depth of the sea prevailed, due in a large measure to the defective
sounding apparatus of the time. Thus Captain Durham, in 1852, recorded a
depth of 7,730 fathoms in the South Atlantic, and Lieutenant Parker
mentions one of 8,212 fathoms--depths which the _Challenger_ and the
_Gazelle_ corrected to 2,412 and 2,905 fathoms respectively. The deepest
parts of the sea, as revealed by recent research, do not lie, as many
have thought, in or near the centres of the great oceans, but in the
neighbourhood of, or at no great distance from, the mainland, or in the
vicinity of volcanic islands. One of the deepest ‘pockets’ yet found is
probably that sounded by the American expedition on board the
_Tuscarora_ (1873-1875) east of Japan, when bottom was only reached at a
depth of 4,612 fathoms. More recently, soundings of 5,035 fathoms have
been recorded in the Pacific, in the neighbourhood of the Friendly
Islands, and south of these again, one of 5,113 fathoms; but the deepest
of all lies north of the Carolines, and attains a depth of 5,287
fathoms. It thus appears that there are ‘pockets’ or pits in the sea
whose depth below the surface of the water is about equal to the height
of the highest mountains taken from the sea-level. Both are
insignificant in comparison with the mass of the globe; and it is
sometimes said that, were the seas gathered up, and the earth shrunk to
the size of an orange, the mountain ranges and abysmal depths would not
be more striking than are the small elevations and intervening
depressions on the skin of the fruit.
But it is not with these exceptional abysses that we have to do; they
are as rare and as widely scattered as great mountain-ranges on land. It
is with the deep sea, as opposed to shoal water and the surface layers,
that this article is concerned; but the depth at which the sea becomes
‘deep’ is to some extent a matter of opinion. Numerous attempts, headed
by that of Edward Forbes, have been made to divide the sea into zones or
strata; and, just as the geological strata are characterized by peculiar
species, so, in the main, the various deep-sea zones have their peculiar
fauna. These zones, however, are not universally recognized; and their
limits, like those of the zoogeographical regions on land, whilst
serving for some groups of animals, break down altogether as regards
others. There are, however, two fairly definite regions in the sea; and
the limit between them is the very one for our purpose. This limit
separates the surface waters, which are permeable by the light of the
sun and in which owing to this life-giving light, _algæ_ and vegetable
organisms can live, from the deeper waters which the sun’s rays cannot
reach, and in which no plant can live. The regions pass imperceptibly
into one another; there is no sudden transition. The conditions of life
gradually change, and the precise level at which vegetable life becomes
impossible varies with differing conditions. With strong sunlight and a
smooth sea, the rays penetrate further than if the light be weak and the
waters troubled.
Speaking generally, we may place the dividing-line between the surface
layer and the deep sea at 300 fathoms. Below this no light or heat from
the sun penetrates; and it is the absence of these factors that gives
rise to most of the peculiarities of the deep sea. It is a commonplace,
which every schoolboy now knows, that all animal life is ultimately
dependent on the food-stuffs stored up by green plants; and that the
power which such plants possess of fixing the carbonic acid of the
surrounding medium, and building it up into more complex food-stuffs,
depends upon the presence of their green colouring matter (chlorophyll),
and is exercised only in the presence of sunlight. But, as we have
pointed out, ‘the sun’s perpendicular rays’ do not ‘illumine the depths
of the sea’; they hardly penetrate 300 fathoms. This absence of sunlight
below a certain limit, and the consequent failure of vegetable life,
gave rise at one time to the belief that the abysses of the ocean were
uninhabited and uninhabitable; but, as we have already seen, this view
has long been given up.
The inhabitants of the deep sea cannot, any more than other creatures,
be self-supporting. They prey on one another, it is true; but this must
have a limit, or very soon there would be nothing left to prey upon.
Like the inhabitants of great cities, the denizens of the deep must have
an outside food-supply, and this they must ultimately derive from the
surface layer.
