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!