The Abyssal regions of the sea contain many of the most interesting Crustacea. Families entirely confined to the abyss are the Eryonidae, Pylochelidae, and certain Caridean Prawns (Psalidopodidae, etc.), but there are a great number of normally littoral genera which have representatives in deep water. If we draw the limit between the littoral and abyssal regions at about 200 metres, we can characterise the latter as absolutely dark except for the presence of phosphorescent organisms, with the temperature at a little above zero, and with a comparative lack of dissolved oxygen in the water. These conditions bring about remarkable modifications in the structure and life-histories of the inhabitants of the deep sea; we have already touched on the modifications of the visual organs and on the presence of phosphorescence in many of the animals; other points to be noticed are the usually uniform yellowish or bright red coloration, the frequent delicacy of the tissues without much calcification, variations in the structure of the breathing organs, e.g. in Bathynomus giganteus and Encephaloides armstrongi, and the loss of the larval development. Owing to the similarity of conditions in the deep sea all over the globe most of its inhabitants are universally distributed. It is also a striking fact that species are found in the deep sea of the tropics whose nearest allies occur, not in the littoral seas of the tropics, but in those of the temperate region. This fact has already been alluded to in dealing with the distribution of the Lithodinae. Alcock[[163]] remarks that between 50–500 fathoms in the Indian Ocean are found Crabs such as Maia, Latreillia, and Homola, regarded as characteristic of the north temperate seas; the lobster Nephrops andamanica, taken at 150–400 fathoms, is closely allied to the Norwegian N. norwegica; and nine species of “Schizopoda,” which are certainly temperate forms, occur in the Indian Ocean at depths of 500–1750 fathoms.

B. Fresh-Water.[[164]]

If we except the Crayfishes and River-crabs, the Crustacean fauna of running water is exceedingly poor, but in all standing fresh-water, from the smallest pond to the large lakes and inland seas, Crustacea, especially Entomostraca, are abundant and characteristic, and form an important item in the food of fresh-water fishes. In small ponds a vast assemblage of Cladocera is met with; these animals multiply with great rapidity by parthenogenesis, especially during spring and summer, but on the advent of untoward conditions sexual individuals are produced, which lay fertilised winter-eggs which lie dormant until favourable conditions again arise. As Weismann first pointed out, the frequency with which sexual individuals are produced in the various species is closely correlated with the liability of the water in which they live to dry up; so that the Cladocera which inhabit small ponds usually have at least two “epidemics” of sexual individuals, one during early summer and the other before the onset of winter.

Besides Cladocera, the Phyllopoda (e.g. Apus, Artemia, etc.) inhabit small pools; and also a great number of Cyclopidae. Of the other fresh-water families of Copepoda, viz. Centropagidae and Harpacticidae, inhabitants of small pieces of water are Diaptomus castor, as opposed to the other species of Diaptomus which are pelagic, and a number of Harpacticidae (Canthocamptus), the members of this family living in the weed or mud of either small ponds or else on the shores of the larger lakes. The greater number of Ostracoda are found in similar situations.

A district like the Broads of Norfolk, which consists partly of slowly-moving streams and partly of extensive stretches of shallow water, supports a Crustacean fauna intermediate in character between that found in small ponds and the truly pelagic fauna characteristic of deep lakes. A very complete list of the Crustacea of the Norfolk Broads, with an interesting commentary on their distribution, is given by Mr. Robert Gurney.[[165]] We miss here the pelagic Cladocera, such as Leptodora, Bythotrephes, Holopedium, etc., which form so characteristic a feature of large lakes; at the same time, besides a rich development of the Cladocera, Cyclopidae, and Harpacticidae, which haunt the weeds and mud of shallow waters, we find such species as Polyphemus pediculus and Bosmina longirostris among Cladocera, which are otherwise confined to large bodies of water, and a few pelagic Diaptomus, e.g. D. gracilis. The fauna is also complicated in this district by the proximity to the sea and the frequently high salinity of the water, which allows a number of typically marine Copepods to pass up the estuaries and intermingle with typically fresh-water species; such are Eurytemora affinis among the Centropagidae, and several species of Harpacticidae (see p. [62]).

