The animal symbol of this intertidal world of coral is a group of snails whose whole structure and being represent an extraordinary contrast to the way of life typical of this class of mollusks. They are called the vermetid or "wormlike" snails. The shell is no ordinary gastropod spire or cone, but a loose uncoiled tube very like the calcareous tubes built by many worms. The species that inhabit this intertidal zone have become colonial, and their tubes form closely packed and intertwined masses.
The very nature of these vermetid snails and their departure from the form and habits of related mollusks are eloquent of the circumstances of their world and of the readiness of life to adapt itself to a vacant niche. Here on this coral platform the tide ebbs and flows twice daily, and each flood brings renewed food supplies from offshore. There is but one perfect way to exploit such rich supplies: to remain in one place and fish the currents as they stream by. This is done on other shores by such animals as the barnacles, the mussels, and the tube-building worms. It is not ordinarily a snail's way of life, but in adaptation these extraordinary snails have become sedentary, abandoning the typical roaming habit. No longer solitary, they have become gregarious to an extreme degree, living in crowded colonies, with shells so intertwined that early geologists called their formations "worm rock." And they have given up the snail habits of scraping food from the rocks or of hunting and devouring other animals of large size; instead they draw the sea water into their bodies and strain out the minute planktonic food organisms. The tips of their gills are thrust out and drawn through the water like nets—an adaptation probably unique in all the group of snail-like mollusks. The vermetids give their own clear demonstration of the plasticity of the living organism and its responsiveness to the world about it. Again and again, in group after group of widely different and unrelated animals, the same problem has been met and solved by the evolution of diverse structures that function for a common purpose. So the legions of the barnacles sweep food from the tides on a New England shore, using a modification of what in their relatives would be a swimming appendage; mole crabs gather by the thousand where surf sweeps the southern beaches, straining out food with the bristles of their antennae; and here on this coral shore the crowded masses of this strange snail filter the waters of the incoming tide through their gills. By becoming the imperfect, the atypical snail, they have become the perfectly adapted exploiter of the opportunities of their world.
The edge of the low tide is a dark line traced by colonies of short-spined, rock-boring sea urchins. Every hole and every depression in the coral rock bristles with their small dark bodies. One spot in the Keys lives in my memory as an urchin paradise. This is the seaward shore of one of the eastern group of islands, where the rock drops in an abrupt terrace, somewhat undercut and deeply eroded into holes and small caves, many with their roofs open to the sky. I have stood on the dry rock above the tide and looked down into these little water-floored, rock-walled grottoes, finding twenty-five to thirty urchins in one of these caverns that was no larger than a bushel basket. The caves shine with a green water-light in the sun, and in this light the globular bodies of the urchins have a reddish color of glowing, luminous quality, in rich contrast to the black spines.
A little beyond this spot the sea bottom slopes under water more gradually, with no undercutting. Here the rock borers seem to have taken over every niche that can afford shelter; they give the illusion of shadows beside each small irregularity of bottom. It is not certain whether they use the five short stout teeth on their under surfaces to scrape out holes in the rock, or perhaps merely take advantage of natural depressions to find a safe anchorage against the occasional storms that sweep this coast. For some inscrutable reason, these rock-boring urchins and related species in other parts of the world are bound to this particular tidal level, linked to it precisely and mysteriously by invisible ties that prevent their wandering farther out over the reef flat, although other species of urchins are abundant there.
Above and below the zone of the rock-boring urchins, closely crowded throngs of pale brown tubular creatures push up through the chalky sediment. When the tide leaves them their tissues retract and all that proclaims them to be animals is hidden; then one might pass them by as some strange marine fungi. With the return of the water their animal nature is revealed, and from each fawn-colored tube a crown of tentacles, of purest emerald green, is unfolded as each of these anemone-like creatures begins to search the tide for the food it has brought. Living where their very existence depends on keeping the delicate tissues of the tentacles above the choking dust of sediment, these zoanthids are able to stretch their bodies into slender threads where the sediments are deep, though normally their tubes are short and stout.
