Most corals build colonies of many individuals. All the individuals of any one colony, however, are derived from a single fertilized ovum that matured and then began to form new polyps by budding. The colony has a shape characteristic of the species—branched, boulderlike, flatly encrusting, or cup-shaped. Its core is solid, for only the surface is occupied by living polyps, which may be widely separated in some species or closely crowded in others. It is often true that the larger and more massive the colony, the smaller the individuals that compose it; the polyps of a branching coral taller than a man may themselves be only an eighth of an inch high.
The hard substance of the coral colony is usually white, but may take on the colors of minute plant cells that live within the soft tissues in a relation of mutual benefit. There is the exchange usual in such relations, the plants getting carbon dioxide and the animals making use of the oxygen given off by the plants. This particular association may have a deeper significance, however. The yellow, green, or brown pigments of the algae belong to the group of chemical substances known as carotinoids. Recent studies suggest that these pigments in the imprisoned algae may act on the corals, serving as "internal correlators" to influence the processes of reproduction. Under normal conditions, the presence of the algae seems to benefit the coral, but in dim light the coral animals rid themselves of the algae by excreting them. Perhaps this means that in weak light or in darkness the whole physiology of the plant is changed and the products of its metabolism are altered to something harmful, so that the animal must expel the plant guest.
Within the coral community there are other strange associations. In the Florida Keys and elsewhere in the West Indian region, a gall crab makes an oven-shaped cavity on the upper surface of a colony of living brain coral. As the coral grows the crab manages to keep open a semicircular entrance through which, while young, it enters and leaves its den. Once full grown, however, the crab is believed to be imprisoned within the coral. Few details of the existence of this Florida gall crab are known, but in a related species in corals of the Great Barrier Reef only the females form galls. The males are minute, and apparently visit the females in the cavities where they are imprisoned. The female of this species depends on straining food organisms from indrawn currents of sea water and its digestive apparatus and appendages are much modified.
Everywhere, throughout the whole structure of the reef as well as inshore, the horny corals or gorgonians are abundant, sometimes outnumbering the corals. The violet-hued sea fan spreads its lace to the passing currents, and from all the structure of the fan innumerable mouths protrude through tiny pores, and tentacles reach out into the water to capture food. The little snail known as the flamingo tongue, wearing a solid and highly polished shell, often lives on the sea fans. The soft mantle, extended to cover the shell, is a pale flesh color with numerous black, roughly triangular markings. The gorgonians known as sea whips are more abundant, forming dense stands of undersea shrubbery, often waist-high, sometimes as tall as a man. Lilac, purple, yellow, orange, brown, and buff are the colors worn by these gorgonians of the coral reefs.
Encrusting sponges spread their mats of yellow, green, purple, and red over the walls of the reef; exotic mollusks like the jewel box and the spiny oyster cling to it; long-spined sea urchins make dark, bristling patches in the hollows and crevices; and schools of brightly colored fishes twinkle along the façade of the reef where the lone hunters, the gray snapper and the barracuda, wait to seize them.
At night the reef comes alive. From every stony branch and tower and domed façade, the little coral animals, who, avoiding daylight, had remained shrunken within their protective cups until darkness fell, now thrust out their tentacled heads and feed on the plankton that is rising toward the surface. Small crustacea and many other forms of microplankton, drifting or swimming against a branch of coral, are instant victims of the myriad stinging cells with which each tentacle is armed. Minute though the individual plankton animals be, the chances of passing unharmed through the interlacing branches of a stand of elkhorn coral seem slender indeed.
Other creatures of the reef respond to night and darkness and many of them emerge from the grottoes and crevices that served as daytime shelter. Even that strange hidden fauna of the massive sponges—the small shrimps and amphipods and other animals that live as unbidden guests deep within the canals of the sponge—at night creep up along those dark and narrow galleries and collect near their thresholds as though looking out upon the world of the reef.
On certain nights of the year, extraordinary events occur over the reefs. The famed palolo worm of the South Pacific, moved to gather in its prodigious spawning swarms on a certain moon of a certain month—and then only—has its less-known counterpart in a related worm that lives in the reefs of the West Indies and at least locally in the Florida Keys. The spawning of this Atlantic palolo has been observed repeatedly about the Dry Tortugas reefs, at Cape Florida, and in several West Indian localities. At Tortugas it takes place always in July, usually when the moon reaches its third quarter, though less often on the first quarter. The worms never spawn on the new moon.
The palolo inhabits burrows in dead coral rock, sometimes appropriating the tunnelings of other creatures, sometimes excavating its burrow by biting away fragments of rock. The life of this strange little creature seems to be ruled by light. In its immaturity the palolo is repelled by light—by sunlight, by the light of the full moon, even by paler moonlight. Only in the darkest hours of the night, when this strong inhibition of the light rays is removed, does it venture from its burrow, creeping out a few inches in order to nibble at the vegetation on the rocks. Then, as the season for spawning approaches, remarkable changes take place within the bodies of the worms. With the maturing of the sex cells, the segments of the posterior third of each animal take on a new color, deep pink in the males, greenish gray in the females. Moreover, this part of the body, distended with eggs or sperm, becomes exceedingly thin-walled and fragile, and a noticeable constriction develops between this and the anterior part of the worm.
