body in an inflexible case, from which fins and tail protruded as the only movable parts. From tip of tail to the little forward-projecting, bovine horns, these small cowfish were the green of the grass in which they lived.
Especially where they border the channels between the Keys, the shoals carpeted with marine grasses are visited from time to time by sea turtles, which live in some numbers about the outer reef. The hawksbill wanders far out to sea, and seldom turns landward; but the green and loggerhead turtles often swim into the shallow waters of Hawk Channel or seek the passages between the Keys, where the tides race swiftly. When these turtles visit the grassy shoals they are usually seeking those inflated sand dollars, the sea biscuits, whose home is among the grass, or they may seize some of the conchs. Apart from others of their own kind, the conchs probably have no more dangerous enemies than the big turtles.
However far they may wander, loggerhead, green, or hawksbill all must return to land for the spawning season. There are no spawning places on the Keys of coral rock or limestone, but on some of the sand keys of the Tortugas group the loggerhead and the green turtles emerge from the ocean and lumber over the sand like prehistoric beasts to dig their nests and bury their eggs. The chief spawning pla
ces of the turtles, however, are on the beaches of Cape Sable and other sand strands of Florida, and farther north in Georgia and the Carolinas.
If the predatory visits of the big turtles to the sea-grass meadows are sporadic, it is far otherwise with the ceaseless, day-by-day preying of the various conchs, one upon another, and all upon mussels or oysters, sea urchins and sand dollars. The chief predator of all the conchs is the dusky-red spindle-shaped one called the horse conch. One has only to see it feeding to realize how powerful it is; when the massive body, brick-red like the shell, is extended to enfold and overwhelm its prey, it seems impossible to believe that so much flesh can ever be drawn back into the shell again. Even the king crown conch, itself a predator on many other conchs, is no match for it. No other American gastropods approach its size. (One-foot individuals are fairly common and the giants of its kind are two feet long.) The big cask shells also are victims of the horse conch, while they themselves feed usually on urchins. Yet I have felt little awareness of this relentless predation on making a casual visit to the habitat of the conchs. There are long periods of somnolence and repletion, and the grassy world by day seems a peaceful place. A conch gliding over the coral sand, a sea cucumber burrowing sluggishly among the roots of the grasses, or the dark and swiftly fleeting forms of sea hares in sudden passage may be the only visible signs of life and motion. For by day life is in retreat; life is buried and hidden in crevices and corners of ledge and rock; life has crept under or within the shelter of sponge or gorgonian or coral or empty shell. In the shallow waters of the shore, many creatures must avoid the penetrating sunlight that irritates sensitive tissues and reveals prey to predator.
The Edge of the Sea Page 21