The careful investigation of life in the sea has shown that not only the
surface layer, but all the intermediate zones teem with life. Nowhere is
there a layer of water in which animals are not found. But, as we have
seen, the _algæ_ upon which the life of marine animals ultimately
depends, live only in the upper waters; below 100 fathoms they begin to
be rare, and below 200 fathoms they are absent. Thus it is evident that
those animals which live in the surface layers have, like an
agricultural population, their food-supply at hand, while those that
live in the depths must, like dwellers in towns, obtain it from afar.
Many of the inhabitants of what may be termed the middle regions are
active swimmers, and these undoubtedly from time to time visit the more
densely peopled upper strata. They also visit the depths and afford an
indefinite food-supply to the deep-sea dwellers.
But probably by far the larger part of the food consumed by abysmal
creatures consists of the dead bodies of animals which sink down like
manna from above. The surface layers of the ocean teem with animal and
vegetable life. Every yachtsman must at times have noticed that the sea
is thick as a _purée_ with jelly-fish, or with those little transparent,
torpedo-shaped creatures, the _Sagitta_. What he will not have noticed,
unless he be a microscopist, is that at almost all times the surface is
crowded with minute organisms, foraminifera, radiolaria, diatoms. These
exist in quite incalculable numbers, and reproduce their kind with
astounding rapidity. They are always dying, and their bodies sink
downwards like a gentle rain.[1] In such numbers do they fall, that
large areas of the ocean bed are covered with a thick deposit of their
shells. In the shallower waters the foraminifera, with their calcareous
shells, prevail, but over the deeper abysses of the ocean they take so
long in falling that the calcareous shells are dissolved in the water,
which contains a considerable proportion of carbonic acid gas, and their
place is taken by the siliceous skeletons of the radiolarians and
diatoms. Thus there is a ceaseless falling of organisms from above, and
it must be from these that the dwellers of the deep ultimately obtain
their food. As Mr. Kipling in his ‘Seven Seas,’ says of the deep-sea
cables:
‘The wrecks dissolve above us; their dust drops down from afar--
Down to the dark, to the utter dark, where the blind
white sea-snakes are.’
In trying to realize the state of things at the bottom of the deep sea,
it is of importance to recognize that there is a wonderful uniformity of
physical conditions _là-bas_. Climate plays no part in the life of the
depths; storms do not ruffle their inhabitants; these recognize no
alternation of day or night; seasons are unknown to them; they
experience no change of temperature. Although the abysmal depths of the
polar regions might be expected to be far colder than those of the
tropics, the difference only amounts to a degree or so--a difference
which would not be perceptible to us without instruments of precision.
The following data show how uniform temperature is at the bottom of the
sea.
In June, 1883, Nordenskiöld found on the eastern side of Greenland the
following temperatures: at the surface 2·2° C.; at 100 metres 5·7° C.;
at 450 m. 5·1° C. In the middle of December, 1898, the German deep-sea
expedition, while in the pack-ice of the Antarctic, recorded the
following temperatures: at the surface -1° C.; at 100 m.-1·1° C.; at 400
m. 1·6° C.; at 1,000-1,500 m. 1·6° C.; at 4,700 m.-0·5° C. These may be
compared with some records made in the Sargasso Sea by the Plankton
Expedition in the month of August, when the surface registered a
temperature of 24° C.; 195 m. one of 18·8° C.; 390 m. one of 14·9° C.;
and 2,060 m. one of 3·8° C. It is thus clear that the temperature at the
bottom of the deep sea varies but a few degrees from the freezing-point;
and, whether in the tropics or around the poles, this temperature does
not undergo anything like the variations to which the surface of the
earth is subjected.
There are, however, some exceptions to this statement. The
Mediterranean, peculiar in many respects, is also peculiar as to its
bottom temperature. In August, 1881, the temperature, as taken by the
_Washington_, was at the surface 26° C.; at 100 m. 14·5° C.; at 500 m.
