The Edge of the Sea
Copepods (oar-footed) are very small crustaceans with rounded bodies, jointed tails, and oarlike legs with which to propel themselves jerkily. In spite of their minute size (from microscopic to half-inch) the copepods form one of the basic populations of the sea, and are food for an immense variety of other animals. They are an indispensable link in the food chain by which the nutrient salts of the sea are eventually made available (via plant plankton, animal plankton, carnivores) to larger animals such as fishes and whales. Copepods of the genus Calanus, known as "red feed," redden large areas of ocean surface and are eaten in prodigious numbers by herring and mackerel and also by certain whales. Birds of the open sea such as petrels and albatrosses are plankton feeders and sometimes subsist largely on copepods. In their turn, the copepods graze on diatoms, eating sometimes as much as their own weight in a day.
Amphipods are small crustaceans that are flattened from side to side, while isopods are flattened from upper to lower surface. The names are a scientific reference to the kinds of appendages possessed by these small creatures. The amphipods have feet that can be used both for swimming and walking or crawling. The isopods, or "equal-footed" animals, have appendages that show little difference in size and shape from one end of the body to the other.
On the shore the amphipods include the beach hoppers, or sand fleas, that rise in clouds (leaping, not flying) from masses of seaweed when they are disturbed, and others that live offshore in seaweed and under rocks. They eat fragments and bits of organic debris and are themselves eaten in great number by fish, birds, and other larger creatures. Many amphipods wriggle along on their sides when out of water. Sand hoppers use their tails and posterior legs as a spring and progress by leaps; other species swim.
Isopods of the shore (closely related to the familiar sow bugs of the garden) include the slaters (sea roaches, wharf rats, quay lice) often seen running over rocks and wharf pilings. These have left the water and seldom return to it; they drown if long submerged. Others live offshore, often in seaweeds whose color and form they mimic. Still others swarm in tide pools, sometimes nipping the skin of human waders to produce a tingling or itching sensation. Most are scavengers; some are parasites; and some form habitual associations (commensalism) with an animal of unrelated species.
Both amphipods and isopods carry their young in brood chambers instead of liberating eggs into the sea. This habit has helped some in each group to live high on the shore and is a necessary preliminary to land existence.
The barnacles belong to the order Cirripedia (Latin, cirrus—a ringlet or curl), presumably named because of their gracefully curving feathery appendages. The larval stages are free-living and resemble the larvae of many other crustaceans, but the adults are attached, living in a shell of calcareous material, fixed to rocks or other hard objects. The gooseneck barnacles are attached by a leathery stalk; the rock or acorn barnacles are attached directly. The gooseneck barnacles are often oceanic, attaching themselves to ships and floating objects of all sorts. Some of the acorn barnacles grow-on the hide of whales or the shells of sea turtles.
The large crustaceans—shrimps, crabs, and lobsters—not only are most familiar but display the typical arthropod plan of body most clearly. The head and thoracic regions usually are fused and covered with a hard shell, or carapace; only the appendages indicate the division into segments. The flexible abdomen or "tail," on the other hand, is divided into segments and usually is an important aid to swimming. Crabs, however, keep the tail segments folded under the body.
The hard shell of an arthropod must be shed periodically as the animal grows. The creature gets out of the old shell through a slit that opens up usually across the back. Underneath is the new shell, much folded and wrinkled, soft and tender. The crustacean, after shedding, may spend days in seclusion, hiding from enemies until its armor has hardened.
The class Arachnoidea includes in one group the horseshoe crabs, and in another diverse one the spiders and mites, only a few of which are marine. The horseshoe, or king, crab has a peculiar distribution, being very abundant on the Atlantic coast of America, absent from Europe, and represented by three species on the Asiatic coast from India to Japan. Its larval stages closely resemble the ancient trilobites of Cambrian times and as a reminder of those past ages it is often called a living fossil. Horseshoe crabs are abundant along the shores of bays and other relatively quiet waters, where they eat clams, worms, and other small animals. They come out on beaches early in the summer to lay eggs in depressions scooped out in the sand.
