CHAPTER VII

FLOATING CRUSTACEA OF THE OPEN SEA

It is only rarely that the floating organisms of the surface of the sea are so large or so abundant as to catch the attention of the casual observer. Except for an occasional shoal of porpoises or of flying-fish, the waste of waters seen from the deck of a ship in mid-ocean usually seems to be barren of life. Nevertheless, there is probably no region of the ocean where the tow-net will not reveal the existence of a more or less varied fauna and flora. Sometimes, indeed, these organisms, though minute, are so numerous as to discolour the water over large areas; whalers in the Arctic seas know by the appearance of "whale-food" where whales are likely to be found, and Herring or Mackerel fishermen recognize the changes in colour of the water among the "signs" which guide them when and where to shoot their nets.

The organisms which make up this "pelagic" fauna and flora may be grouped into two classes, which may be termed the "swimmers," or Necton, and the "drifters," or Plankton. The former include the larger and more active animals, such as fish, whales, and the like, whose movements are more or less independent of the movements of the water; the latter comprise the plant-life and the floating or feebly swimming animals that drift at the mercy of waves and currents. A great deal of attention has been given in recent years to the study of the plankton, and it has come to be recognized as filling a very important place in the balance of life in the sea. In the sea, as on land, all the animals are ultimately dependent on plants for their food. The larger and more conspicuous sea-weeds which grow on the sea-bottom, however, can only flourish in comparatively shallow water, and the region which they occupy forms only a narrow fringe round the land-masses of the globe. It is only necessary to look at a map of the world, showing the depth of the sea, to realize what an insignificant part of the area of the oceans contributes in this way to the food-supply of marine animals. The microscopic plant-life of the plankton, however, makes up for the individual minuteness of its constituents by their incalculable numbers. The lowly organisms known as "diatoms," familiar to the microscopist from the beauty of their flinty skeletons, are among the most numerous and important of these, and they are associated with a great variety of other single-celled algæ and allied organisms, some of them so minute that they pass through the finest silk plankton-nets, and have to be sought for by special methods of collection recently devised for the purpose. All these organisms possess the green colouring matter (chlorophyll) that enables them to live, as the higher plants do, on the carbon dioxide and other substances dissolved in the water. The smaller animals of the plankton feed on these vegetable organisms, and in their turn serve as food for larger animals. The Herring, the Mackerel, the gigantic Basking Shark, and the still more gigantic Greenland Whale, all feed directly on the animal plankton, and we have already seen that the animals of the deep sea depend entirely on the same source of food-supply. Further, very many of the bottom-living animals of shallow water swim at the surface in the early stages of their life, and feed on the other plankton animals and plants. Indeed, it is no exaggeration to say that "all fish is diatom" in the same physiological sense as "all flesh is grass," and the study of the plankton is thus of practical importance as well as of scientific interest.

Of all the minute animals that form the intermediate links in the chain between diatom and fish or whale, the Crustacea are the most important and the most numerous both in species and in individuals. The Copepoda are more richly represented than any of the other groups, and it would be difficult to find a sample of marine plankton from which they were altogether absent. Associated with them we find one or two species of Cladocera, a larger number of Ostracoda (chiefly of the family Halocypridæ), a few Mysidacea, the Amphipoda of the suborder Hyperiidea, the Euphausiacea, and some of the shrimp-like Decapods; while the larval stages of these and other groups also form an important part of the plankton.

