1911 Encyclopædia Britannica/Plankton
PLANKTON, a name invented by Professor Victor Hensen for the drifting population of the sea. This is a convenient heading under which to discuss not only plankton proper, but the benthos, or crawling population of the sea-bottom. Scientific investigation of these subjects dates from the reports of the “Challenger” expedition, which, despite its many successors, still stands out as the most important of the oceanographic expeditions, alike by the work achieved, the distance traversed, the time occupied, and the money devoted to the publication of the results. It laid the foundation of our knowledge of the physics and chemistry of ocean water, of oceanic and atmospheric currents, of the contour of the sea-bottom, and of the main features of distribution of deep-sea life. Later work has confirmed and expanded, but not revoked, the conclusions thus attained. But, in spite of this and of several subsequent expeditions, it cannot be pretended that we are in a position to formulate general canons of marine distribution other than of the most tentative character. Two fallacies underlie many attempts to define distributional oceanic areas for special groups: the one, that such areas can be made to bear some relation to existing geographical or even national divisions; the other, that what is true for one group of the animal kingdom must hold good equally for another. It is necessary at the outset to divest oneself of these errors; oceanic conditions depend only very indirectly upon the distribution of the land, and strongly swimming or freely floating animals are not to be confined by the same factors as determine the distribution of sessile forms, whose range is governed by a variety of circumstances.
As Wyville Thomson pointed out long ago, there is but one ocean. This surrounds the southern half of the globe, and has two large gulfs, generally called the Atlantic and Pacific Oceans, which meet through narrow channels in the small Arctic Ocean, and a half gulf, the Indian Ocean. The Atlantic and Pacific exhibit a striking homology of atmospheric pressure and of prevalent wind and current; the Indian, to a great extent, resembles the southern half of a larger one, but this resemblance is modified by the neighbourhood of vast land masses. The prevalent winds, dependent on the fairly constant distribution of atmospheric pressure over the great oceans, are the most important determinant of currents. As at most points in the ocean the temperature, salinity and chemical composition of the water are mainly determined by the currents—that is, by the condition at the place whence the water came—it is obvious that a study of currents must precede any general view of the distribution of marine forms.
Regard must be had not merely to the superficial currents indicated in fig. 1, but also to the movements of the deeper layer. Ice melting at the poles, together with polar precipitation of hail, snow and rain, yields large quantities of water of low salinity and very low temperature; this water sinks under the warmer salter surface water drifted from lower latitudes, and, creeping slowly north and south from the poles, covers the bottom of all the great open oceans at very uniformly low temperatures (in some cases as low as 30° F.). Between surface and bottom the temperature gradually decreases (except where affected by local circumstances), and in the middle layers the existence of slow currents is suspected. The cold bottom water wells up to the surface in certain areas, replacing the surface water drained away by currents, notably to the westward of the great land masses. Ocean water is remarkably uniform as regards its contained salts and gases, and it does not seem likely that we can look to these to explain the facts of distribution. In its temperature, on the contrary, there is enormous variation. While the bottom water of the ocean is very cold, and the midwater of a more or less intermediate temperature, the surface water, according as it has drifted from the equator polewards or in the reverse direction, has a mean annual temperature somewhere between 84° and 30° F., losing or gaining heat on its way. In the case of narrow or “closed” seas, and near land masses, sea-water does not exhibit that uniformity of composition which characterizes the open ocean; but even in such cases the temperature is largely influenced by adjacent currents, and, though less obviously than in the open ocean, seems to be a very important agent in distribution.
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Fig. 1.—Diagram of the Atlantic Ocean, showing the Main Surface Currents (some are seasonal only): the corresponding Indian and Pacific currents are cited in parentheses; they are rarely so strongly marked as in the Atlantic. | |
| 1. | Counterequatorial (also 1′ Pacific and Indian). |
| 2. | North Equatorial (also Pacific). |
| 2′ | The Equatorial (also 2″ Pacific and Indian). |
| 3. | Gulf Stream proper (Japan Stream). |
| 3′. | Brazil Current (Australian Current). |
| 3″. | Mozambique Current (recurved off Cape Agulhas). |
| 4. | Labrador Current (Kamchatka Current). |
| 4′. | Falkland Current. |
| 5. | North Atlantic Drift, generally called Gulf Stream (North Pacific Drift). |
| 5′. | South Atlantic Drift, ill defined (South Pacific Drift). |
| 6. | North African Current (Mexico Current). |
| 6′. | Benguela Current. |
| 6″. | Peru Current. |
| 7. | Antarctic Circumpolar Drift. 7′, its northerly branches on the west sides of Africa and South America. |
The fauna of the sea is divisible into the plankton, the swimming or drifting fauna which never rests on the bottom (generally taken now to include E. Haeckel's nekton, the strong swimmers, such as fish and cephalopods), and the benthos, which is fixed to or crawls upon the bottom. These groups require a further subdivision according to depth—the more necessarily since, to some zoologists, any water over 100 fathoms is “deep” or even “abyssal.” It is simplest to begin with the benthos. From the shore seawards we may distinguish several zones. Even the tidal zone, between high and low water-mark, is sub divisible by its fauna and flora. There generally follows on this a very gentle slope to the depth of about 100 fathoms, locally subdivisible into many lesser zones. It has been termed the continental shelf or littoral zone, not very appropriately, since it occurs round many oceanic islands, and even away from any land. In this zone, if near land, fall to the bottom the heavy materials produced by land Waste and river drainage. The fauna of this zone, generally very well characterized, may be 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.