Bryozoa: Moss Animals, Sea Laces
THE BRYOZOA are a group of uncertain position and relationships, including rather diverse forms. They may appear as fluffy plantlike growths often mistaken for seaweeds, especially when found dried on the shore. Another form grows as flat hard patches encrusting seaweeds or rocks and having a lacy appearance. Still another type is a branched and upright growth of gelatinous texture. All of these are colonial forms or associations of many individual polyps, all living in adjoining cells or embedded in a unifying matrix.
The encrusting Bryozoa, or sea laces, are beautiful mosaics of closely set compartments, each inhabited by a small tentacled creature that superficially resembles the hydroid polyp, but possesses a complete digestive system, a body cavity, simple nervous system, and many other features of higher animals. The individuals of a bryozoan colony are largely independent of each other, instead of being connected as the hydroids are.
The Bryozoa are an ancient group dating from the Cambrian. They were considered seaweeds by early zoologists, and later were classified as hydroids. There are about 3000 marine species, compared with only about 35 in fresh waters.
Echinodermata: Starfish, Sea Urchins, Brittle Stars, Sea Cucumbers
OF ALL the invertebrates, the echinoderms are most truly marine, for among their nearly 5000 species not one lives in fresh water or on land. They are an ancient group, dating from the Cambrian, but in all the hundreds of millions of years since then none has even attempted to make the transition to a land existence.
The earliest echinoderms were the crinoids, or sea lilies, stalked forms that lived attached to the floor of Paleozoic seas. Some 2100 fossil species of crinoids are known, in contrast to about 800 living species. Today most crinoids live in East Indian waters; a few occur in the West Indian region and come as far north as Cape Hatteras, but there are none in the shallow waters of New England.
The common echinoderms of the shore represent the four remaining classes of the phylum: the sea stars, the brittle and serpent stars, the sea urchins and sand dollars, and the holothurians, or sea cucumbers. In all members of the group there is a recurrent insistence on the number five, many of the structures occurring in fives or multiples of five, so that the figure is almost a symbol of the group.
The sea stars, or starfish, have flattened bodies, many in the conventional five-pointed shape, though the number of arms varies. The skin is roughened by hard limy plates from which short spines grow. In most species the skin also bears structures like minute forceps on flexible stalks (called pedicellaria); with these the animal keeps the skin clear of sand grains and also picks off larvae of sedentary forms that try to settle there. This is necessary because the delicate breathing organs—soft rosettes of tissue—also project through the skin.
Like all other echinoderms, the starfish possess a so-called water-vascular system that functions in locomotion and secondarily in other ways, and consists of a series of water-filled tubes running to all parts of the body. Intake of sea water is accomplished, in starfish, through a conspicuous perforated plate on the upper surface—the madreporite (mother of pores). The fluid passes along the water canals and eventually into the many short flexible tubes (tube feet) that occupy the long grooves on the under surface of the arms. Each tube bears a sucker at its tip. The tube feet can be lengthened or contracted by changes of hydrostatic pressure—when extended, the suckers grip the underlying rock or other hard surface and the animal pulls i
tself along. The tube feet are used also to grip the shells of mussels or other bivalve mollusks on which the starfish preys. As the starfish moves, any of its various arms may Brittle star go first and thus serve as temporary "head."
In the slender, graceful brittle stars and serpent stars the arms are not grooved and the tube feet are reduced. However, these animals progress rapidly by writhing motions of the arms. They are active predators and feed on a variety of small animals. Sometimes they lie in "beds" of many hundreds of animals on the sea bottom offshore—a living net through which scarcely any small creature can safely reach bottom.
