CHAPTER III

THE RELATIONS OF THE ABYSMAL ZONE AND THE ORIGIN OF ITS FAUNA

In the study of the geographical distribution of terrestrial animals one of the great difficulties met with is the impossibility of defining exactly the limits of the regions into which we divide the surface of the earth. In a general way we recognise that there is an Australian region, an Ethiopian region, &c.; but, when we come to discuss the exact position of the frontier lines that separate these regions from their neighbours, we find all kinds of difficulties to overcome and inconsistencies to meet.

For the sake of convenience it is useful to adopt certain arbitrary limits for these regions, notwithstanding these difficulties and inconsistencies, but we must recognise the fact that nature recognises no such limits, that every region overlaps its neighbours to a greater or less extent, and that there are many debateable grounds in the world where the fauna characteristic of one region is mixed with that characteristic of another.

But this difficulty in defining the exact limits of the terrestrial faunistic regions is even more pronounced in the case of the regions and zones of the marine fauna.

On the dry land we find mountain ranges, forests, deserts, and other barriers, that to a very considerable extent prevent the mixing of one fauna with another, but in the sea there are no barriers of anything like the same importance, but one fauna gradually merges into the neighbouring fauna according to the temperature, the pressure, the amount of light, the salinity of the water or the food supply. This then is one of the difficulties met with in the study of the geographical distribution of the marine fauna.

But there is another that leads to almost greater complications. In considering terrestrial life it is customary to refer only to regions of geographical, or perhaps it would be more correct to call it—superficial distribution. It would be quite possible, however, to subdivide the geographical areas into zones of elevation above the sea-level, not very clearly marked off from one another, it is true, but nevertheless each showing a number of characteristic features. This idea is expressed, for example, when we speak of the Alpine fauna, the Himalayan fauna, or the fauna of the great Andes.

In the study of the marine fauna and flora we must notice, it is the depth of the water, or in other words the depression of the habitats below the sea-level, that forms the most important consideration. Geographical sub-regions may be recognised and defined with a certain amount of accuracy, especially in the case of the fauna of the shallow waters, but by far the most important changes in the general characters of the fauna are found when we pass from one ‘zone’ of depression to another. Thus in describing any particular marine fauna we should mention first of all its zone or sub-zone of depression and then its geographical region and sub-region. For example, we may speak of the fauna of the pelagic zone of the British sub-region of the European region, or the fauna of the abysmal zone of the Northern sub-region of the Atlantic region.

We can recognise three primary zones of the marine fauna which we may call the ‘Pelagic,’ the ‘Neritic,’ and the ‘Abysmal’ zones.

The Pelagic zone includes the superficial waters of all seas extending from the surface to a depth which cannot at present be very accurately determined, but is probably the same as the limit of the influence of direct sunlight.

The animals of this zone are frequently characterised by a general transparency of their tissues, a white or sea water (i.e. blue or green) colour, an organisation capable of prolonged swimming or floating movement, and by giving birth to floating eggs which hatch out transparent larvæ or embryos.

The pelagic zone may be divided into several geographical regions and sub-regions, which it would be beyond the scope of this book to enumerate here, but there is one that calls for a few brief remarks. In many parts of the ocean there may be found vast areas of floating sea-weed, which carry with them a population of crustacea and other animals peculiarly their own. This ‘sargasso’ fauna presents so many characteristics and so many features different from that of the ordinary pelagic fauna, that the tracts of sea bearing this weed must be considered to rank as a special region of the pelagic zone, which may be called the Sargasso region.

The zone of shallow water for which we shall adopt Professor Haeckel’s term—the Neritic zone—embraces all parts of the seas of less depth than 500 fathoms, including the inland seas, the shores of great continents and islands, and the shallow banks in the great oceans. It does not include the superficial waters—which belong to the pelagic zone—but extends only from the actual bottom to a distance of a few fathoms above it. The fauna of this zone is extremely varied, consisting of animals that swim, crawl, or are permanently fixed to the bottom, animals of almost every variety of colour and marking, and of every size and shape.