14·1° C.; and from 2,500 m. to 3,550 m. 13·3° C. These observations
agree, within one-fifth of a degree, with those recorded later by Chun
in the same waters. There are also certain areas near the Sulu Islands
where, with a surface temperature of 28° C., the deep sea, from 730 m.
to 4,660 m., shows a constant temperature of 10·3° C.; and again, on the
westerly side of Sumatra, the water, from 900 m. downwards, shows a
constant temperature of 5·9° C.; whilst in the not far distant Indian
Ocean it sinks at 1,300 m. to 4° C., and at 1,700 m. to 3° C. In spite
of these exceptions, we may roughly say that all deep-sea animals live
at an even temperature, which differs by but a few degrees from the
freezing-point. Indeed, the heating effect of the sun’s rays is said not
to penetrate, as a rule, further than 90 to 100 fathoms, though in the
neighbourhood of the Sargasso Sea it undoubtedly affects somewhat deeper
layers. In the Mediterranean the heat-rays probably do not penetrate
more than 50 fathoms. Below these limits all seasonable variations
cease. Summer and autumn, spring and winter, are unknown to the dwellers
of the deep; and the burning sun of the tropical noonday, which heats
the surface water to such a degree that the change of temperature from
the lower waters to the upper proves fatal to many delicate animals when
brought up from the depths, has no effect on the great mass of water
below the 100-fathom line.
Again, in the depths the waters are still. A great calm reigns. The
storms which churn the upper waters into tumultuous fury have but a
superficial effect, and are unfelt at the depth of a few fathoms. Even
the great ocean currents, such as the Gulf Stream, are but surface
currents, and their influence is probably not perceptible below 200
fathoms. There are places, as the wear and tear of telegraphic cables
show, where deep-sea currents have much force; but these are not common.
We also know that there must be a very slow current flowing from the
poles towards the Equator. This replaces the heated surface waters of
the tropics, which are partly evaporated and partly driven by the
trade-winds towards the poles. Were there no such current, the waters
round the Equator, in spite of the low conductivity of salt water,
would, in the course of ages, be heated through. But this current is
almost imperceptible; on the whole, no shocks or storms disturb the
peace of the oceanic abyss.
An interesting result of this is that many animals, which in shallower
waters are subject to the strain and stress of tidal action or of a
constant stream, and whose outline is modified by these conditions, are
represented in the depths by perfectly symmetrical forms. For instance,
the monaxonid sponges from the deep sea have a symmetry as perfect as a
lily’s, whilst their allies from the shallower seas, subject as they are
to varying tides and currents, are of every variety of shape, and their
only common feature is that none of them are symmetrical. This radial
symmetry is especially marked in the case of sessile animals, those
whose ‘strength is to sit still,’ attached by their base to some rock or
stone, or rooted by a stalk into the mud. Such animals cannot move from
place to place, and, like an oyster, are dependent for their food on
such minute organisms as are swept towards them in the currents set by
the action of their cilia. A curious and entirely contrary effect is
produced by this stillness on certain animals, which, without being
fixed, are, to say the least, singularly inert. The sea-cucumbers or
holothurians, which can be seen lying still as sausages in any shallow
sub-tropical waters, are nevertheless rolled over from time to time, and
present now one, now another, surface to the bottom. These have retained
the five-rayed symmetry, which is so eminently characteristic of the
group Echinoderma, to which they belong. But the holothurians in the
deep sea, where nothing rolls them about, continue throughout life to
present the same surface to the bottom; and these have developed a
secondary bilateral symmetry, so that, like a worm or a lobster, they
have definite upper and lower surfaces. These bilateral holothurians
first became known by the dredgings of the _Challenger_, and formed one
of the most important additions to our knowledge of marine zoology for
which we are indebted to that expedition.
At the bottom of the sea there is no sound--
‘There is no sound, no echo of sound, in the deserts of the deep,
Or the great grey level plains of ooze where the shell-burred
cables creep.’
The world down there is cold and still and noiseless. Nevertheless, many
of the animals of the depths have organs to which by analogy an
auditory function has been assigned. But it must not be forgotten that
even in the highest land-vertebrates the ear has two functions. It is at
once the organ of hearing and of balancing. Part of the internal ear is
occupied with orientating the body. By means of it we can tell whether
we are keeping upright, going uphill or descending, turning to the right
or to the left; and it is probably this function which is the chief
business of the so-called ears of marine animals. Professor Huxley once
said that, unless one became a crayfish, one could never be sure what
the mental processes of a crayfish were. This is doubtless true; but
experiment has shown, both in crayfishes and cuttlefishes, that, if the
auditory organ be interfered with or injured, the animal loses its sense
of direction and staggers hither and thither like a drunken man. It is
obvious that animals which move about at the bottom require such
balancing organs quite as much as those which skim the surface, and it
is in no wise remarkable that such organs should be found in those
dwellers in the deep which move from place to place.
