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 loggerhead 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.