III. The Rocky Shores 2 страница

When, through the attacks of fish, predatory worms, or snails, or through natural causes, the barnacle’s life comes to an end, the shells remain attached to the rocks. These become shelter for many of the minute beings of the shore. Besides the baby periwinkles that regularly live there, the little tide‑pool insects often hurry into these shelters if caught by the rising tide. And lower on the shore, or in tide pools, the empty shells are likely to house young anemones, tube worms, or even new generations of barnacles.

The chief enemy of the barnacle on these shores is a brightly colored carnivorous marine snail, the dog whelk. Although it preys also on mussels and even occasionally on periwinkles, it seems to prefer barnacles to all other food, probably because they are more easily eaten. Like all snails, the whelk possesses a radula. This is not used, periwinkle fashion, to scrape the rocks, but to drill a hole in any hard‑shelled prey. It can then be pushed through the hole it has made, to reach and consume the soft parts within. To devour a barnacle, however, the whelk need only envelop the cone within its fleshy foot and force the valves open. It also produces a secretion that may have a narcotic effect. This is a substance called purpurin. In ancient times the secretion of a related snail in the Mediterranean was the source of the dye Tyrian purple. The pigment is an organic compound of bromine that changes in air to form a purple coloring matter.

Although violent surf excludes them, the dog whelks appear in numbers on most open shores, working up high into the zone of the barnacles and mussels. By their voracious feeding they may actually alter the balance of life on the shore. There is a story, for example, about an area where the whelks had reduced the number of barnacles so drastically that mussels came in to fill the vacant niche. When the whelks could find no more barnacles they moved over to the mussels. At first they were clumsy, not knowing how to eat the new food. Some spent futile days boring holes in empty shells; others climbed into empty shells and bored from inside. In time, however, they adjusted to the new prey and ate so many mussels that the colony began to dwindle. Then the barnacles settled anew on the rocks and in the end the snails returned to them.

In the middle stretches of shore and even down toward the low‑tide line the whelks live under the dripping curtains of weed hanging down from the rock walls, or within the turf of Irish moss or among the flat, slippery fronds of the red seaweed, dulse. They cling to the under sides of overhanging ledges or gather in deep crevices where salt water drips from weeds and mussels, and little streams of water trickle over the floor. In all such places the whelks collect in numbers to pair and lay their eggs in straw‑colored containers, each about the size and shape of a grain of wheat and tough as parchment. Each capsule stands alone, attached by its own base to the substratum, but usually they are crowded so closely together that they form a pattern or mosaic.

A snail takes about an hour to make one capsule but seldom completes more than 10 in twenty‑four hours. It may produce as many as 245 in a season. Although a single capsule may contain as many as a thousand eggs, most of these are unfertilized nurse eggs that serve as food for the developing embryos. On maturing, the capsules become purple, being colored by the same chemical purpurin that is secreted by the adult. In about four months embryonic life is completed, and from 15 to 20 young dog whelks emerge from the capsule. Newly hatched young seldom if ever are found in the zone where the adults live, although the capsules are deposited and development takes place there. Apparently the waves carry the young snails down to low‑tide level or below. Probably many are washed out to sea and lost, but the survivors are to be found at low water. They are very small–about one‑sixteenth of an inch high–and are feeding on the tube worm, Spirorbis. Apparently the tubes of these worms are easier to penetrate than the cones of even very small barnacles. Not until the dog whelk is about one‑fourth or three‑eighths of an inch high does it migrate higher on the shore and begin feeding on barnacles.