At last there comes a night when these worms—so changed in their physical beings—respond in a new way to the light of the moon. No longer does the light repel and hold them prisoners within their burrows. Instead, it draws them out to the performance of a strange ritual. The worms back out of their burrows, thrusting out the swollen, thin-walled posterior ends, which immediately begin a series of twisting movements, writhing in spiral motions until suddenly the body breaks at the weak point and each worm becomes two. The two parts have different destinies—the one to remain behind in the burrow and resume the life of the timid forager of the dark hours, the other to swim up toward the surface of the sea, to become one of a vast swarm of thousands upon thousands of worms joining in the spawning activities of the species.
During the last hours of the night the number of swarming worms increases rapidly, and when dawn comes the sea over the reef is almost literally filled with them. When the first rays of the sun appear, the worms, strongly stimulated by the light, begin to twist and contract violently, their thin-walled bodies burst open, and the eggs from some and sperm from others are cast into the sea. The spent and empty worms may continue to swim weakly for a short time, preyed upon by fish that gather for a feast, but soon all that remain have sunk to the bottom and died. But floating at the surface of the sea are the fertilized eggs, drifting over areas many feet deep and acres in extent. Within them swift changes have begun—the division of cells, the differentiation of structure. By evening of that same day the eggs have yielded up tiny larvae, swimming with spiral motions through the sea. For about three days the larvae live at the surface; then they become burrowers in the reefs below until, a year hence, they will repeat the spawning behavior of their kind.
Some related worms that swarm periodically about the Keys and the West Indies are luminous, creating beautiful pyrotechnic displays on dark nights. Some people believe that the mysterious light reported by Columbus as seen
by him on the night of October 11, "about four hours before making the landfall and an hour before moonrise," may have been a display of some of these "fireworms."
The tides pouring in from the reefs and sweeping over the flats come to rest against the elevated coral rock of the shore. On some of the Keys the rock is smoothly weathered, with flattened surfaces and rounded contours, but on many others the erosive action of the sea has produced a rough and deeply pitted surface, reflecting the solvent action of centuries of waves and driven salt spray. It is almost like a stormy sea frozen into solidity, or as the surface of the moon might be. Little caves and solution holes extend above and below the line of the high tide. In such a place I am always strongly aware of the old, dead reef beneath my feet, and of the corals whose patterns, now crumbling and blurred, were once the delicately sculptured vessels that held the living creatures. All the builders now are dead—they have been dead for thousands of years—but that which they created remains, a part of the living present.
Crouching on the jagged rocks, I hear little murmurings and whisperings born of the movements of air and water over these surfaces—the audible voice of this nonhuman, intertidal world. There are few obvious signs of life to break the spell of brooding desolation. Perhaps a dark-bodied isopod—a sea roach—darts across the dry rock to disappear into one of the small sea caves, daring exposure to light and to sharp-eyed enemies only for the moment of its swift passage from one dark recess to another. There are thousands of its kind in the coral rock, but not until darkness covers the shore will they come out in numbers to search for the bits of animal and vegetable refuse that are their food.
At the high-tide line, growths of microscopic plants darken the coral rock, tracing that mysterious black line that marks the sea's edge on all rocky coasts of the world. Because of the irregular surface and deep dissection of the coral rock, the sea runs in under the high-tide rocks by way of crevices and depressions, and so the black zone darkens the jagged peaks and the rims of holes and little caves, while lighter rock of a yellowish-gray hue lines the depressions below that controlling tidal level.
Small snails whose shells are boldly striped or checked in black and white—the neritas—crowd down into cracks and cavities in the coral or rest on open rock surfaces waiting for the return of the tide when they can feed. Others, in rounded shells with roughly beaded surfaces, belong to the periwinkle tribe. Like many others of their kind, these beaded periwinkles are making a tentative invasion of the land, living under rocks or logs high on the shore or even entering the fringe of land vegetation. Black horn shells live in numbers just below the line of the high tides, feeding on the algal film over the rocks. The living snails are held by some intangible bonds to this tidal level, but the shells discarded after their death are found and taken as habitations by the smallest of the hermit crabs, who then carry them down onto the lower levels of the shore.
