Like the anemones, the soft coral hangs its thimble-sized colonies on the under side of ledges. Limp and dripping at low tide, they suggest nothing of the life and beauty to which the returning water restores them. Then from all the myriad pores of the surface of the colony, the tentacles of little tubular animals appear and the polyps thrust themselves out into the tide, seizing each for itself the minute shrimps and copepods and multiformed larvae brought by the water.
The soft coral, or sea finger, secretes no limy cups as the distantly related stony, or reef, corals do, but forms colonies in which many animals live embedded in a tough matrix strengthened with spicules of lime. Minute though the spicules are, they become geologically important where, in tropical reefs, the soft corals, or Alcyonaria, mingle with the true corals. With the death and dissolution of the soft tissues, the hard spicules become minute building stones, entering into the composition of the reef. Alcyonarians grow in lush profusion and variety on the coral reefs and flats of the Indian Ocean, for these soft corals are predominantly creatures of the tropics. A few, however, venture into polar waters. One very large species, tall as a tall man and branched like a tree, lives on the fishing banks off Nova Scotia and New England. Most of the group live in deep waters; for the most part the intertidal rocks are inhospitable to them and only an occasional low-lying ledge, rarely and briefly exposed on the low spring tides, bears their colonies on dark and hidden surfaces.
In seams and crevices of rock, in little water-filled pools, or on rock walls briefly exposed by the tide's low ebbing, colonies of the pink-hearted hydroid Tubularia form gardens of beauty. Where the water still covers them the flowerlike animals sway gracefully at the ends of long stalks, their tentacles reaching out to capture small animals of the plankton. Perhaps it is where they are permanently submerged, however, that they reach their fullest development. I have seen them coating wharf pilings, floats, and submerged ropes and cables so thickly that not a trace of the substratum could be seen, their growth giving the illusion of thousands of blossoms, each as large as the tip of my little finger.
Below the last clumps of Irish moss, a new kind of sea bottom is exposed. The transition is abrupt. As though a line had been drawn, suddenly there is no more moss, and one steps from the yielding brown cushion onto a surface that seemingly is of stone. Except that the color is wrong, the effect is almost that of a volcanic slope—there is the same barren nakedness. Yet this is not rock that we see. The underlying rock is coated on every surface, vertical or horizontal, exposed or hidden, with a crust of coralline algae, so that it wears a rich old-rose color. So intimate is the union that the plant seems part of the rock. Here the periwinkles wear little patches of pink on their shells, all the rock caverns and fissures are lined with the same color, and the rock bottom that slants away into green water carries down the rose hue as far as the eye can follow.
The coralline algae are plants of unusual fascination. They belong to the group of red seaweeds, most of which live in the deeper coastal waters, for the chemical nature of their pigments usually requires the protection of a screen of water between their tissues and the sun. The corallines, however, are extraordinary in their ability to withstand direct sunlight. They are able to incorporate carbonate of lime into their tissues so that they have become hardened. Most species form encrusting patches on rocks, shells, and other firm surfaces. The crust may be thin and smooth, suggesting a coat of enamel paint; or it may be thick and roughened by small nodules and protuberances. In the tropics the corallines often enter importantly into the composition of the coral reefs, helping to cement the branching structures built by the coral animals into a solid reef. Here and there in the East Indies they cover the tidal flats as far as eye can see with their delicately hued crusts, and many of the "coral reefs" of the Indian Ocean contain no coral but are built largely of these plants. About the coasts of Spitsbergen, where under the dimly lit waters of the north the great forests of the brown algae grow, there are also vast calcareous banks, stretching mile after mile, formed by the coralline algae. Being able to live not only in tropical warmth but where water temperatures seldom rise above the freezing point, these plants flourish all the way from Arctic to Antarctic seas.
Where these same corallines paint a rose-colored band on the rocks of the Maine coast, as though to mark the low water line of the lowest spring tides, visible animal life is scarce. But although little else lives openly in this zone, thousands of sea urchins do. Instead of hiding in crevices or under rocks as they do at the higher levels, they live fully exposed on the flat or gently shelving rock faces. Groups of a score or half a hundred individuals lie together on the coralline-coated rocks, forming patches of pure green on the rose background. I have seen such herds of urchins lying on rocks that were being washed by a heavy surf, but apparently all the little anchors formed by their tube feet held securely. Though the waves broke heavily and poured back in a turbulent rush of waters, there the urchins remained undisturbed. Perhaps the strong tendency to hide and to wedge themselves into crevices and under boulders, as displayed by urchins in tide pools or up in the rockweed zone, is not so much an attempt to avoid the power of the surf as a means of escaping the eager eyes of the gulls, who hunt them relentlessly on every low tide. This coralline zone where the urchins live so openly is covered almost constantly with a protective layer of water; probably not more than a dozen daytime tides in the entire year fall to this level. At all other times, the depth of water over the urchins prevents the gulls from reaching them, for although a gull can make shallow plunges under water, it cannot dive as a tern does, and probably cannot reach a bottom deeper than the length of its own body.