If we could descend to the depths and look about us, we should find the
bottom of the sea near the land carpeted with deposits washed down from
the shore and carried out to sea by rivers, and dotted over with the
remains of animals and plants which inhabit shoal waters. This deposit,
derived from the land, extends to a greater or less distance around our
coast-line. In places this distance is very considerable. The Congo is
said to carry its characteristic mud 600 miles out to sea, and the
Ganges and the Indus to carry theirs 1,000 miles; but sooner or later we
should pass beyond the region of coast mud and river deposit, the
seaward edge of which is the ‘mud-line’ of Sir John Murray.
When we get beyond the mud-line, say a hundred miles from the Irish or
American coast, we should find that the character of the sea-bottom has
completely changed. Here we should be on Rudyard Kipling’s ‘great grey
level plains of ooze.’ All around us would stretch a vast dreary level
of greyish-white mud, due to the tireless fall of the minute globigerina
shells mentioned above. This rain of foraminifera is ceaseless, and
serves to cover rock and stone alike. It is probably due to this chalky
deposit that so many members of the ‘Benthos’--a term used by Haeckel to
denote those marine animals which do not swim about or float, but which
live on the bottom of the ocean either fixed or creeping about--are
stalked. Many of them, whose shoal-water allies are without a pedicel,
are provided with stalks; and those whose shallow-water congeners are
stalked are, in the depths, provided with still longer stalks. Numerous
sponges--the alcyonarian _Umbellula_, the stalked ascidians, and, above
all, the stalked crinoids--exemplify this point.
Flat as the Sahara, and with the same monotony of surface, these great
plains stretch across the Atlantic, dotted here and there with a yet
uncovered stone or rock dropped by a passing iceberg. In the deeper
regions of the ocean--where, as we have already seen, occasional pits
and depressions occur, and great ridges arise to vex the souls of the
cable-layers--the globigerina ooze is replaced by the less soluble
siliceous shells of the radiolarians and diatoms. The former are largely
found in pits in the Pacific, the latter in the Southern Seas. But there
is a third deposit which occurs in the deeper parts of the ocean--the
red clay. This is often partly composed of the empty siliceous shells
just mentioned; but over considerable areas of the Pacific the number of
these shells is very small, and here it would seem that the red clay is
largely composed of the ‘horny fragments of dead surface-living
animals, of volcanic and meteoric dust, and of small pieces of
water-logged pumice-stone.’ On whichever deposit we found ourselves,
could we but see the prospect, we should be struck with the monotony of
a scene as different as can well be imagined from the variegated beauty
of a rock-pool or a coral island lagoon.
There is, however, an abundance of animal life. The dredge reveals a
surprising variety and wealth of form. Sir John Murray records ‘at
station 146 in the Southern Ocean, at a depth of 1,375 fathoms, that 200
specimens captured belonged to 59 genera and 78 species.’ He further
states that this was ‘probably the most successful haul, as regards
number, variety, novelty, size, and beauty of the specimens,’ up to the
date of the dredging; but even this was surpassed by the captures from
the depths at station 147. The Southern Ocean is particularly well
populated. The same writer says: ‘The deep-sea fauna of the Antarctic
has been shown by the _Challenger_ to be exceptionally rich, a much
larger number of species having been obtained than in any other region
visited by the expedition; and the _Valdivia’s_ dredgings, in 1898,
confirm this.’ There seems to be no record of such a wealth of species
in depths of less than 50 fathoms, and we are justified in the belief
that the great depths are extremely rich in species.
