distinguished as the epibenthos. As with the shallowest or tidal zone, its nature varies much more according to latitude and the character of the coast than the deeper zones. Everywhere, however, the epibenthic fauna is exposed to certain definite environmental conditions, as compared with a deeper fauna: namely, a high or fairly high temperature (except near the poles); a fairly good light, with its important consequence, a vegetable basis of food supply; tide and current to distribute the larvae to a suitable habitat, which the varied nature of the bottom near land is likely to furnish. Passing farther seawards, we find a steeper slope to about the 500-fathom line, the so-called continental slope. In this zone the environment is absolutely different. The water, no longer subject to seasonal variations of temperature, or to direct sunlight, is cold, and of a nearly uniform annual temperature (300 fathoms, ± 44.7° F.). Light has disappeared from all but the shallower part, and with it plant life, tide and current are no longer felt. To the latter fact is due, however, a great part of the food supply, which maintains in this zone an abundant fauna: a great quantity of organic matter, brought down by river action, produced by disintegrated sea-weed, and due to the death of surface organisms, together with the finer clayey materials of land waste, settles to the bottom in quiet water, near the 100-fathom contour, thus making the mud-line the richest feeding-ground in the ocean (Murray). The mud-line is the real upper limit of this zone; it typically begins at about 100 fathoms, but may begin at 5 to 20 fathoms in deep sheltered firths, or be pushed down to 300 fathoms where currents are strong. The fauna of this zone may be termed the mesobenthos; it is not so abundant, nor so sharply characterized, as the epibenthos, and yet is sufficiently distinct to deserve at any rate a provisional name. Another difference of condition between epibenthos and mesobenthos is the pressure of the water; at a depth of 500 fathoms this is, roughly speaking, half a ton to the square inch. It is very doubtful whether this enormous pressure makes the slightest difference to marine invertebrates, the tissues of which are uniformly permeated by fluids, so that the pressure is uniform in every direction; but animals with free gases naturally require time to adjust the gas-pressure when altering their levels. As regards the penetration of light, assimilative rays useful to plant life probably do not reach beyond 150 fathoms. Photographic rays have been detected as low as 220 fathoms, and if any light penetrate beyond this depth, it will consist only of blue, violet and ultra-violet rays: it has been suggested that the red colour prevalent in many deep-sea animals may be a screen from these hurtful rays. Below the 500-fathom line the ocean bottom exhibits almost uniform conditions everywhere, varied only by the character of the bottom deposit and the amount of food supply. In this zone, which extends from about 500 fathoms to the greatest depths (which may in some cases exceed 5000 fathoms, or more than 5½ m.), the temperature at any given point is uniform throughout the year, and is always very low: the mean at 2200 fathoms is 35.2° F.; at greater depths and in special circumstances less than 30° F. has been recorded. The darkness is probably absolute; for food the animals are dependent upon each other and upon the incessant rain of dead plankton from higher levels; the pressure may be anything between half a ton and five tons per square inch. To the fauna which lives in these remarkable circumstances the name hypobenthos may be applied.
|
|
Fig. 2.—Mean Annual Surface Isotherms of the Atlantic. (After Buchan, “Challenger” Report on “Oceanic Circulation”). On the north-east and south-west sides they are deflected polewards by the warm North Atlantic Drift and Brazil Current; on the south-east and north-west sides equatorwards by the cold Labrador and Benguela Currents. Note the markedly different latitudes of the same isotherms east and west of South America and Africa; also the effect of the Falkland Current against the Brazil Current. |
That each of the three benthic groups is well characterized by a special fauna is shown by the following table, out of the total numbers of species captured by the “Challenger” at seventy stations in these three zones.—
| Species confined to this Zone. |
Species occurring in other Zones. | |
| Epibenthos | 91% | 8% |
| Mesobenthos | 74% | 25% |
| Hypobenthos | 61% | 38% |
Out of the 25% of its species which the mesobenthos shares with other zones, 59% occur also in the epibenthos, about 40% in the hypobenthos; the mesobenthos, therefore, on these figures, may be taken to consist of 74% of peculiar species, 15% shared with the epibenthos, 10% with the hypobenthos. Speaking of the benthos as a whole, it may be said that the following statement holds good: The number of individuals, the proportion of species to genera, and the number of individuals of a given species, all decrease with increasing depth. Animal life also tends to diminish with increasing distance from land; this may be partly due to the greater food supply near land, partly to the fact that population is obviously thinnest on the advancing fringe of a migration.
The plankton can be subdivided into at least two groups. The fauna to which light and warmth are more or less necessary, which feeds either upon plants or upon organisms nearly dependent upon plant life, may be termed the epiplankton. This fauna is capable of a good deal of vertical movement upwards and downwards, the causes of which are still obscure, but most of its members seem rarely to descend lower than about 100 fathoms. Below this depth the fauna may be called the mesoplankton. In every area this appears to have its peculiar species, but the careful study by opening and closing tow-nets of the distribution of the mesoplankton is of so recent a growth that no statistics, such as we have of the benthos, are available. It is now generally admitted that the mesoplankton extends to the lowest depths yet searched (2730 to 2402 fathoms, Valdivia); but the number of specimens decreases rapidly after 200 fathoms, and below 1000 fathoms very little is captured. The conditions of light, temperature, pressure, &c., are practically those of the corresponding depths of the benthos; as regards the food, however, the mesoplankton can only depend on intercepting dead organisms which are falling from higher horizons, or on capturing the scanty prey of its own zone. It is possible that the plankton immediately over the bottom may prove to be sufficiently distinct to be separately classed as hypoplankton.
The main subdivisions of the marine fauna having thus been briefly sketched, it is advisable to consider them in somewhat Epibenthos. more detail. The epibenthos is obviously that fauna to which, except in polar regions, light and warmth are necessary; and the absence of these at greater depths is
