The thin larval shell takes form, but soon it is replaced by another, double-valved as in adult mussels. By this time the velum has disintegrated, and the mantle, foot, and other organs of the adult have begun their development.
From early summer these tiny shelled creatures live in prodigious numbers in the seaweeds of the shore. In almost every bit of weed I pick up for microscopic examination I find them creeping about, exploring their world with the long tubular organ called the foot, which bears an odd resemblance to the trunk of an elephant. The infant mussel uses it to test out objects in its path, to creep over level or steeply sloping rocks or through seaweeds, or even to walk on the under side of the surface film of quiet water. Soon, however, the foot assumes a new function: it aids in the work of spinning the tough silken threads that anchor a mussel to whatever offers a solid support and insurance against being washed away in the surf.
The very existence of the mussel fields of the low-tide zone is evidence that this chain of circumstances has proceeded unbroken to its consummation untold millions upon millions of times. Yet, for every mussel surviving upon the rocks, there must have been millions of larvae whose setting forth into the sea had a disastrous end. The system is in delicate balance; barring catastrophe, the forces that destroy neither outweigh nor are outweighed by those that create, and over the years of a man’s life, as over the ages of recent geologic time, the total number of mussels on the shore probably has remained about the same.
In much of this low-water area the mussels live in intimate association with one of the red seaweeds, Gigartina, a plant of low-growing, bushy form and almost cartilaginous texture. Plants and mussels unite inseparably to form a tough mat. Very small mussels may grow about the plants so abundantly as to obscure their basal attachment to the rocks. Both the stems and the repeatedly subdivided branches of the seaweed are astir with life, but with life on so small a scale that the human eye can see its details only with the aid of a microscope.
Snails, some with brightly banded and deeply sculptured shells, crawl along the fronds, browsing on microscopic vegetable matter. Many of the basal stems of the weed are thickly encrusted with the bryozoan sea lace, Membranipora; from all its compartments the minute, be-tentacled heads of the resident creatures are thrust out. Another bryozoan of coarser growth, Flustrella, also forms mats investing the broken stems and stubble of the red weed, the substance of its own growth giving such a stem almost the thickness of a pencil. Rough hairs or bristles protrude from the mat, so that much foreign matter adheres to it. Like the sea laces, however, it is formed of hundreds of small, adjacent compartments. From one after another of these, as I watch through my microscope, a stout little creature cautiously emerges, then unfurls its crown of filmy tentacles as one would open an umbrella. Threadlike worms creep over the bryozoan, winding among the bristles like snakes through coarse stubble. A tiny, cyclopean crustacean, with one glittering ruby eye, runs ceaselessly and rather clumsily over the colony, apparently disturbing the inhabitants, for when one of them feels the touch of the blundering crustacean it quickly folds its tentacles and withdraws into its compartment.
In the upper branches of this jungle formed by the red weed, there are many nests or tubes occupied by amphipod crustaceans known as Amphithoe. These small creatures have the appearance of wearing cream-colored jerseys brightly splotched with brownish red; in each goatlike face are set two conspicuous eyes and two pairs of hornlike antennae. The nests are as firmly and skillfully constructed as a bird’s but are subject to far more continuous use, for these amphipods are weak swimmers and ordinarily seem loath to leave their nests. They lie in their snug little sacs, often with the heads and upper parts of their bodies protruding. The water currents that pass through their seaweed home bring them small plant fragments and thus solve the problem of subsistence.
For most of the year Amphithoe lives singly, one to a nest. Early in the summer the males visit the females (who greatly outnumber them) and mating occurs within the nest. As the young develop the mother cares for them in a brood-pouch formed by the appendages of her abdomen. Often, while carrying her young, she emerges almost completely from her nest and vigorously fans currents of water through the pouch.
The eggs yield embryos, the embryos become larvae; but still the mother holds and cares for them until their small bodies have so developed that they are able to set forth into the seaweeds, to spin their own nests out of the fibers of plants and the silken threads mysteriously fashioned in their own bodies, and to feed and fend for themselves.
As her young become ready for independent life, the mother shows impatience to be rid of the swarm in her nest. Using claws and antennae, she pushes them to the rim and with shoves and nudges tries to expel them. The young cling with hooked and bristled claws to the walls and doorway of the familiar nursery. When finally thrust out they are likely to linger nearby; when the mother incautiously emerges, they leap to attach themselves to her body and so be drawn again into the security of their accustomed nest, until maternal impatience once more becomes strong.
Even the young just out of the brood-sac build their own nests and enlarge them as their growth requires. But the young seem to spend less time than the adults do inside their nests, and to creep about more freely over the weeds. It is common to see several tiny nests built close to the home of a large amphipod; perhaps the young like to stay close to the mother even after they have been ejected from her nest.
At low tide the water falls below the rockweeds and the mussels and enters a broad band clothed with the reddish-brown turf of the Irish moss. The time of its exposure to the atmosphere is so brief, the retreat of the sea so fleeting, that the moss retains a shining freshness, a wetness, and a sparkle that speak of its recent contact with the surf. Perhaps because we can visit this area only in that brief and magical hour of the tide’s turning, perhaps because of the nearness of waves breaking on rocky rims, dissolving in foam and spray, and pouring seaward again to the accompaniment of many water sounds, we are reminded always that this low-tide area is of the sea and that we are trespassers.
Here, in this mossy turf, life exists in layers, one above another; life exists on other life, or within it, or under it, or above it. Because the moss is low-growing and branches profusely and intricately, it cushions the living things within it from the blows of the surf, and holds the wetness of the sea about them in these brief intervals of the low ebbing of the tide. After I have visited the shore and then at night have heard the surf trampling in over these moss-grown ledges with the heavy tread of the fall tides, I have wondered about the baby starfish, the urchins, the brittle stars, the tube-dwelling amphipods, the nudibranchs, and all the other small and delicate fauna of the moss; but I know that if there is security in their world it should be here, in this densest of intertidal jungles, over which the waves break harmlessly.
The moss forms so dense a covering that one cannot see what is beneath without intimate exploration. The abundance of life here, both in species and individuals, is on a scale that is hard to grasp. There is scarcely a stem of Irish moss that is not completely encased with one of the bryozoan sea mats—the white lacework of Membranipora or the glassy, brittle crust of Microporella. Such a crust consists of a mosaic of almost microscopic cells or compartments, arranged in regular rows and patterns, their surfaces finely sculptured. Each cell is the home of a minute, tentacled creature. By a conservative guess, several thousand such creatures live on a single stem of moss. On a square foot of rock surface there are probably several hundred such stems, providing living space for about a million of the bryozoans. On a stretch of Maine shore that the eye can take in at a glance, the population must run into the trillions for this single group of animals.