The exact limits of the Neritic sub-zones are not easy to define. The distinguished naturalist Forbes, to whom the abysmal zone was unknown, divided the seas from 0–50 fathoms in depth into three zones—the littoral zone lying between tide-marks, the laminarian zone extending from 0–15 fathoms, and the coralline zone from 15–50 fathoms.

The first of these will stand as a sub-zone, the animals that are able to withstand exposure to the sun and air either in pools or upon the rocks and sand even for a few minutes frequently possessing features that distinguish them from those dwelling beyond low-water mark, just as those more active creatures that migrate backwards and forwards with the ebb and flow of every tide differ from the dwellers in the open sea. There is, it is true, at every low tide, a migration of part of the fauna of this sub-zone into the next, but still it is sufficiently well defined to be allowed to remain in our category.

The second sub-zone is not so easy to define. The terms ‘laminarian’ and ‘coralline’ used by Forbes are only applicable to certain geographical regions and must be abandoned for general use.

We can only recognise one sub-zone between the littoral sub-zone and the abysmal zone, for notwithstanding the important varieties it exhibits in the nature of the bottom, whether it be rocky, sandy, or weedy, the amount of light, the temperature of the water, and the rapidity of the currents, it is not possible at present to point to any general characters of the fauna of its different parts to justify us in subdividing it.

The name that may be given to this second sub-zone of the neritic zone is the Katantic—the sub-zone of the slopes.

The last well-marked zone is the abysmal, extending from the 500–fathom line to the greatest depths of the ocean, one of enormous superficial area, one that it is most difficult to investigate, and one about which we know but little.

In the present state of our knowledge we cannot divide it into any well-marked sub-zones nor even into geographical regions or sub-regions. It is not divided into sections by any important geographical barriers, and the general characters presented by its fauna are practically the same all the world over.

Professor A. Agassiz has pointed out in his ‘Challenger’ monograph that the deep-sea echinoids of the Atlantic Ocean differ from those living in corresponding depths in the Pacific Ocean, but it is doubtful whether any such well-marked differences can be observed in other groups of animals. If, in the course of time, increased knowledge of deep-sea animals emphasises the difference between the abysmal fauna of the Pacific and that of the Atlantic, then we can divide this zone into two geographical regions; but at present it seems more correct to consider the abysmal zone as one that is indivisible either bathymetrically or geographically.

Before passing on to the consideration of the general characters of the abysmal fauna, there are still one or two points that must be just briefly referred to.

It is the function of every true naturalist to consider animals from every possible point of view. Not only must he regard them as members of a certain species belonging to a genus, a family, an order, and so on, presenting certain peculiarities of structure and development; not only must he regard them as inhabitants of a certain locality or zone of depth, but he must also pay attention to their habits and mode of life.

Now amongst marine animals we can recognise three principal modes of life. Some animals simply float or drift about with the currents of the sea and are unable to determine for themselves, excepting, perhaps, within very small limits, the direction in which they travel. Such are the countless forms of protozoa, the jelly-fishes and medusæ, numerous pelagic worms and crustacea, the pyrosomas and salps, and many other forms well known to those who are in the habit of using the tow-net. This portion of the fauna has recently been called the Plankton.

Then there are the animals that are capable of very considerable swimming movements, animals that are able to stem the tide and migrate at will from one part of the sea to another, such as the cetacea, most fishes, and perhaps also many cephalopods. This portion of the fauna has been called the Nekton.

And lastly we have those animals that remain perfectly fixed to the bottom or are capable only of creeping or crawling over the rocks and sand, such as the sponges, hydroids, sedentary tunicates, gasteropods, most lamellibranchs, and many crustacea. This portion of the fauna has been called the Benthos.

Although it will not be necessary to use these terms very frequently in this little book, it may be advisable for the reader to bear in mind that in any exhaustive treatise on the marine fauna such terms would be employed, and that in the chapters dealing with the fauna of the abysmal zone we should find accounts of the ‘bathybial plankton,’ the ‘bathybial nekton,’ and the ‘bathybial benthos.’

Lastly we must consider quite briefly the views that have been held concerning the origin of the abysmal fauna.

As soon as it became clear to naturalists that there is no part of the ocean, however deep it may be, that deserves the name ‘azoic,’ but that almost every part has a fauna of greater or less density, the problem of the origin of this fauna presented itself.