In the sea urchins the tube feet are arranged in five avenues or rows running from upper to lower apexes of the body, just as the meridians on a globe run from pole to pole. The skeletal plates of the urchins are articulated rigidly to form a globular shell, or test. The only movable structures are the tube feet, which are thrust out through perforations in the test, the pedicellaria, and the spines, which are mounted on protuberances on the plates. The tube feet are retracted when the animal is out of water, but when submerged they may be extended beyond the spines to grasp the substratum or to capture prey. They may also perform some sensory functions. In the various species the spines differ greatly in length and thickness.
The mouth is on the under surface, surrounded by five white, shining teeth used to scrape vegetation off the rocks and also to assist in locomotion. (Although other invertebrates—e.g., the annelids—have biting jaws, the urchins are the first to have grinding or chewing organs.) The teeth are operated by an internally projecting apparatus of calcareous rods and muscles known to zoologists as Aristotle's lantern. On the upper surface the digestive tract opens to the exterior through a centrally placed anal pore. Around this are five petal-shaped plates, each bearing a pore that serves to discharge eggs or sperm. The reproductive organs are arranged in five clusters just under the upper or dorsal surface. They are practically the only soft parts the animal possesses and it is for these that the sea urchins are sought as human food, especially in Mediterranean countries. Gulls hunt the urchins for a similar purpose, often dropping them on the rocks to break the tests so that they can eat out the soft parts.
The eggs of the sea urchins have been used extensively in biological studies of the nature of the cell, and Jacques Loeb in 1899 used them in a historic demonstration of artificial parthenogenesis, causing an unfertilized egg to develop merely by treating it with chemicals or by mechanical stimulus.
The holothurians, or sea cucumbers, are curious echinoderms with soft, elongated bodies. They crawl on one surface with the mouth end foremost and so have secondarily substituted a functional bilateral symmetry for the radial symmetry characteristic of the phylum. Tube feet, where present, are confined to three rows on the functional under surface of the body. Some holothurians are burrowing forms, using small spicules embedded in the body surface to grasp the surrounding mud or sand and aid their progress. The shapes of these spicules vary with the species and often must be studied microscopically before correct identification can be made. The holothurians are large and abundant in tropical seas (they are the trepang, or beche-de-mer, of commerce) and in northern waters are represented by smaller species living on offshore bottoms or among intertidal rocks and seaweeds.
Mollusca: Clams, Snails, Squids, Chitons
BECAUSE OF their endlessly varied shells, often intricately made and beautifully adorned, some of the mollusks probably are better known than any other animals of the shore. As a group they possess qualities different from those of any other invertebrates, although their more primitive members and the nature of their larvae suggest that their remote ancestors may have resembled those of the flatworms. They have soft, unsegmented bodies typically protected by a hard shell. One of the most remarkable and characteristic molluscan structures is the mantle, a cloaklike tissue that encloses the body, secretes the shell, and is responsible for its complex structure and adornment.
The most familiar mollusks are the snail-like gastropods and the clamlike bivalves. The most primitive mollusks are the creeping, sluggish coat-of-mail shells, or chitons, the least known are the tusk shells, or scaphopods, and the most highly developed class the cephalopods, represented by the squids.
The shells of the gastropods are univalve or in one piece, and coiled in more or less spiral fashion. Nearly all snails are "right-handed," that is, the opening is to the right as it faces the observer. One of the exceptions is the "left-handed conch," one of the most common gastropods of Florida beaches. Occasionally a left-handed individual occurs in a normally right-handed species. Some gastropods have reduced the shell to an internal remnant, as in the sea hares, or have lost it entirely, as in the sea slugs or nudibranchs (in which, however, a coiled shell is present in the embryo).
The snails are for the most part active animals, both the vegetarians that move about scraping plant food from the rocks and the carnivores that capture and devour animal prey. The sedentary boat shells, or slipper shells, are exceptions; they attach themselves to shells or to the sea bottom and live on diatoms strained from the water, in the manner of oysters, clams, and other bivalves. Most snails glide about on a flattened muscular "foot," or they may use this same organ to burrow into the sand. When disturbed, or at low tide, they draw back into their shells, the opening being closed by a calcareous or horny plate called the operculum. The shape and structure of the operculum vary greatly in the different species and sometimes it is useful in identification. In common with other mollusks (except the bivalves) the gastropods have a remarkable, tooth-studded band, the radula, on the floor of the pharynx, or, in some species, on the end of a long proboscis. The radula is used to scrape off vegetation or to drill holes in shelled prey.
