epibenthos of warm seas appears to be especially wealthy in such forms as secrete heavy calcareous skeletons; but in colder water, among the epibenthos of polar or sub-polar regions, and the hypobenthos everywhere in open oceans, the predominant forms are those which exhibit little or no calcareous secretion: even the apparent exceptions, Madreporaria and Echinoderma from great depths, tend to develop slighter skeletons than their warm-water congeners. The following table will serve to illustrate this point, and to give an idea of the composition of the epibenthos of cold and warm seas and of the hypobenthos: the figures are the percentages of total species captured in each locality by H.M.S. “Challenger,” the balance being made up by few specimens in scattered groups:—
| Kerguelen Area—over 1260 fm. |
Kerguelen Area—0 to 150 fm. |
Cape York— 0 to 12 fm. | |||||||||||||||||||||||||||||
| Madreporaria | 0.8 | 0.0 |
| ||||||||||||||||||||||||||||
| Alcyonaria | 1.2 | 1.0 | |||||||||||||||||||||||||||||
| Shelled Mollusca | 8.0 | 19.7 | |||||||||||||||||||||||||||||
| Decapoda | 3.6 | 0.8 | |||||||||||||||||||||||||||||
| Echinodermata | 33.6 | 11.7 | |||||||||||||||||||||||||||||
|
6.8 | 4.6 | |||||||||||||||||||||||||||||
| Annelida | 6.8 | 8.0 | |||||||||||||||||||||||||||||
|
16.5 | 25.0 | |||||||||||||||||||||||||||||
| Tunicata | 4.4 | 6.8 | |||||||||||||||||||||||||||||
| 81.7 | 77.6 | ||||||||||||||||||||||||||||||
While the Madreporaria represent only 3.3% of the species at the tropical station, it must be remembered that they probably made up 80% or more of the weight.
The epiplankton is dependent either directly or proximately upon light, warmth and the presence of plant life. The wealth Epiplankton. of minute organisms near the surface is inconceivable to those who have not seen the working of a two-net: it may be gauged by the fact that a single species is sometimes present in such quantities as to colour the sea over an appreciable area, and by the estimate that the skeletons of epiplankton from a square mile of tropical ocean a hundred fathoms deep would yield 16 tons of lime. In the tropics the wealth of species, and towards the poles the number of individuals of comparatively few species, are characteristic of the latitudes. In temperate and tropical regions there is a great difference between the epiplankton near land and that far out at sea: the former is termed neritic; it extends, roughly speaking, at least as far out as the mud-line, and is characterized by the predominance of what may be termed hemibenthic forms, that is, benthic forms with a planktonic larval stage (Decapoda, Polychaeta, &c.), or with a planktonic phase (metagenetic Medusae). The horizontal barriers to the neritic plankton are practically those mentioned as governing the epibenthos; indeed, it would seem that the distribution of hemibenthic adults is determined by that of their more delicate larvae. Special conditions of wind and current may of course carry into the neritic zone forms which are characteristic of the open sea, and vice versa. In the neritic epiplankton of polar waters the larvae of hemibenthic forms are almost absent; indeed, the development of cold-water benthos, whether shallow or abyssal, appears to be in most cases direct, this is, without a larval metamorphosis. The epiplankton of the open sea is described as oceanic; it consists almost entirely of holoplanktonic forms and their larvae. The chief barrier to horizontal distribution, here as elsewhere, is doubtless temperature. For example, through the reports of the “National” cruise (German Plankton Expedition) runs the same story; one fauna characterized their course from Shetland to Greenland and Newfoundland, another the traverse of the Gulf Stream, Sargasso Sea and the Equatorial Currents. The influence of temperature may be gauged in another way: where hot and cold currents meet, occur “frontier” districts, in which the respective organisms are intermingled, and can only exist till their maxima or minima are reached. Well-marked examples of such districts occur off New Jersey (Gulf Stream and Labrador Current), in the China Sea (warm currents of the south-west monsoon and Kamchatka Current), in the Faeroe Channel, south of the Cape (recurving of the Agulhas Current): in some of these the range of variation amounts to as much as 50° F. in the year, with the result of a colossal death-rate of the plankton, and its corollary, a rich bottom fauna, for which food is thus amply supplied. The majority of the oceanic epiplankton appears to be stenothermal; for example, few components of the well-characterized fauna of the Gulf Stream and Sargasso Sea ever reach the British shores alive, although, if current and salinity were the determining factors and not temperature, this fauna should reach to Shetland, and even to Lofoten. It will only be possible to make satisfactory distributional areas for these oceanic forms by such systematic traverses as that of the “National”; at present it would seem that adjacent species have such different maxima and minima that every species must be mapped separately (compare the distribution-maps of the “National” Plankton Expedition). Some members of the epiplankton are, however, extraordinarily eurythermal and eurybathic; for example, Calanus finmarchicus ranges from 76° N. to 52° S. (excepting perhaps for 10° each side of the equator), and is apparently indifferent to depth.
In the first hundred fathoms at sea the fall of temperature is gradual and slight, and forms practically no hindrance to the diurnal oscillation of the oceanic epiplankton—the alleged rise and fall of almost the entire fauna. Roughly speaking, the greatest number of animals is nearest the surface at midnight; but different species sink and rise at different times, and to or from different depths. Apart from this diurnal oscillation, unfavourable conditions at the surface send or keep the fauna down in a remarkable way: for example, in the Bay of Biscay few organisms are to be found in the first fathom in bright sunlight, but on a still, hot day the next few fathoms teem with life; yet after a few minutes' wind or rain these upper layers will be found almost deserted. This leads to the consideration of the hydrostatics of the plankton: apart fro1n strong swimmers, the majority contests the tendency to sink either by some means of diminishing specific gravity (increasing floating power) or by increased frictional resistance. The former is generally attained (a) by increase of bulk through development of a fluid secretion of low specific gravity (vacuoles of Foraminifera, Radiolaria, &c.); (b) or of a gelatinous secretion of low specific gravity (Medusae, Chaetopod and Echinoderm larvae, Chaetognatha, Thaliacea: the characteristic transparency of so many oceanic forms is probably attributable to this); (c) by secretion or retention of air or other gas (Physalia, Minyas, Evadne); (d) by development of oil globules (Copepoda, Cladocera, fish ova). Increased frictional resistance is obtained by flattening out of the body (Phyllosoma, Sapphirina), or by its expansion into lateral processes (Tomopteris, Glaucus), or by the development of long delicate spines or hairs (pelagic Foraminifera, many Radiolaria, many Chaetopod and Decapod larvae). In many cases two or more of these are combined in the same organism. Notwithstanding the above adaptations, some of which are adjustable, it is difficult to understand the mechanics of the comparatively rapid oscillations of the epiplankton, of which both causes and methods are still obscure.
It will be seen from the distribution of the Thecosomatous Pteropoda—a purely oceanic group—how difficult it will prove to draw distributional areas for classes of epiplankton. P. Pelseneer recognizes in all ten such provinces (“Challenger” Reports: “Zool.,” xix., xxiii.) and 42 good species: of the latter 1 is confined to the Arctic, 4 to the Antarctic province, but of the remaining 37 species and eight provinces 30% occur in all eight, 16% in seven, and only 35% have as yet been captured in a single province only.
The mesoplankton has only received serious attention during the last few years. In the “Challenger,” open nets towed at Mesoplankton. various depths seemed to show the existence of a deep-water plankton, but this method gives no certain information as to the horizon of capture, the nets being open in their passage down and up. C. Chun
