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 and 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.