The large lakes of the world, such as the continental lakes of Europe and America, or of our own Lake District, reproduce on a small scale the varied conditions which appertain to the ocean—as in the ocean, we can recognise in these lakes a littoral, a pelagic, and an abyssal region. Our knowledge of the physiography of lakes is largely due to the classical work of Forel,[[166]] and the following account of the physical conditions in the various regions is condensed from his book.

The littoral region is sharply marked off from the others by the relative instability of its physical conditions, owing to the agitation of its waters, the affluence of streams and drainage, and the constant changes of temperature. The water in this region generally contains a good deal of solid matter in suspension, while the shelving banks of the lake support a wealth of vegetable growth, both of Algae and of Phanerogams, down to about 20–25 metres. At this depth the daylight does not penetrate sufficiently to admit of the growth of green plants, so that this region marks the limit, both physical and biological, between the littoral and the abyssal zones. In this littoral region there flourish a great quantity of Entomostraca, most of which are also found in small ponds where similar conditions of life prevail—the pelagic species only penetrating rarely, and by accident, into its waters. At the beginning of July Mr. H. O. S. Gibson and myself found that the weedy littoral region of Grasmere contained almost entirely large quantities of the Cladoceran Eurycercus lamellatus, and a number of Cyclops fuscus and C. strenuus. In the littoral zone of large lakes, Amphipods, Isopods, and fresh-water shrimps may also be met with, but this applies more to the lakes of the Tropics and of the Southern Hemisphere.

The pelagic[[167]] region is distinguished from the littoral by the greater purity and transparency of its waters, and by the relative stability of the temperature, the annual range, even at the surface, in Geneva being from 4°–20° C., while at 100 metres the water has a uniform temperature of 4° or 5° C. The upper strata are, of course, brightly illuminated, but at 20 metres there is hardly sufficient light for green plants to grow, and at 100 metres it is completely dark. The inhabitants of this region, known collectively as plankton, spend their whole life swimming freely in the water, sometimes at the surface and sometimes in the deeper strata. They consist chiefly of Diatoms, Protozoa, Rotifera, and Crustacea. The pelagic Crustacea, especially the Cladocera, are often the most curiously and delicately built creatures. Leptodora hyalina, which is quite transparent, is the largest of them, attaining to three-quarters of an inch in length, though Bythotrephes longimanus is nearly as large if we include the immense spine which terminates the body. Holopedium gibberum, which is the commonest of all in Grasmere lake, but not so frequently met with in the other English lakes, is peculiar in that its body is enveloped in a spherical mass of transparent jelly, sometimes a quarter of an inch in diameter, so that the contents of a tow-net jar full of Holopedium have something of the consistency of boiled sago. The enormous quantities in which these animals often occur during summer is very astonishing; but to be truly appreciated tow-nettings should be taken at the surface of the lake either during night-time when there is not much moonlight, or else on a dark still day when there is a quiet drizzle falling on the surface of the water. In bright sunshine the plankton passes below the surface into the lower strata, and can be usually taken by sinking the tow-net some 10–20 feet, or to even greater depths in the water. The exact reason of these periodic migrations out of the light, and their dependence on other physical conditions, such as temperature and the agitation of the water, is not clearly understood. It appears, however, that when the water is rough, plankton always passes into the deeper regions. Besides the species mentioned, the minute Bosminidae, whose trunked heads are suggestive under the microscope of elephants, and Polyphemus pediculus are among the commonest pelagic Cladocera, though neither Polyphemus nor Bythotrephes ever form shoals. The above-mentioned genera are characteristic of the larger lakes in the Northern Hemisphere. Our knowledge of the Crustacean plankton of tropical lakes and of those of the Southern Hemisphere is limited (but see p. [216]).

A very important constituent of lake-plankton is furnished by the Copepoda, especially of the genus Diaptomus. With the exception of Holopedium, by far the commonest Crustacean in Grasmere during July was found by Mr. Gibson and myself to be D. caeruleus.

At the same time a number of Cyclopidae, e.g. Cyclops strenuus, may occur in the pelagic region in considerable quantities, though they were never found by us in such numbers as Diaptomus.