These deeply eroded rocks are the home of the chitons, whose primitive form harks back to some ancient group of mollusks of which they are the only living representatives. Their oval bodies, covered with a jointed shell of eight transverse plates, fit into depressions in the rocks when the tide is out. They grip the rocks so strongly that even heavy waves can get no hold on their sloping contours. When the high tide covers them, they begin to creep about, resuming their rasping of vegetation from the rocks, their bodies swaying to and fro in time to the scraping motions of the radula or file-like tongue. Month in and month out, a chiton moves only a few feet in any direction; because of this sedentary habit, the spores of algae and the larvae of barnacles and tube-building worms settle upon its shell and develop there. Sometimes, in dark wet caves, the chitons pile up, one on top of another, and each scrapes algae off the back of the one beneath it. In a small way these primitive mollusks may be an agent of geologic change as they feed on the rocks, each removing, along with the algae, minute scrapings of rock particles and so, over the centuries and the millennia in which this ancient race of beings has lived its simple life, contributing to the processes of erosion by which earth surfaces are worn away.
On some of these Keys a small intertidal mollusk called Onchidium lives deep in little rock caverns, the entrances of which are often overgrown by colonies of mussels. Although it is a mollusk and a snail, Onchidium has no shell. It belongs to a group that consists largely of land snails or slugs, in many of which the shell is lacking or concealed. Onchidium inhabits tropical seashores, living usually on beaches of roughly eroded rock. As the tide falls, processions of small black slugs emerge from their doorways, wriggling and pushing their way out through the impeding mussel threads, a dozen or more individuals coming out of a common cave to feed on the rocks, from which they scrape vegetation as the chitons do. As they emerge, each is invested with a tunic of slime that makes it look jet black, wet, and shining; in wind and sun the little slug dries to a deep blue-black, over which is a slight, milky bloom.
On these journeys the slugs seem to follow haphazard or irregular paths over the rocks. They continue feeding as the tide falls to its lowest ebb, and even as it turns and begins to rise. About half an hour before the returning sea has reached them, and before so much as a drop of water has splashed into their nests, all of the slugs cease their grazing and begin to return to the home nest. While the outgoing path was meandering, the return is by a direct route. The members of each community return to their own nest, even though the way may lie over greatly eroded rock surfaces and even though the path may cross the routes of other slugs returning to other nests. All of the individuals belonging to one nest-community, even though they may have been widely separated while feeding, begin the return journey at almost the same moment. What is the stimulus? It is not the returning water, for that has not touched them; when it laps again over their rocks they will be safe within their nests.
The whole pattern of behavior of this little creature is puzzling. Why should it be drawn to live again at the edge of the sea that its ancestors deserted thousands or millions of years ago? It comes forth only when the tide has fallen, then, somehow sensing the impending return of the sea and seeming to remember its recent affinities with the land, it hurries to safety before the tide can find it and carry it away. How has it acquired this behavior, attracted yet repelled by the sea? We can only ask these questions; we cannot answer them.
For its protection during the feeding journeys, Onchidium is equipped with means of detecting and driving away its enemies. Minute papillae on its back are sensitive to light and passing shadows. Other, stouter papillae associated with the mantle are equipped with glands that secrete a milky, highly acid fluid. If the animal is suddenly disturbed, it expels spurting streams of this acid, the streams breaking up in the air to a fine spray that may be thrown five or six inches, or as much as a dozen times the length of the animal. The old German zoologist Semper, who studied a species of Onchidium in the Philippines, believed this dual equipment served to protect the slug from the beach-hopping blenny, a fish of many tropical mangrove coasts that leaps along above the tide, feeding on Onchidium and crabs. Semper thought the slugs could detect the shadow of an approaching fish and drive off the enemy by discharging the white acid spray. In Florida or elsewhere in the West Indian region there is no fish that comes out of water to pursue its prey. On the rocks where Onchidium must feed there are, however, scrambling crabs and isopods whose jostlings might well push the slugs into the water, for they have no means of gripping the rocks. For whatever reason, the slugs react to the crabs and to the isopods as to dangerous enemies, responding to their touch by discharging the repellant chemical.
In the strip between tropical tide lines, conditions are difficult for nearly all forms of life. The heat of the sun increases the hazards of exposure during the withdrawal of the tide. The shifting layers of choking sediment, accumulating on flat or gently sloping surfaces, discourage many plants and animals of types that inhabit rocky shores in the clearer, cooler waters of the north. Instead of the vast barnacle an
d mussel fields of New England there are only scattering patches of these creatures, varying from Key to Key but never really abundant. Instead of the great rockweed forests of the north, there are only scattered growths of small algae, including various brittle, lime-secreting forms, none of which offer shelter or security to any considerable number of animals.
If the area marked out by the advance and retreat of the neap tides is in general inhospitable, there are nevertheless two forms of life—one plant, one animal—that are thoroughly at home there, and live in profusion nowhere else. The plant is a peculiarly-beautiful alga that resembles spheres of green glass clustered together in irregular masses. It is Valonia, the sea bottle, a green alga that forms large vesicles filled with a sap that bears a definite chemical relation to the water about it, varying the proportions of its contained ions of sodium and potassium according to variations in the intensity of sunlight, the exposure to surf, and other conditions of its world. Under overhanging rock and in other sheltered places it forms sheets and masses of its emerald globules, lying half buried in deep drifts of sediment.