The bivalves, with few exceptions, are sedentary. Some (e.g., the oyster) fix themselves permanently to a hard surface. Mussels and some others anchor themselves by secreting silklike byssus threads. The scallops and the lima clams are examples of the few bivalves that possess the ability to swim. The razor clams have a slender pointed foot by means of which they dig deeply and with incredible speed into the sand or mud.
Bivalves that bury deeply in the substratum are able to do so because they possess a long breathing tube, or siphon, through which they draw in water and so receive oxygen and food. Although most are suspension feeders, filtering minute food organisms from the water, some, including the tellins and coquina clams, live on detritus that accumulates on the sea floor. There are no carnivorous bivalves.
The shells of gastropods and bivalves are secreted by the mantle. The basic chemical material of molluscan shells is calcium carbonate, which forms the outer layer of calcite, and the inner layer of aragonite, which is a heavier and harder substance although it has the same chemical composition. Calcium phosphate and magnesium carbonate also are contained in mollusk shells. The limy materials are laid down on an organic matrix of conchiolin, a substance chemically allied to chitin. The mantle contains pigment-forming cells as well as shell-secreting cells. The rhythm of activity of these two kinds of cells results in the marvelous sculpturing and color patterns of molluscan shells. Although shell formation is affected by many factors in the environment and in the physiology of the animal itself, the basic hereditary pattern is so strongly determined that each species of mollusk has its characteristic shell by which it may be identified.
A third class of the mollusk phylum consists of the cephalopods, so unlike the snails and clams that superficially it is hard to reconcile the relationship. Although ancient seas were dominated by shelled cephalopods, all but one (the chambered nautilus) have now lost the external shell, retaining only an inconspicuous internal remnant. One large group, the decapods, have cylindrical bodies with ten arms; they are represented by the squids, the ramshorn shell, and the cuttlefish. Another group, the octopods, have baglike bodies with eight arms; examples are the octopus and the argonaut.
The squids are strong and agile; over short distances they are probably
the swiftest animals of the sea. They swim by expelling a jet of water through the siphon, controlling the direction of motion by pointing the siphon forward or backward. Some of the smaller species swim in schools. All squids are carnivorous, preying on fish, crustaceans, and various small invertebrates. They are sought by cod, mackerel, and other large fish, and are a favorite bait. The giant squid is the largest of all invertebrates. The record specimen, taken on the Grand Banks of Newfoundland, measured about 55 feet including the arms.
Octopuses are nocturnal animals and, according to those most familiar with their habits, are timid and retiring. They live in holes or among rocks, feeding on crabs, mollusks, and small fish. Sometimes the location of an octopus den may be discovered by the pile of empty mollusk shells near the entrance.
The chitons belong to a primitive order of mollusks, the Amphineura. Most of them wear a shell consisting of eight transverse plates bounded by a tough band, or girdle. They creep sluggishly over rocks, scraping off vegetation. At rest, they settle into a depression, blending so well with their surroundings that they are easily overlooked. They are sought as food (sea beef) by West Indian natives.
The fifth class of mollusks consists of the little-known scaphopods (tooth shells or tusk shells), which form shells resembling an elephant's tusk, from one to several inches long and open at both ends. They dig into sandy bottoms, using a small, pointed foot. Some specialists think their structure may be similar to that of the ancestors of all mollusks. However, this is a field for speculation, since the principal classes of mollusks were all defined early in the Cambrian, and clues to the nature of the ancestral forms are exceedingly vague. The tooth shells number about 200 species, and are widely distributed in all seas. None, however, are intertidal.