It is necessary to make a distinction between the "neritic" plankton of shallow water near the coast and the "oceanic" plankton of the open sea. In the inshore waters the plankton consists not only of organisms that pass the whole of their life at or near the surface, but also, and very largely, of the free-swimming larvæ of bottom-living species, and of others that make occasional and temporary excursions to the surface. For example, if the tow-net be used a short distance from land--say in some sheltered bay on our own coasts--the catch will often be found to consist largely of larval Crustacea. The zoëa and megalopa stages of Crabs, the zoëa and schizopod stages of Prawns and Shrimps, are often conspicuous by their numbers, or we may find swarms of the nauplius and cypris larvæ of Barnacles. Sometimes, and especially at night, numbers of Cumacea may be found in the tow-net; and it is noteworthy that these are usually males, which leave the females burrowing in the mud at the bottom, and swarm to the surface for a brief period of activity. Besides all these more or less temporary visitors, however, there are numerous species, even in the inshore waters, which are adapted to a floating life, and pass their whole existence as members of the plankton. Copepoda of many kinds, some Mysidæ, Amphipods like _Hyperia_--which is commonly found sheltering under large jellyfish--some species of actively swimming Isopods, and many other forms, are only to be captured by the tow-net; and now and then, in certain localities, winds and currents may drive into coastal waters shoals of species whose proper home is the open ocean.

In a similar way the strictly neritic forms may sometimes be carried far out to sea, so that it is nowhere possible to draw a hard-and-fast line between the regions occupied by the neritic and the oceanic plankton. With increasing distance from land, however, the larval stages of bottom-living species become fewer, and finally disappear altogether, and there is left an assemblage of animals whose whole existence is passed floating at the surface or at the intermediate depths. How far down from the surface this floating fauna actually descends is a question which has been much debated. It appears now to be certain that there is no stratum of water between the surface and the bottom of the ocean which is devoid of life, although the upper layers (not at, but some distance below, the surface) are probably much more densely populated than those of the abyss. Many of the species appear to undertake more or less extensive migrations in a vertical direction, coming nearer the surface at certain stages of their life-history, and sinking into deeper water at others. Further, some species at least seem to rise to the surface at night, and to sink again during the day. Apart from these vertical movements, which are as yet only imperfectly understood, it is desirable to distinguish between the "epiplankton," comprising the organisms which inhabit the superficial strata of the ocean down to about 100 fathoms, and the "mesoplankton," found at greater depths. The plant-life which is dependent on sunlight belongs to the epiplankton, while the animals of the mesoplankton are dependent, like the bottom animals of the deep sea, on the supply of dead food material falling from above. A third division, the "hypoplankton," has been established for those animals which live immediately above the bottom, but its distinctness from the mesoplankton has not yet been satisfactorily established. Indeed, many of the swimming forms which have already been mentioned in dealing with the Crustacea of the deep sea are probably rather to be considered as belonging to the deep mesoplankton--at least, where their size and swimming powers do not entitle them to be ranked with the "necton."

Many of the modifications in structure characteristic of pelagic animals may be traced to the necessity for keeping continuously afloat with a minimum of exertion. The Crustacea of the plankton never carry the heavy armour found in bottom-living species. Thus, the thick-shelled Ostracoda of the bottom are represented in the plankton chiefly by the family Halocypridæ (Fig. 48), in which the shell is thin, uncalcified, and almost membranous. Many species, particularly of the Copepoda, are seen, under the microscope, to have large globules of oil distributed through the tissues of the body, and these no doubt serve as floats, increasing the buoyancy of the animal. The same purpose is probably served, in many cases, by having large spaces, filled with fluid, within the body. This is characteristic of pelagic animals, and is well seen in many of the Crustacea in which the viscera and muscles occupy a relatively small part of the interior of the animals, the intervening spaces being filled with colourless transparent fluid. Many of the Hyperid Amphipoda show this peculiarity--for example, the relatively gigantic _Cystisoma_, which is mesoplanktonic in deep water; and it reaches its extreme in _Mimonectes_ (Fig. 49), in which the anterior part of the body is, as it were, blown out into a balloon, giving the animal the aspect of a small jellyfish rather than an Amphipod.

