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.