On the seaward side of many of the Keys the bottom slopes gently, with wading depths for perhaps a quarter of a mile or more. Once beyond the rock-boring sea urchins, the vermetid snails, and the green and brown jewel anemones, the bottom of coarse sand and coral fragments begins to be marked by dark patches of turtle grass, and larger animals begin to inhabit the reef flats. Sponges, dark and bulky, grow in water only deep enough to cover their massive forms. Small, shallow-water corals, somehow able to survive the rain of sediments that would be fatal to the larger reef-builders, erect their hard structures, stoutly branched or domed, on the floor of coral rock. The gorgonians, plantlike in their habit of growth, are a low shrubbery of delicate rose and brown and purple hues. And within and among and beneath them all is the infinitely varied fauna of a tropical coast, as many creatures that wander freely through the waters of this warm sea crawl or swim or glide over the flats.
Massive and inert, the loggerhead sponges by their appearance suggest nothing of the activity that goes on within their dark bulks. There is no sign of life for the casual passer-by to read, although if he waited and watched long enough he might sometimes see the deliberate closing of some of the round openings, large enough to admit an exploring finger, that penetrate the flat upper surface. These and other openings are the key to the nature of the giant sponge which, like even the smallest of its group, can exist only as long as it can keep the waters of the sea circulating through its body. Its vertical walls are pierced by intake canals of small diameter, groups of them covered by sieve plates with numerous perforations. From these the canals lead almost horizontally into the interior of the sponge, branching and rebranching into tubes of progressively smaller bore, to penetrate all the massive bulk of the sponge and finally to lead up to the large exit canals. Perhaps these exit holes are kept free of choking sediment by the strength of the outbound currents; at any rate they are the only part of the sponge that shows a pure black color, for the flour-like whiteness of the reef sediments has been sifted over all the sooty black surface of the body.
In its passage through the sponge, the water leaves a coating of minute food organisms and organic detritus on the walls of the canals; the cells of the sponge pick up the food, pass the digestible materials along from cell to cell, and return waste material to the flowing currents. Oxygen passes into the sponge cells; carbon dioxide is given off. And sometimes small sponge larvae, having undergone the early stages of their development within the parent sponge, detach themselves and enter the dark flowing river, to pass with it into the sea.
The intricate passageways, the shelter and available food they offer, have attracted many small creatures to live within the sponge. Some come and go; others never leave the sponge once they have taken up residence within it. One such permanent lodger is a small shrimp—one of the group known as snapping shrimp because of the sound made by snapping the large claw. Although the adults are imprisoned, the young shrimp, hatched from eggs adhering to the appendages of their mothers, pass out with the water currents into the sea and live for a time in the currents and tides, drifting, swimming, perhaps carried far afield. By mischance they may occasionally find their way into deep water where no sponges grow. But many of the young shrimp will in time find and approach the dark bulk of some log
gerhead sponge and, entering it, will take up the strange life of their parents. Wandering through its dark halls, they scrape food from the walls of the sponge. As they creep along these cylindrical passageways, they carry their antennae and their large claws extended before them, as though to sense the approach of a larger and possibly dangerous creature, for the sponge has many lodgers of many species—other shrimps, amphipods, worms, isopods—and their numbers may reach into the thousands if the sponge is large.
There, on the flats off some of the Keys, I have opened small loggerheads and heard the warning snapping of claws as the resident shrimps, small, amber-colored beings, hurried into the deeper cavities. I had heard the same sound filling the air about me, as, on an evening low tide, I waded in to the shore. From all the exposed reef rock there were strange little knockings and hammerings, yet the sounds, to a maddening degree, were impossible to locate. Surely this nearby hammering came from this particular bit of rock; yet when I knelt to examine it closely there was silence; then from all around, from everywhere but this bit of rock at hand, all the elfin hammering was resumed. I could never find the little shrimps in the rocks, yet I knew they were related to those I had seen in the loggerhead sponges. Each has one immense hammer claw almost as long as the rest of its body. The movable finger of the claw bears a peg that fits into a socket in the rigid finger. Apparently the movable finger, when raised, is held in position by suction. To lower it, extra muscular force must be applied, and when the suction is overcome, it snaps into place with audible sound, at the same time ejecting a spurt of water from its socket. Perhaps the water jet repels enemies and aids in capturing prey, which may also be stunned by a blow from the forcibly retracted claw. Whatever the value of the mechanism, the snapping shrimps are so abundant in the shallows of tropical and subtropical regions, and snap their claws so incessantly, that they are responsible for much of the extraneous noise picked up on underwater listening devices, filling the water world with a continuous sizzling, crackling sound.