If, as seems probable, the body-fluid of these animals is of a lower specific gravity than the sea-water, it will act like the oil-globules of the Copepoda in keeping the animals afloat. Even if the specific gravity be the same, however, the distension of the body with fluid acts in another way, by increasing the surface exposed to friction with the surrounding water, and so retarding sinking. The principle involved is illustrated by the fact that a soap-bubble sinks much more slowly through the air than the drop of water into which it collapses. The same result is produced if the surface is increased by outstanding spines or hairs, just as, for instance, a downy feather sinks slowly through the air, but drops rapidly if it is rolled into a ball between the fingers. This is, no doubt, one function of the spines with which plankton Crustacea, and particularly larvæ, are frequently provided, though they may also serve in some cases as a protection against enemies. The spines have been already alluded to in describing the various larvæ, but it may be noted here that they are most strongly developed in larvæ which live in the open ocean; for example, the most elaborately armed of all Decapod larvæ are the zoëa stages of _Sergestes_ (Fig. 50), which, like the adults, belong to the oceanic plankton. The nauplius larvæ of Cirripedes are all more or less spiny, and the spines reach an exaggerated development in the larvæ of the genus _Lepas_ (Fig. 51), of which the adults are attached to floating drift-wood or the like, and belong to the oceanic fauna, although hardly to be classed with the plankton.

The large feathered bristles that decorate the limbs or tail of many plankton Copepoda have no doubt the same function in assisting flotation. In the genus _Calocalanus_ (Fig. 52), for example, the tail setæ are large and brilliantly coloured feathery plumes, and in one species, _C. plumulosus_, one of these setæ is of relatively enormous size, five or six times as long as the body of the animal itself.

Among the most singular of plankton Crustacea are the _Phyllosoma_ larvæ (see Fig. 28, p. 72) of the Spiny Lobsters and their allies (Scyllaridea), which have been already described. These larvæ are sometimes found far out at sea, and it seems likely that their larval life is unusually prolonged, and that they may be drifted to great distances by ocean currents. At all events, they are well adapted for pelagic life, since the broad flat body, hardly thicker than a sheet of paper, can be sustained in the water like a "hydroplane" by comparatively slight efforts of the swimming legs.

The watery character of the body, together with the thinness of the exoskeleton, helps to explain the glassy transparency which is a feature of most plankton Crustacea. This transparency has been regarded as a protective adaptation rendering the animals inconspicuous in the water, and it has indeed that effect to human eyes, but it is very doubtful whether the animals derive much benefit from this. Many of the animals--such as Herring and other pelagic fishes--that prey upon plankton Crustacea appear to swallow them in bulk, without much selection; and the Greenland Whale, as it swims open-mouthed through the sea, is not likely to be guided by the greater or less visibility of the Copepods that it sifts out on its baleen plates. Further, this glass-like transparency is by no means universal, for many plankton Copepoda are brightly coloured. In some, as in the beautiful blue _Anomalocera_, common in British waters, the colour is due to pigment in the fluids and tissues of the body; in others the feathery hairs on the body and limbs show brilliant metallic colours, produced, like the colours of a peacock's feather, not by pigments, but by the diffraction of light in the texture of the organ. The most beautiful of all Copepoda is _Sapphirina_, in which the surface of the body absolutely sparkles with iridescent colours.

The striking phenomenon known as the "phosphorescence of the sea" is familiar to every ocean voyager, and is seen from time to time on our own coast. On a dark night the crest of every wave often seems to break in a pale glow, the wake of the vessel is a trail of light, and an oar dipped in the water seems on fire. This luminosity is due to the animals of the plankton, largely to the lowly Protozoa and the jellyfishes, but in part also to certain Crustacea. A number of pelagic Copepoda have been shown by Giesbrecht to secrete, from special glands on the surface of the body, a substance which becomes luminous on coming in contact with the water. Even specimens which had been dried were found to give out light on being wetted. Some pelagic Ostracods of the family Halocypridæ have been observed to emit clouds of a luminous secretion from a gland in the neighbourhood of the mouth. A similar habit has been seen, as already mentioned, in certain deep-sea Prawns and Mysidacea, which may perhaps belong to the deeper part of the mesoplankton rather than to the bottom fauna. The complex light-producing organs of the Euphausiacea have already been described in dealing with deep-sea Crustacea. A great many species of this group, however, are members of the epiplankton, and in these the phosphorescent apparatus is quite as fully developed as in species coming from greater depths. _Meganyctiphanes norvegica_ (Fig. 24, p. 56), which is one of the largest of the Euphausiacea, is common at no great depths in many places in British seas. If a jar of sea-water in which specimens of this species are swimming be brought into a dark room, a tap on the glass will cause the photophores to flash out like a row of tiny lamps along the side of the body. After shining for a few seconds the light dies out, to appear again if the tapping be repeated.

