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 descended 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.