Down in the middle sections of the shore the limpets become abundant. They appear sprinkled over the open rock surfaces, but most live numerously in shallow tide pools. A limpet wears a simple cone of shell the size of a fingernail, unobtrusively mottled with soft browns and grays and blues. It is one of the most ancient and primitive types of snails, and yet the primitiveness and the simplicity are deceptive. The limpet is adapted with beautiful precision to the difficult world of the shore. One expects a snail to have a coiled shell; the limpet has instead a flattened cone. The periwinkles, which have coiled shells, are often rolled around by the surf unless they have hidden themselves securely in crevices or under weeds. The limpet merely presses its cone against the rocks and the water slides over the sloping contours without being able to get a grip; the heavier the waves, the more tightly they press it to the rocks. Most snails have an operculum to shut out enemies and keep moisture inside; the limpet has one in infancy and then discards it. The fit of the shell to the substratum is so close that an operculum is unnecessary; moisture is retained in a little groove that runs around just inside the shell, and the gills are bathed in a small sea of their own until the tide returns.

Ever since Aristotle reported that limpets leave their places on the rocks and go out to feed, people have been recording facts about their natural history. Their supposed possession of a sort of homing sense has been widely discussed. Each limpet is said to possess a “home” or spot to which it always returns. On some types of rock there may be a recognizable scar, either a discoloration or a depression, to which the contours of the shell have become precisely fitted. From this home the limpet wanders out on the high tides to feed, scraping the small algae off the rocks by licking motions of the radula. After an hour or two of feeding it returns by approximately the same path, and settles down to wait out the period of low water.

Many nineteenth‑century naturalists tried unsuccessfully to discover by experiment the nature of the sense involved and the organ in which the homing sense resides, much as modern scientists try to find a physical basis for the homing abilities of birds. Most of these studies dealt with the common British limpet, Patella, and although no one could explain how the homing instinct worked, there seemed to be little doubt in anyone’s mind that it did work, and with great precision.

In recent years American scientists have investigated the matter with statistical methods, and some have concluded that Pacific coast limpets do not “home” very well at all. (No careful studies of homing have been made among New England limpets.) Other recent work in California, however, supports the homing theory. Dr. W. G. Hewatt marked a large number of limpets and their homes with identifying numbers. He found that on each high tide all the limpets left home, wandered about for some two and a half hours, then returned. The direction of their excursions changed from tide to tide, but they always returned to the home spot. Dr. Hewatt tried filing a deep groove across one animal’s homeward path. The limpet halted on the edge of the groove and spent some time confronting this dilemma, but on the next tide it moved around the edge of the groove and returned home. Another limpet was taken about nine inches from its home and the edges of its shell were filed smooth. It was then released on the same spot. It returned to its home, but presumably the exact fit of shell to rock home had been destroyed by the filing and the next day the limpet moved about twenty‑one inches away and did not return. On the fourth day it had taken up a new home and after eleven days it disappeared.

The limpet’s relations with other inhabitants of the shore are simple. It lives entirely on the minute algae that coat the rocks with a slippery film, or on the cortical cells of larger algae. For either purpose, the radula is effective. The limpet scrapes the rocks so assiduously that fine particles of stone are found in its stomach; as the teeth of the radula wear away under hard use they are replaced by others, pushed up from behind. To the algal spores swarming in the water, ready to settle down and become sporelings and then adult plants, the limpets stand in the relation of enemy, since they keep the rocks scraped fairly clean where they are numerous. By this very act, however, they render a service to barnacles, making easier the attachment of their larvae. Indeed, the paths radiating out from a limpet’s home are sometimes marked by a sprinkling of the starlike shells of young barnacles.

In its reproductive habits this deceptively simple little snail seems again to have defied exact observation. It seems certain, however, that the female limpet does not make protective capsules for her eggs in the fashion typical of many snails, but commits them directly to the sea. This is a primitive habit, followed by many of the simpler sea creatures. Whether the eggs are fertilized within the mother’s body or while floating at sea is uncertain. The young larvae drift or swim for a time in the surface waters; the survivors then settle down on rocky surfaces and metamorphose from the larval to the adult form. Probably all young limpets are males, later transforming to females–a circumstance not at all uncommon among mollusks.