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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 probably the chief barrier to its vertical extension; the food supply is sufficiently plentiful in, at any rate, the upper parts of the mesobenthic zone to present no obvious barrier. The chemical constitution of the water (except to animals in brackish water near river mouths) and the pressure appear to exert little or no influence; and only those species which attach themselves to clean hard substances would be repelled by the mud-line. The chief barrier to a horizontal extension of the epibenthos is undoubtedly temperature. As an example of its distribution may be taken the Gastropod and Lamellibranch Molluscs, as groups of which the distribution has been studied for many years by specialists. The shallow-water species fall into provinces (compare Cooke, Camb. Nat. Hist. vol. “Molluscs,” ch. xii), and a comparison of figs. 1 and 3 shows at once the profound influence upon them of the great currents. Taking the Atlantic Ocean, we find the Arctic species, tempted southwards by the cold Labrador Current, repelled northwards by the warm North Atlantic Drift. The Boreal or sub-Arctic species, many of which are identical on both sides of the ocean (2 and 2′, fig. 3), lie much farther southwards on the west than on the east side, from the same causes. The warm-water molluscs of West Africa (5) are cut off from those of the east side (7) by the cold water from the great easterly Antarctic Drift, which impinges on the Cape, giving it a special fauna (6). On the South American coasts the tropical and temperate fauna reach respectively to 28° S. and 45° S on the east coast, owing to the warm Brazil Current, but the corresponding groups on the west coast only to 5° S. and 37° S., being kept back by cold upwelling and Humboldt's Current. This influence is visible in individual species as well as in the facies of a fauna: Purpura lapillus, a temperate form, reaches on the east side of the Pacific to 24° N. and on the East Atlantic to 32° N.; but on the West Pacific only to 41° N and the West Atlantic to 42° N., being repelled by the Japan stream (and other warm currents of the south-west monsoon) and Gulf Stream respectively.
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Fig. 3.—Diagram showing the Coastwise (not seaward) Extension of the Provinces of Epibenthic Gastropods and Lamellibranchs Provinces.— | |||||
| 1. Arctic. | 7. Indo-Pacific. | 14. Peruvian | 16. Argentinian. | ||
| 2. Boreal of East Atlantic. | 8. Japanese. |
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17. Caribbean. | ||
| 2′. Boreal of West Atlantic. | 9. Australian. | 18. Transatlantic. | |||
| 3. Celtic. | 10. New Zealand. | ||||
| 4. Lusitanian. | 11. Aleutian. | ||||
| 5. West African. | 12. Californian. | ||||
| 6. South African. | 13. Panama. | ||||
But while some species may be confined to a bay, others to a province, others to an ocean, there are cosmopolitan species which either vertical or horizontal barriers, or both, fail to restrain. In relation to temperature the wide-ranging species are termed eurythermal, the limited, stenothermal (Moebius); the terms are useful to record a fact, but are not explanatory. It seems to be the case that to every organism is assigned a minimum temperature below which it dies, a maximum temperature above which it dies, and an optimum temperature at which it thrives best; but these have to be studied separately for every species. Similarly, in regard to depth, species have been classed as eurybathic and stenobathic, but, since increased depth practically means diminished temperature, these are probably merely expressions of the same fact in another form. That an Arctic shallow-water species should stretch to considerable depths is not surprising, but it is remarkable to find such forms as, for example, Venus mesodesma on a New Zealand beach at 55° F. and in 1000 fathoms at 37° F. off Tristan d'Acunha. The provinces of zoological distribution, like the geographical divisions of mankind, must be taken merely to indicate the facies of a well-characterized fauna, not to imply the restriction of all its habitants to that area.