It was on the reef flats off Ohio Key, on a day early in May, that I had my first, startled encounter with tropical sea hares. I was wading over a part of the flat that had an unusually heavy growth of rather tall seaweeds when sudden movement drew my eyes to several heavy-bodied, foot-long animals moving among the weeds. They were a pale tan color, marked with black rings, and when I touched one cautiously with my foot, it responded instantly by expelling a concealing cloud of fluid the color of cranberry juice.
I had met my first sea hare years before on the North Carolina coast. It was a small creature about as long as my little finger, browsing peacefully among some seaweeds near a stone jetty. I slipped my hand under it and gently brought it toward me, then, its identity confirmed, I returned the little creature carefully to the algae, where it resumed its grazing. Only by drastic revision of my mental image could I accept these tropical creatures, which seemed to belong in some book of mythology, as relatives of that first little elfin being.
The large West Indian sea hares inhabit the Florida Keys as well as the Bahamas, Bermuda, and the Cape Verde Islands. Within their range they usually live offshore, but at the spawning season they move in to the shallows, where I had found them, to attach their eggs, in tangled threads, to the weeds near the low-tide mark. They are marine snails of a sort, but have lost their external shells and possess only an internal remnant, hidden by the soft mantle tissue. Two prominent tentacles suggestive of ears, and the rabbit-like body shape, are responsible for the common name (see page 235).
Whether because of its strange appearance, or because of its defensive fluids, often thought to be poisonous, the Old World sea hare has long had a secure place in folk lore, superstition, and witchcraft. Pliny declared it was poisonous to the touch, and recommended as an antidote asses' milk and ground asses' bones, boiled together. Apuleius, known chiefly as the author of The Golden Ass, became curious about the internal anatomy of the sea hare and persuaded two fishermen to bring him a specimen; whereupon he was accused of witchcraft and poisoning. Some fifteen centuries were to pass before anyone else ventured to publish a description of the internal anatomy of the creature—then Redi in 1684 described it, and although popular belief called it sometimes a worm, sometimes a holothurian, sometimes a fish, he placed it correctly, at least as to general relationships, as a marine slug. For the past century or more the harmless nature of the sea hares has been recognized for the most part, but although they are fairly well known in Europe and Great Britain, the American sea hares, largely confined to tropical waters, are less familiar animals.
Perhaps this anonymity is due in part to the infrequency of their spawning migrations into tidal waters. An individual animal is both male and female; it may function as either sex, or as both. In laying its eggs, the sea hare extrudes a long thread in little spurts, about an inch at a time, continuing the slow process until the string has reached a length sometimes as great as 65 feet, and contains about 100,000 eggs. As the pink or orange-colored thread is expelled it curls about the surrounding vegetation, forming a tangled mass of spawn. The eggs and the resulting young meet the common fate of marine creatures; many eggs are destroyed, being eaten by crustacea or other predators (even by their own kind), and many of the hatching larvae fail to survive the dangers of life in the plankton. In the drift of the currents the larvae are carried offshore, and when they undergo metamorphosis to the adult form and seek the bottom they are in deep water. Their color changes with changing food as they migrate shoreward: first they are a deep rose color, then they are brown, then olive-green like the adults. For one of the European species, at least, the known life history suggests a curious parallel with that of the Pacific salmon. With maturity, the sea hares turn shoreward to spawn. It is a journey from which there is no return; they do not reappear on the offshore feeding grounds, but apparently die after this single spawning.
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.