The lives of many of these creatures of the low-tide rocks are bound together by interlacing ties, in the relation of predator to prey, or in the relation of species that compete for space or food. Over all these the sea itself exercises a directing and regulating force.
The urchins seek sanctuary from the gulls at this low level of the spring tides, but in themselves stand in the relation of dangerous predators to other animals. Where they advance into the Irish moss zone, hiding in deep crevices and sheltering under rock overhangs, they devour numbers of periwinkles, and even attack barnacles and mussels. The number of urchins at any particular level of shore has a strong regulating effect on the populations of their prey. The starfish and a voracious snail, the common whelk, like the urchins, have their centers of population in deep water offshore and make predatory excursions of varying duration into the intertidal zone.
The position of the prey animals—the mussels, barnacles, and periwinkles—on sheltered shores has become difficult. They are hardy and adaptable, able to live at any level of the tide. Yet on such shores the rockweeds have crowded them out of the upper two-thirds of the shore, except for scattered individuals. At and just below the low-tide line are the hungry predators, so all that remains for these animals is the level near the low-water line of the neap tides. On protected coasts it is here that the barnacles and mussels assemble in their millions to spread their cover of white and blue over the rocks, and the legions of the common periwinkle gather.
But the sea, with its tempering and modifying effect, can alter the pattern. Whelks, starfish, and urchins are creatures of cold water. Where the offshore waters are cold and deep and the tidal flow is drawn from these icy reservoirs, the predators can range up into the intertidal zone, decimating the numbers of their prey. But when there is a layer of warm surface water the predators are confined to the cold deep levels. As they retreat seaward, the legions of their prey follow down in their wake, descending as far as they may into the world of the low spring tides.
Tide pools contain mysterious worlds within their depths, where all the beauty of the sea is subtly suggested and portrayed in miniature. Some of the pools occupy deep crevices or fissures; at their seaward ends these crevices disappear under water, but toward the land they run back slantingly into the cliffs and their walls rise higher, casting deep
shadows over the water within them. Other pools are contained in rocky basins with a high rim on the seaward side to hold back the water when the last of the ebb drains away. Seaweeds line their walls. Sponges, hydroids, anemones, sea slugs, mussels, and starfish live in water that is calm for hours at a time, while just beyond the protecting rim the surf may be pounding.
The pools have many moods. At night they hold the stars and reflect the light of the Milky Way as it flows across the sky above them. Other, living stars come in from the sea: the shining emeralds of tiny phosphorescent diatoms—the glowing eyes of small fishes that swim at the surface of the dark water, their bodies slender as matchsticks, moving almost upright with little snouts uplifted—the elusive moonbeam flashes of comb jellies that have come in with a rising tide. Fishes and comb jellies hunt the black recesses of the rock basins, but like the tides they come and go, having no part in the permanent life of the pools.
By day there are other moods. Some of the most beautiful pools lie high on the shore. Their beauty is the beauty of simple elements—color and form and reflection. I know one that is only a few inches deep, yet it holds all the depth of the sky within it, capturing and confining the reflected blue of far distances. The pool is outlined by a band of bright green, a growth of one of the seaweeds called Enteromorpha. The fronds of the weed are shaped like simple tubes or straws. On the land side a wall of gray rock rises above the surface to the height of a man, and reflected, descends its own depth into the water. Beyond and below the reflected cliff are those far reaches of the sky. When the light and one's mood are right, one can look down into the blue so far that one would hesitate to set foot in so bottomless a pool. Clouds drift across it and wind ripples scud over its surface, but little else moves there, and the pool belongs to the rock and the plants and the sky.
In another high pool nearby, the green tube-weed rises from all of the floor. By some magic the pool transcends its realities of rock and water and plants, and out of these elements creates the illusion of another world. Looking into the pool, one sees no water but instead a pleasant landscape of hills and valleys with scattered forests. Yet the illusion is not so much that of an actual landscape as of a painting of one; like the strokes of a skillful artist's brush, the individual fronds of the algae do not literally portray trees, they merely suggest them. But the artistry of the pool, as of the painter, creates the image and the impression.
Little or no animal life is visible in any of these high pools—perhaps a few periwinkles and a scattering of little amber isopods. Conditions are difficult in all pools high on the shore because of the prolonged absence of the sea. The temperature of the water may rise many degrees, reflecting the heat of the day. The water freshens under heavy rains or becomes more salty under a hot sun. It varies between acid and alkaline in a short time through the chemical activity of the plants. Lower on the shore the pools provide far more stable conditions, and both plants and animals are able to live at higher levels than they could on open rock. The tide pools, then, have the effect of moving the life zones a little higher on the shore. Yet they, too, are affected by the duration of the sea's absence, and the inhabitants of a high pool are very different from those of a low-level pool that is separated from the sea only at long intervals and then briefly.