In considering the effect of temperature (and this applies to plankton as well as to benthos down to 100 fathoms), attention must be directed not only to the question of general warmth or cold as expressed by the mean annual temperature, but also to the range between the annual extremes: these ranges of variation have been carefully mapped by Sir J. Murray (Geog. Journ. xii. 113; compare ibid. xiv. 34). Still more important to the death-rate than these is the suddenness with which such variations occur: many animals are known to endure great extremes of heat and cold if exposed to them gradually, but to succumb to rapid alterations of temperature. Hence the frontier districts (Mischgebiete) between opposing currents are characterized by a heavy death-rate, and constitute marked barriers. A conspicuous instance of such a barrier in distribution is afforded at the Cape. The warm Mozambique Current, with a southwesterly direction off Natal, meets a north-east branch of the cold Antarctic Drift, and is beaten back eastwards: a result of the constant warring of these hot and cold currents is a high range of sudden temperature variation. Hence the Cape fauna consists mainly of only such species from neighbouring provinces as can endure high sudden variations; and the district is practically impassable. For example, nineteen species of Echinoids are known from the Cape district. Of these twelve are peculiar to the Indo-Pacific province, which stretches from East Africa to the Sandwich Islands and from Japan to Australia; two species are Southern Ocean forms, all but confined to south of 40° S.; four species are peculiar to the Atlantic Ocean: of these eighteen not one gets past the Cape into the next province; the nineteenth is practically a cosmopolitan (A. Agassiz, “Challenger” Reports: “Echinoidea”; compare also C. Chun, Aus den Tiefen des Weltmeeres, pp. 157, 158).
Among the barriers to the horizontal extension of epibenthos must be mentioned a wide deep ocean. The Indo-Pacific fauna ranges from East Africa to about 108° W., stepping from island to island over the Pacific; but this continuity is then broken by 37 degrees of longitude and more than 2000 fathoms of water, and such sessile species as are most Mollusca (cf. fig. 3) are unable to reach the American coast. This is presumably due to the fact that the planktonic larvae of epibenthic adults must settle on a suitable bottom within a certain period or die. In spite of the direct set of the currents from Florida to the British Isles, the epibenthos of the two is absolutely dissimilar; the similarity of the two Boreal provinces (2 and 2′, fig. 3) is to be assigned to a former continuity by way of Greenland, Iceland and Faeroe; a similar continuity, still unbroken, is exhibited by the Aleutian province on both sides of the Pacific. Though larvae cannot cross wide oceans, adults may no doubt traverse great stretches occasionally on floating timber, &c.
This barrier by distance may be instanced in another way. In the Arctic regions land masses are continuous or contiguous, and there are many circumpolar species, as, for example, Rhynchonella psittacea; towards the South Pole the southern continent is almost ice-bound, and the available land consists only of the tips of the continents and of the few oceanic islands. Hence few if any littoral species are circumpolar. For example, not a single littoral Ophiurid surrounds the South Pole, but five or six species are circumpolar in the northern hemisphere.
Taking next the mesobenthos and hypobenthos, living at depths where temperature is constant and current practically negligible, Mesobenthos; Hypobenthos. there appears theoretically to be no reason why an organism which can thrive at 500 fathoms should not have a world-wide range over the bottom of all oceans. Yet this is not often, although occasionally, known to be the case; and although perhaps, speaking generally, hypobenthic species have wider ranges than epibenthic, still they also seem to be limited. It must, however, be remembered that the ocean is large, deep hauls of trawl or dredge few, and individuals at great depths scattered, so that too much stress must not be laid on this point. The “Challenger” results seem to allow of at least one generalization—the deeper the fauna, the wider its range. This is shown by the following table of the “Challenger” benthos: the first column gives the number of benthos species captured at depths indicated in fathoms by the second column, the percentage of these species which is known to have been captured between the tropics, as well as south and north of the tropics, is shown in the third column:—
| Number of Specimens. |
Horizon. | S. T. N. |
| 4248 | 0-100 | 0.6 |
| 1887 | 100-500 | 2 |