Like the animal life of this coast, the seaweeds tell a silent story of heavy surf. Back from the headlands and in bays and coves the rockweeds may grow seven feet tall; here on this open coast a seven‑inch plant is a large one. In their sparse and stunted growth, the seaweed invaders of the upper rocks reveal the stringent conditions of life where waves beat heavily. In the middle and lower zones some hardy weeds have been able to establish themselves in greater abundance and profusion. These differ so greatly from the algae of quieter shores that they are almost a symbol of the wave‑swept coast. Here and there the rocks sloping to the sea glisten with sheets composed of many individual plants of a curious seaweed, the purple laver. Its generic name, Porphyra, means “a purple dye.” It belongs to the red algae, and although it has color variations, on the Maine coast it is most often a purplish brown. It resembles nothing so much as little pieces of brown transparent plastic cut out of someone’s raincoat. In the thinness of its fronds it is like the sea lettuce, but there is a double layer of tissue, suggesting a child’s rubber balloon that has collapsed so that the opposite walls are in contact. At the stem of the “balloon” Porphyra is attached strongly to the rocks by a cord of interwoven strands–hence its specific name, “umbilicalis.” Occasionally it is attached to barnacles and very rarely it grows on other algae instead of directly on hard surfaces. When exposed at ebb tide under a hot sun, the laver may dry to brittle, papery layers, but the return of the sea restores the elastic nature of the plant, which, despite its seeming delicacy, allows it to yield unharmed to the push and pull of waves.

Down at the lower tidal levels is another curious weed–Leathesia, the sea potato. It grows in roughly globular form, its surface seamed and drawn into lobes, forming fleshy, amber‑colored tubers that are any size up to an inch or two in diameter. Usually it grows around the fronds of moss or another seaweed, seldom if ever attaching directly to the rocks.

The lower rocks and the walls of low tide pools are thickly matted with algae. Here the red weeds largely supplant the browns that grow higher up. Along with Irish moss, dulse lines the walls of the pools, its thin, dull red fronds deeply indented so that they bear a crude resemblance to the shape of a hand. Minute leaflets sometimes haphazardly attached along the margins create a strangely tattered appearance. With the water withdrawn, the dulse mats down against the rocks, paper‑thin layers piled one upon another. Many small starfish, urchins, and mollusks live within the dulse, as well as in the deeper growth of Irish moss.

Dulse is one of the algae that have a long history of usefulness to man, as food for himself and his domestic animals. According to an old book on seaweeds, it used to be said in Scotland that “he who eats of the Dulse of Guerdie and drinks of the wells of Kildingie will escape all maladies except black death.” In Great Britain, cattle are fond of it and sheep wander down into the tidal zone at low water in search of it. In Scotland, Ireland, and Iceland people eat dulse in various ways, or dry it and chew it like tobacco; even in the United States, where such foods are usually ignored, it is possible to buy dulse fresh or dried in some coastal cities.

In the very lowest pools the Laminarias begin to appear, called variously the oarweeds, devil’s aprons, sea tangles, and kelps. The Laminarias belong to the brown algae, which flourish in the dimness of deep waters and polar seas. The horsetail kelp lives below the tidal zone with others of the group, but in deep pools also comes over the threshold, just above the line of the lowest tides. Its broad, flat, leathery frond is frayed into long ribbons, its surface is smooth and satiny, and its color richly, glowingly brown.

The water in these deep basins is icy cold, filled with dusky, swaying plants. To look into such a pool is to behold a dark forest, its foliage like the leaves of palm trees, the heavy stalks of the kelps also curiously like the trunks of palms. If one slides his fingers down along such a stalk and grips just above the holdfast, he can pull up the plant and find a whole microcosm held within its grasp.