The highest of the pools scarcely belong to the sea at all; they hold the rains and receive only an occasional influx of sea water from storm surf or very high tides. But the gulls fly up from their hunting at the sea's edge, bringing a sea urchin or a crab or a mussel to drop on the rocks, in this way shattering the hard shelly covering and exposing the soft parts within. Bits of urchin tests or crab claws or mussel shells find their way into the pools, and as they disintegrate their limy substance enters into the chemistry of the water, which then becomes alkaline. A little one-celled plant called Sphaerella finds this a favorable climate for growth—a minute, globular bit of life almost invisible as an individual, but in its millions turning the waters of these high pools red as blood. Apparently the alkalinity is a necessary condition; other pools, outwardly similar except for the chance circumstance that they contain no shells, have none of the tiny crimson balls.
Even the smallest pools, filling depressions no larger than a teacup, have some life. Often it is a thin patch of scores of the little seashore insect, Anurida maritima—"the wingless one who goes to sea." These small insects run on the surface film when the water is undisturbed, crossing easily from one shore of a pool to another. Even the slightest rippling causes them to drift helplessly, however, so that scores or hundreds of them come together by chance, becoming conspicuous only as they form thin, leaflike patches on the water. A single Anurida is small as a gnat. Under a lens, it seems to be clothed in blue-gray velvet through which many bristles or hairs protrude. The bristles hold a film of air about the body of the insect when it enters the water, and so it need not return to the upper shore when the tide rises. Wrapped in its glistening air blanket, dry and provided with air for breathing, it waits in cracks and crevices until the tide ebbs again. Then it emerges to roam over the rocks, searching for the bodies of fish and crabs and the dead mollusks and barnacles that provide its food, for it is one of the scavengers that play a part in the economy of the sea, keeping the organic materials in circulation.
And often I find the pools of the upper third of the shore lined with a brown velvety coating. My fingers, exploring, are able to peel it off the rocks in thin smooth-surfaced sheets like parchment. It is one of the brown seaweeds called Ralfsia; it appears on the rocks in small, lichen-like growths or, as here, spreading its thin crust over extensive areas. Wherever it grows its presence changes the nature of a pool, for it provides the shelter that many small creatures seek so urgently. Those small enough to creep in under it—to inhabit the dark pockets of space between the encrusting weed and the rock—have found security against being washed away by the surf. Looking at these pools with their velvet lining, one would say there is little life here—only a sprinkling of periwinkles browsing, their shells rocking gently as they scrape at the surface of the brown crust, or perhaps a few barnacles with their cones protruding through the sheet of plant tissue, opening their doors to sweep the water for food. But whenever I have brought a sample of this brown seaweed to my microscope, I have found it teeming with life. Always there have been many cylindrical tubes, needle-fine, built of a muddy substance. The architect of each is a small worm whose body is formed of a series of eleven infinitely small rings or segments, like eleven counters in a game of checkers, piled one above another. From its head arises a structure that makes this otherwise drab worm beautiful—a fanlike crown or plume composed of the finest feathery filaments. The filaments absorb oxygen and also serve to ensnare small food organisms when thrust out of the tube. And always, among this microfauna of the Ralfsia crust, there have been little fork-tailed crustaceans with glittering eyes the color of rubies. Other crustaceans called ostracods are enclosed in flattened, peach-colored shells fashioned of two parts, like a box with its lid; from the shell long appendages may be thrust out to row the creatures through the water. But most numerous of all are the minute worms hurrying across the crust—segmented bristle worms of many species and smooth-bodied, serpent-like ribbon worms or nemerteans, their appearance and rapid movements betraying their predatory errands.
A pool need not be large to hold beauty within pellucid depths. I remember one that occupied the shallowest of depressions; as I lay outstretched on the rocks beside it I could easily touch its far shore. This miniature pool was about midway between the tide lines, and for all I could see it was inhabited by only two kinds of life. Its floor was paved with mussels. Their shells were a soft color, the misty blue of distant mountain ranges, and their presence lent an illusion of depth. The water in which they lived was so clear as to be invisible to my eyes; I could detect the interface between air and water only by the sense of coldness on my fingertips. The crystal water was filled with sunshine—an infusion and distillation
of light that reached down and surrounded each of these small but resplendent shellfish with its glowing radiance.
The mussels provided a place of attachment for the only other visible life of the pool. Fine as the finest threads, the basal stems of colonies of hydroids traced their almost invisible lines across the mussel shells. The hydroids belonged to the group called Sertularia, in which each individual of the colony and all the supporting and connecting branches are enclosed within transparent sheaths, like a tree in winter wearing a sheath of ice. From the basal stems erect branches arose, each branch the bearer of a double row of crystal cups within which the tiny beings of the colony dwelt. The whole was the very embodiment of beauty and fragility, and as I lay beside the pool and my lens brought the hydroids into clearer view they seemed to me to look like nothing so much as the finest cut glass—perhaps the individual segments of an intricately wrought chandelier. Each animal in its protective cup was something like a very small sea anemone—a little tubular being surmounted by a crown of tentacles. The central cavity of each communicated with a cavity that ran the length of the branch that bore it, and this in turn with the cavities of larger branches and with those of the main stem, so that the feeding activities of each animal contributed to the nourishment of the whole colony.