| 616 | 500-1000 | 4 |
| 493 | 1000-1500 | 7 |
| 394 | 1500-2000 | 7 |
| 247 | 2000-2500 | 9 |
| 153 | over 2500 | 9 |
We can only guess at the causes of the apparently limited range of many deep-sea types. (a) One of these is probably the limited food supply: presumably, as with a land fauna, there are as many mouths in a given area as it will support, and an equilibrium of species is maintained which will at least hinder the extension of any one. For food the bulk of the deep-water fauna is dependent upon the rain of dead organisms falling from higher levels, these, slowly disintegrating (probably under chemical, not bacterial, action), seem to form with the bottom deposit a kind of nitrogenous ooze, through which many deep-sea organisms slowly swallow their way, as an earthworm goes through earth extracting nutriment. (b) Another hindrance to the extension of many deep-sea species is that they are holobenthic, that is, do not pass through a free-swimming larval stage; the means of dispersal is therefore regulated by the animal's own power of locomotion. Generally speaking, as might be expected, the freely-moving hypobenthos, fish and crustacean, have the widest ranges, and even these are not helped by currents, as are epibenthic or planktonic forms. The larval history of deep-water forms is, however, unfortunately obscure. (c) Lastly, extension of area of a species being at best difficult in deep water for non-swimmers, the place and date of their first migration must be taken into account; forms which have comparatively recently adopted deep-water life cannot be expected to have spread far from their original centre. As regards this point, in the first place, it is with migration, not with local evolution, that we have to deal: no classes and orders, only a few families and genera, rarely sub-orders, are peculiar to the hypobenthos; the deep members of each group consist for the most part of widely separated genera, the species do not grade into each other, as is so often the case in the epibenthos; and evolution could hardly have produced these species and genera under the uniformity of their present environment. This migration downwards from the mud-line has no doubt occurred all over the world, notably in the Southern Ocean, if we may judge by the richness of the deep-water fauna there to-day; probably also largely in Arctic and sub-Arctic regions, less so in tropical and temperate zones. As to the date of migration, the following fact seems to show that it is of comparatively recent origin, and is indeed still in progress: taking the “Challenger” species from the epibenthos, from the mesobenthos, and then from zones of 500 fathoms down to 2500, each zone shares a larger percentage of species with the zone above it than with that below it (except in one case where they are nearly equal). But it is not to be supposed that all our present-day deep-water forms began their migration simultaneously, and we can say with fair certainty that migration to deep water did not begin before the close of the Mesozoic epoch. Had it begun earlier, we should find typical Mesozoic and even older forms, or their conveners, at great depths: so far is this from being the case that the most venerable animals of to-day—Lingula, Amphioxus, Limulus, 75% of Crinoids, 90% of Brachiopoda, &c.—are epibenthic or mesobenthic. On the other hand, it is extremely likely that the Cretaceous epoch marked the commencement of migration. The hexactinellidan sponges are known to have lived in quite shallow water at the date of deposition of the Inferior Oolite; to-day none occur at a less depth than 95 fathoms; and as only two genera are known from the shallow Tertiary deposits, it would seem that the migration began about Cretaceous times (“Challenger” Reports: “Hexactinellida, ” F. E. Schulze). In 1881 (A. Agassiz, “Challenger” Reports: “Echinoidea”) 105 living genera of Echinoidea were admitted; of these 23% were known from Cretaceous but not from Tertiary deposits, 35% from Tertiary but not Cretaceous, and 40% as Recent only. The species of Cretaceous genera constituted only 29% of the epibenthic Echinoids, 44% of the mesobenthic, and no less than 55% of the hypobenthic. These species of Cretaceous genera were distributed fairly evenly over all three zones, but 72% of the species of Tertiary genera and 55% of the Recent forms were confined to the epibenthos. As out of the twenty-five living genera known from the Cretaceous only seven are known also from Jurassic deposits, it is obvious that the close relationship is between Cretaceous and hypobenthos, rather than between any other geological and bathymetric horizons. Other instances, such as that of the Eryonidae, seem to point to similar conclusions.