One of these laminarían holdfasts is something like the roots of a forest tree, branching out, dividing, subdividing, in its very complexity a measure of the great seas that roar over this plant. Here, finding secure attachment, are plankton‑strainers like mussels and sea squirts. Small starfish and urchins crowd in under the arching columns of plant tissue. Predacious worms that have foraged hungrily during the night return with the daylight and coil themselves into tangled knots in deep recesses and dark, wet caverns. Mats of sponge spread over the holdfasts, silently, endlessly at their work of straining the waters of the pool. One day a larval bryozoan settles here, builds its tiny shell, then builds another and another, until a film of frosty lace flows around one of the rootlets of the seaweed. And above all this busy community, and probably unaffected by it, the brown ribbons of the kelp roll out into the water, the plant living its own life, growing, replacing torn tissues as best it may, and in season sending clouds of reproductive cells streaming into the water. As for the fauna of the holdfasts, the survival of the kelp is their survival. While it stands firm their little world holds intact; if it is torn away in a surge of stormy seas, all will be scattered and many will perish with it.

Among the animals almost always inhabiting the holdfasts of the tide‑pool kelps are the brittle stars. These fragile echinoderms are well named, for even gentle handling is likely to cause them to snap off one or more arms. This reaction may be useful to an animal living in a turbulent world, for if an arm is pinned down under a shifting rock, the owner can break it off and grow a new one. Brittle stars move about rapidly, using their flexible arms not only in locomotion, but also to capture small worms and other minute sea life and carry them to their mouths.

The scale worms also belong to the holdfast community. Their bodies are protected by a double row of plates forming armament over the back. Under these large plates is an ordinary segmented worm, bearing laterally projecting tufts of golden bristles on each segment. There is a suggestion of primitiveness in the armor plate that is reminiscent of the quite unrelated chitons. Some of the scale worms have developed interesting relations with their neighbors. One of the British species always lives with burrowing animals, although it may change associates from time to time. When young, it lives with a burrowing brittle star, probably stealing its food. When older and larger, it moves into the burrow of a sea cucumber or the tube of the much larger, plumed worm, Amphitrite.

Often the holdfast grips one of the large horse mussels, which have heavy shells and may be four or five inches long. The horse mussel lives only in the deep pools or farther offshore; it is never found in the upper zones with the smaller blue mussel, and it occurs only on or among rocks, where its attachment is relatively secure. Sometimes it constructs a small nest or den as a refuge, using tough byssal threads spun in typical mussel fashion, with pebbles and shell fragments matted among the strands.

A small clam common in laminarían holdfasts is the rock‑borer, which some English writers call the “red‑nose” because of its red siphons. Ordinarily it is a boring form, living in cavities it excavates in limestone, clay, or concrete. Most of the New England rocks are too hard for boring, and so on this coast the clam lives in crusts of coralline algae or among the holdfasts of the kelp. On British coasts it is said to bore rocks that resist mechanical drills. And it does so without recourse to the chemical secretions some borers use, working entirely by repeated and endless mechanical abrasion with its sturdy shell.

The smooth and slippery fronds of the kelps support other populations, less abundant and less varied than those of the holdfasts. On the flat blades of the oarweeds, as well as on rock faces and under ledges, the golden‑star tunicate, Botryllus, lays its spangled mats. Over a field of dark green gelatinous substance are sprinkled the little golden stars that mark the position of clusters of individual tunicates. Each starry cluster may consist of three to a dozen individual animals radiating around a central point; many clusters go to make up this continuous, encrusting mat, which may be six to eight inches long.

Beneath the surface beauty there is marvelous complexity of structure and function. Over each star there are infinitesimal disturbances in the water–little currents funneling down, one to each point of the star, and there being drawn in through a small opening. One heavier, outward‑moving current emerges from the center of the cluster. The indrawn currents bring in food organisms and oxygen, and the outflowing current carries away the metabolic waste products of the group.

At first glance a colony of Botryllus may seem no more complex than a mat of encrusting sponge. In actual fact, however, each of the individuals comprising the colony is a highly organized creature, in structure almost identical with such solitary ascidians as the sea grape and the sea vase, found so abundantly on wharves and sea walls. The individual Botryllus, however, is only one‑sixteenth to one‑eighth of an inch long.