There are certain peculiarities in the structure of the eyes in some plankton Crustacea which suggest that the sense of sight is of special importance to their possessors, although we can hardly do more than guess at their special significance. Most Copepoda have only a single eye in the middle of the head, corresponding to the single eye of the nauplius larva, and of far simpler structure than the paired compound eyes of most other Crustacea. In many plankton species, however, this simple eye becomes much enlarged and complicated in various ways. The three parts of which it is normally made up may become separated from each other, and are sometimes increased in number to five, while lenses serving to concentrate the light are often developed by thickening of the overlying cuticle. The most elaborately constructed eyes are found in the family Corycæidæ. In _Copilia_ (Fig. 53) a pair of eyes of relatively enormous size are present. Each has in front a large biconvex lens set at the end of a conical tube which extends backwards to a smaller lens (like a telescope with object-glass and eyepiece), behind which, again, are the sensory cells, corresponding to the retina, enclosed in a tube of dark pigment, the whole apparatus being more than half the length of the body. These eyes, although paired, do not correspond to the paired compound eyes of other Crustacea, but have arisen by the separation and enlargement of two of the three divisions of the typical median Copepod eye.

A peculiarity of the paired compound eyes found in plankton Crustacea of several different orders consists in the division of each eye into two parts, which differ in structure. In many Euphausiacea and Mysidacea, especially in those haunting the deeper strata (mesoplankton), this division of the eyes is well marked, a frontal or dorsal part having the separate elements of the eye (ommatidia) greatly lengthened and with reduced pigment, while the lateral part is of more normal structure. It seems probable, from the researches of Professor Chun, that the fronto-dorsal division is adapted for the perception of very faint light, while the lateral division will give a more accurate image of brightly illuminated objects.

In the pelagic Amphipoda, forming the suborder Hyperiidea, the eyes are of very large size, generally occupying almost the whole surface of the head, and giving the animals a very characteristic appearance, in contrast to the small-eyed, bottom-living Gammaridea. In the family Phronimidæ (Fig. 54) the eyes are each divided into two parts, differing in structure in the way just described.

There are a few Crustacea living habitually on the high seas which cannot be reckoned as belonging either to the true plankton or to the necton, since they depend on outside help for keeping themselves afloat. Among these are the Barnacles which cluster on logs of drift-wood, and are among the most important causes of the "fouling" of ships' hulls on long voyages. The stalked Barnacles of the genus _Lepas_ are especially common in such situations, and the characters of their larvæ have been already alluded to. Certain species of sessile Barnacles are constantly found attached to large marine animals. For example, _Chelonobia_ adheres to the shell of Turtles, while _Coronula_ and some allied genera are found on Whales.

The little "Gulf-weed Crab" (_Planes minutus_--Plate XIX.) is found clinging to floating drift-weed nearly everywhere throughout the temperate and tropical seas of the globe, and is especially common in the area known as the Sargasso Sea, in mid-Atlantic. It is occasionally drifted to the south coasts of the British Islands. In Sloane's "Natural History of Jamaica," published in 1707-1725, it is stated of the Gulf-weed Crab that "Columbus, finding this alive on the Sargasso floating in the sea, conceived himself not far from some land, on the first voyage he made on the discovery of the West Indies."

A few other Crustacea also form part of the peculiar fauna which is associated with the Sargasso weed, notably a swimming Crab, _Neptunus sayi_, and two or three species of Prawns. All of these are coloured olive-green, like the weed among which they live.