Whence came the curious creatures that live mostly in total darkness and can sustain without injury to their delicate and complicated organisation the enormous pressure of the great depths? Are they the remnants of the fauna of shallow prehistoric seas that have reached their present position by the gradual sinking of the ocean basins? Or, are we to look upon the abysmal region as the nursery of the marine fauna, the place whence the population of the shallow waters was derived? Neither of these answers is supported by the facts with which we are now well acquainted. The fauna of the abysmal region does not show a close resemblance to that of any of the past epochs as revealed to us by geology, nor are we justified in assuming without much stronger evidence than we now possess, that the oceans have undergone any such great depression as this first theory presupposes.

Nor can we consider for a moment that the abyss was the original source of the shallow-water fauna; for not only do we find but few types that can be considered to be, in any sense of the word, ancestral in character; but on the contrary most of the animals of the deep sea seem to be specially modified types of shallow-water forms. The most probable explanation of the origin of the deep-sea fauna is the one that was put forward by Moseley and has been since supported by almost every authority on the subject, namely, that the fauna of the deep sea has been derived from successive immigrations of the animals from the shallow water.

This view is supported by the fact that the deep-sea fauna is much richer in the neighbourhood of land than it is in regions more remote from it. Many examples could be given to illustrate this point. The extraordinary richness of the deep-sea fauna on the western slopes of the floor of the Atlantic has been frequently commented on by the naturalists connected with the expeditions of the American vessels, the ‘Blake,’ the ‘Fish Hawk,’ and the ‘Albatross.’ Moseley called attention several years ago to a few localities in the neighbourhood of the land especially rich in deep-sea forms in comparatively shallow waters, such as one near the island of Sombrero in the Danish West Indies, where within sight of the lighthouse a haul of the dredge in 450 fathoms brought up a rich fauna of blind crustacea, corals, echinoderms, sponges, &c. Another off Kermadec in 630 fathoms brought up numerous curious blind fishes, ascidians, cuttlefishes, crustacea, _Pentacrinus_, and large vitreous sponges, and there are similar localities lying between Aru and Ke and between the Nanusa archipelago and the Talaut islands. The deep water off the Norwegian, Scotch, Irish, and Portuguese coasts also seems to be particularly rich in various forms of animal life. The same is probably true of the deep sea of many other regions in the neighbourhood of land, and, although it cannot be taken to be a rule without exceptions—the abysmal fauna off the western coasts of the Panama region being, according to the recent researches of Alexander Agassiz in the ‘Albatross,’ particularly poor—yet we can assert as a statement of very general application that the further removed from continental land, the poorer is the abysmal fauna.

Another argument that has been brought forward by Moseley in support of his view is that there is a certain relationship between the deep-sea fauna of any particular region and the shallow-water fauna of the nearest coasts. This is a point that is not easy to illustrate by examples, but as Moseley’s argument has not, so far as I am aware, been disputed by any of the naturalists who have followed him in this line of work, and the recent results of the ‘Albatross’ in comparing the deep-sea fauna of the eastern and western sides of the isthmus of Panama seem if anything to support it, we can take it as a point in favour of his view of the origin of the abysmal fauna.

It is impossible to say at present at what time in the world’s history these migrations commenced, but, as Agassiz points out, none of the palæozoic forms are found in the deep sea, and this seems to indicate that the fauna did not commence its existence earlier than the cretaceous period.

It is quite possible, however, that part of the fauna of the deep sea has been derived directly from the pelagic zone. The occurrence of bathybial Radiolaria, Foraminifera and Siphonophora, and among fishes genera and species of the pelagic families Sternoptychidæ and Scopelidæ, suggest that this zone may have contributed very largely to the fauna of the abyss.

Much of course still remains to be done before we can consider any of these interesting problems connected with the deep-sea fauna to be definitely solved. All we can do at present is to speculate upon the direction in which the facts at our disposal seem to point, and by following up one clue after another hope that we may eventually arrive at the truth. The task may be a difficult one, but it will reward our efforts. If truth is hard to find when it lies at the bottom of a well, how much more inaccessible must it be when it lies hidden in the darkness of the sea’s abyss!