One of these entire colonies, comprising perhaps hundreds of star clusters (and so perhaps a thousand or more individuals), may arise from a single fertilized ovum. In the parent colony, eggs are formed early in the summer, are fertilized, and begin their development while remaining within the parental tissues. (Each individual Botryllus produces both eggs and sperms, but since in any one animal they mature at different times, cross fertilization is insured, the spermatozoa being carried in the sea water and drawn in along with the water currents.) Presently the parent releases minute larvae shaped like tadpoles, with long, swimming tails. For perhaps an hour or two such a larva drifts and swims, then settles down on some ledge or weed and makes itself fast. Soon the tissues of the tail are absorbed and all suggestion of ability to swim is lost. Within two days the heart begins to beat in that curious tunicate rhythm–first driving the blood in one direction, pausing briefly, then reversing the direction of the flow. After nearly a fortnight, this small individual has completed the formation of its own body and begins to bud off other individuals. These, in turn, bud off others. Each new creature has its separate opening for the intake of water, but all retain connections with a central vent for the outflow of wastes. When the individuals clustered around this common opening become too crowded, one or more newly formed buds are pushed out into the surrounding mat of gelatinous tissue, where they begin new star clusters. In this way the colony spreads.

The intertidal zone is sometimes invaded by a deep‑water laminarian, the sea colander. It is a representative of those brown seaweeds that flourish in the cold waters of the Arctic, and has come down from Greenland as far as Cape Cod. Its appearance is strikingly different from that of the sea moss and horsetail kelp among which it sometimes appears. The wide frond is pierced by innumerable perforations; these are foreshadowed in the young plant by conical papillae, which later break through to form the perforations.

Beyond the rims of the lowest pools, growing on the rock walls that slope away steeply into deep water, is another laminarian seaweed, Alaria, the winged kelp, called the murlin in Great Britain. Its long, ruffled, streaming fronds rise with each surge and fall as the water pours away seaward. The fertile pinnae, in which the reproductive cells mature, are borne at the base of the frond, for in a plant so exposed to violent surf this location is safer than the tips of the main blade. (In the rockweeds, living higher on the shore and less subject to savage wave action, the reproductive cells are formed at the tips of the fronds.) Almost more than any of the other seaweeds, Alaria is a plant conditioned to constant pounding by the waves. Standing on the outermost point that gives safe footing, one can see its dark ribbons streaming out into the water, being tugged and tossed and pounded. The larger and older plants become much frayed and worn, the margins of the blade splitting or the tip of the midrib being worn off. By such concessions the plant saves some of the strain on its holdfasts. The stipe can withstand a relatively enormous pull, but severe storms tear away many plants.

Still farther down, one can sometimes and in some places get a glimpse of the dark, mysterious forests of the kelps, where they go down into deep water. Sometimes these giant kelps are cast ashore after a storm. They have a stiff, strong stipe from which the long ribbon of the frond extends. The sea tangle or sugar kelp, Laminaria saccharina, has a stipe up to 4 feet long, supporting a relatively narrow frond (6 to 18 inches wide) that may extend out and upward into the sea as much as 30 feet. The margin is greatly frilled and a powdery white substance (mannitol, a sugar) forms on the dried fronds. The long‑stalked laminaria (Laminaria longicruris ) has a stem comparable to the trunk of a small tree, being 6 to 12 feet long. The frond is up to 3 feet wide and 20 feet long, but may sometimes be shorter than the stipe.

The stands of sea tangles and long‑stalked laminarias are, in their way, an Atlantic counterpart of the great submarine jungles of the Pacific, where the kelps rise like giant forest trees, 150 feet from the floor of the sea to the surface.