Excepting the essential air-breathers, practically every phylum and class and most orders are represented in the benthos. The 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 |
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| Alcyonaria | 1.2 | 1.0 | |||||||||||||||||||||||||||||
| Shelled Mollusca | 8.0 | 19.7 | |||||||||||||||||||||||||||||
| Decapoda | 3.6 | 0.8 | |||||||||||||||||||||||||||||
| Echinodermata | 33.6 | 11.7 | |||||||||||||||||||||||||||||
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6.8 | 4.6 | |||||||||||||||||||||||||||||
| Annelida | 6.8 | 8.0 | |||||||||||||||||||||||||||||
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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 constructed the first efficient net which could be opened and shut at known depths, using a propeller mechanism (Bibl. Zool. vol. i.); and he improved his original pattern for the “National” and “Valdivia” expeditions. The present writer has devised a net, of which the opening and closing are effected from the deck by heavy weights; this has been used successfully on the “Siboga” expedition and in cruises of the “Research” (Proc. Zool. Soc., 1898). W. Garstang has constructed an ingenious net which is useful in comparatively shallow water, but is open to criticism as being too light for depths beyond 100 fathoms, and several other types are in use. The existence of a mesoplankton, that is, of a plankton living between 100 fathoms from the surface and the bottom, has been generally considered as definitely proved by these nets. On the other hand, A. Agassiz, using the Tanner tow-nets, contends that while a mixture of surface and bottom species may occur in a closed sea near land, there is no intermediate fauna in the open ocean between about 200 fathoms from the surface and the bottom; his conclusions, based on negative evidence, have not met with general acceptance. Animals captured below the first hundred fathoms in the open sea (the Mediterranean, for special physical reasons, is on a special footing) are divisible into at least three categories: (1) those which are eurythermal and eurybathic, e.g. Calanus finmarchicus, (2) those which, so far as we know, are purely mesoplanktonic and never come to the surface, for example, the Radiolarian family Tuscaroridae; (3) those which, like some Schizopoda, spend a larval period in the epiplankton, and seek deeper water when adult, rising to the surface, if at all, only at night. But until the publication of the results of expeditions provided with efficient mesoplankton nets, generalizations about this fauna had better be stated with all reserve There is, however, a certain amount of evidence to show that the mesoplankton includes different organisms in different latitudes, that surface animals of the north and south, unable to spread into the warmer surface water of lower latitudes, there sink into the cooler waters of the mesoplankton; the distributional area of such an organism will be in three dimensions bounded by isotherms (isobathytherms) and isothermobaths. As with the hypobenthos, there seems to be no theoretical reason against the universal distribution of the mesoplankton.
When a more systematic investigation of the various horizons has been carried out, many of the present cases of supposed discontinuous distribution will doubtless disappear. There are, however, undoubted cases of discontinuity where physical barriers have cut across a distributional area, an example of which may be cited here. The Isthmus of Panama was apparently only upraised about Miocene time, having been previously an archipelago through which a great circumequatorial current could pass, consequently the benthos of the Panama region shows marked alliance with the Caribbean, with which it was formerly continuous, but practically none with the Indo-Pacific. To the same cause is doubtless attributable the distribution of the five Decapoda which are characteristic of the Sargasso Sea, which are circumequatorial oceanic types, only occasionally littoral: three of these are known only from the Atlantic, one occurs in the Atlantic and Pacific, one in the Atlantic, Pacific and Indian Oceans. The damming of a great circumequatorial current by the Isthmus of Panama is probably also responsible for that dislocation of currents which resulted in the present relations of the Gulf Stream and North Atlantic Drift to the Labrador Current and cut the Atlantic Boreal fauna into two discontinuous districts (2 and 2′, fig. 3).
Under the head of discontinuous distribution, the alleged phenomenon known as bipolarity must be mentioned. In summarizing the work of the “Challenger,” Sir John Murray maintained on the basis of the reports that numerous species occurred in both polar and sub-polar areas which were absent from the tropic. He regarded them as the hardy survivors of a universal fauna which had withstood that polar cooling which set in towards the close of the Mesozoic period (Murray, Trans. Roy. Soc. Edin. vol. xxxviii., 1896; G. Pfeffer, Verh. deutsch. Zool. Gesellsch. ix. 1899) This view and the facts on which it was based have been acutely contested, and the question is still far from settlement (for lists of the literature see A. E. Ortmann, Am. Nat. xxxiii. 583; and Miss E. M. Pratt, Mem. Manchester Soc. vol. xlv., 1901). As regards the purely epibenthic and sessile fauna, there are a few undoubted instances of actual specific identity; in some classes, however, such as the Echinoderms, this does not appear to hold (Hamburger Magalhaensche Sammelreise, and F. Römer and F. Schaudinn's Fauna arctica); but even in these the general composition of the fauna and the presence of certain identical and peculiar genera seem to point to something more than a mere “convergence” due to similar environment. As regards the plankton of the two polar regions and such epibenthic forms as extend also into deep water, the suggestion has been made that the Arctic and Antarctic benthos and plankton are really continuous by way of deep water in the main oceans, where the organisms can find a suitably low temperature. As an instance of this, C. Chun (Bezieh. zwischen dem arkt. und antarkt. Plankton, 1897) cites Krohnia hamata, a characteristic Arctic and sub-Arctic constituent of the epiplankton and mesoplankton, known only from the mesoplankton in the tropics, but rising to 38 fathoms at 40° S. 26° E. More exact information, such as may be expected from the various Antarctic expeditions, is required to settle this interesting question with its far-reaching corollaries.
See also Zoological Distribution: § Marine.