On all rocky coasts, this laminarian zone just below low water has been one of the least‑known regions of the sea. We know little about what lives here throughout the year. We do not know whether some of the forms that disappear from the intertidal area in winter may merely move down into this zone. And perhaps some of the species we think have died out in a particular region, perhaps because of temperature changes, have gone down among the Laminarias. The area is obviously difficult to explore, with heavy seas breaking there most of the time. Such an area on the west coast of Scotland was, however, explored by helmet divers working with the British biologist, J. A. Kitching. Below the zone occupied by Alaria and the horsetail kelp, from about two fathoms below low water and beyond, the divers moved through a dense forest of the larger Laminarias. From the vertical stipes an immense canopy of fronds was spread above their heads. Although the sun shone brightly at the surface, the divers were almost in darkness as they pushed through this forest. Between three and six fathoms below low water of the spring tides the forest opened out, so that the men could walk between the plants without great difficulty. There the light was stronger, and through misty water they could see this more open “park” extending farther down the sloping floor of the sea. Among the holdfasts and stipes of the laminarias, as among the roots and trunks of a terrestrial forest, was a dense undergrowth, here formed of various red algae. And as small rodents and other creatures have their dens and runways under the forest trees, so a varied and abundant fauna lived on and among the holdfasts of the great seaweeds.

In quieter waters, protected from the heavy surf of coasts that face the open ocean, the seaweeds dominate the shore, occupying every inch of space that the conditions of tidal rise and fall allow them and by the sheer force of abundant and luxuriant growth forcing other shore inhabitants to accommodate to their pattern.

Although the same bands of life are spread between the tide lines whether the coast be open or sheltered, in their relative development the zones vary greatly on the two types of shore.

Above the high‑tide line there is little change and on the shores of bays and estuaries, as elsewhere, the microplants blacken the rocks and the lichens come down and tentatively approach the sea. Below high water of spring tides, pioneering barnacles trace occasional white streaks in token occupation of the zone they dominate on open coasts. A few periwinkles graze on the upper rocks. But on sheltered coasts the whole band of shore marked out by the tides of the moon’s quarters is occupied by a swaying submarine forest, sensitive to the movements of the waves and the tidal currents. The trees of the forest are the large sea weeds known as the rockweeds or sea wracks, stout of form and rubbery of texture. Here all other life exists within their shelter–a shelter so hospitable to small things needing protection from drying air, from rain, and from the surge of the running tides and the waves, that the life of these shores is incredibly abundant.

When covered at high tide, the rockweeds stand erect, rising and swaying with a life borrowed from the sea. Then, to one standing at the edge of a flooding tide, the only sign of their presence may be a scattering of dark patches on the water close inshore, where the tips of the weeds reach up to the surface. Down below those floating tips small fishes swim, passing between the weeds as birds fly through a forest, sea snails creep along the fronds, and crabs climb from branch to branch of the swaying plants. It is a fantastic jungle, mad in a Lewis Carroll sort of way. For what proper jungle, twice every twenty‑four hours, begins to sag lower and lower and finally lies prostrate for several hours, only to rise again? Yet this is precisely what the rockweed jungles do. When the tide has retreated from the sloping rocks, when it has left the miniature seas of the tide pools, the rockweeds lie flat on the horizontal surfaces in layer above layer of sodden, rubbery fronds. From the sheer rock faces they hang down in a heavy curtain, holding the wetness of the sea, and nothing under their protective cover ever dries out.

By day the sunlight filters through the jungle of rockweeds to reach its floor only in shifting patches of shadow‑flecked gold; by night the moonlight spreads a silver ceiling above the forest–a ceiling streaked and broken by the flowing tide streams; beneath it the dark fronds of the weeds sway in a world unquiet with moving shadows.

But the flow of time through this submarine forest is marked less by the alternation of light and darkness than by the rhythm of the tides. The lives of its creatures are ruled by the presence or absence of water; it is not the fall of dusk or the coming of dawn but the turn of the tide that brings transforming change to their world.