The peculiar conditions under which the Benthos live have had a marked
influence on their structure. Representatives of nearly all the great
divisions of the animal kingdom which occur in the sea are found in the
depths. Protozoa, sponges, cœlenterata, round-worms, annelids,
crustacea, polyzoa, brachiopoda, molluscs, echinoderms, ascidians,
fishes, crowd the sea-bottom. The _Valdivia_ has brought home even
deep-sea ctenophores and sagittas, forms hitherto associated only with
life at the surface. The same expedition also secured adult examples of
the wonderful free-swimming holothurian, _Pelagothuria ludwigi_, which
so curiously mimics a jelly-fish. It was taken in a closing-net at 400
to 500 fathoms near the Seychelles. Most of these animals bear their
origin stamped on their structure, so that a zoologist can readily pick
out from a miscellaneous collection of forms those which have a deep-sea
home. We have already referred to a certain ‘stalkiness,’ which lifts
the fixed animals above the slowly deepening ooze. Possibly the
long-knobbed tentacles of the deep-sea jelly-fish, _Pectis_, on the tips
of which it is thought the creature moves about, may be connected with
the same cause. The great calm of the depths and its effect upon the
symmetry of the body have also been mentioned; but greater in its effect
on the bodies of the dwellers in the ocean abysses is the absence of
sunlight.
No external rays reach the bottom of the sea, and what light there is
must be supplied by the phosphorescent organs of the animals themselves,
and must be faint and intermittent. A large percentage of animals taken
from the deep sea show phosphorescence when brought on deck; and it may
be that this emission of light is much greater at a low temperature, and
under a pressure of 1 to 2 tons on the square inch, than it is under the
ordinary atmospheric conditions of the surface. The simplest form which
these phosphorescent organs take is that of certain skin-glands which
secrete a luminous slime. Such a slime is cast off, according to Filhol,
by many of the annelids; and a similar light-giving fluid is exuded from
certain glands at the base of the antenna and elsewhere in some of the
deep-sea shrimps. But the most highly developed of the organs which
produce light are the curious eye-like lanterns which form one or more
rows along the bodies of certain fishes, notably of members of the
Stomiadæ, a family allied to the salmons. From head to tail the
miniature bull’s-eyes extend, like so many portholes lit up, with
sometimes one or two larger organs in front of the eyes, like the port
and starboard lanterns of a ship, so that when one of these fishes swims
swiftly across the dim scene it must, to quote Kipling again, recall a
liner going past ‘like a grand hotel.’ Sometimes the phosphorescent
organ is at the tip of a barbel or tentacle, and it is interesting to
note that the angler-fish of the deep sea has replaced its white lure,
conspicuous in shallow water, but invisible in the dark, by a luminous
process, the investigation of which leads many a creature into the
enormous, toothed mouth of the fish.
A peculiar organ, known by the name ‘phæodaria,’ exists in the body of
certain radiolarians found only in the deep seas. It has been suggested
that this structure gives forth light; and, if this be the case, the
floor of the ocean is strewn with minute glow-lamps, which perhaps give
forth as much light as the surface of the sea on a calm summer’s night.
There is, however, much indirect evidence that, except for these
intermittent sources, the abysses of the ocean are sunk in an
impenetrable gloom.
When physical conditions change, living organisms strive to adapt
themselves to the changed conditions. Hence, when the inhabitants of the
shallower waters made their way into the darker deeps, many of them, in
the course of generations, increased the size of their eyes until they
were out of all proportion to their other sense-organs. Others gave up
the contest on these lines, and set about replacing their visual organs
by long tactile tentacles or feelers, which are extraordinarily
sensitive to external impressions. Like the blind, they endeavour to
compensate for loss of sight by increased tactile perception; and in
these forms the eyes are either dwindling or have quite disappeared. An
instance in point is supplied by the crustacea, many of whom have not
only lost their eyes, but have also lost the stalk which bore them; but
amongst the crustacea some genera, such as _Bathynomus_, have enormous
eyes with as many as four thousand facets. It is noticeable that this
creature has its eyes directed downwards toward the ground and not
upwards, as is the case with its nearest allies. On the whole the
crustacea lose their eyes more readily, and at a less depth, than
fishes. Many of the latter--_e.g._, _Ipnops_--are blind, and in others
the eyes seem to be disappearing. Thus, amongst the deep-sea cod,
_Macrurus_, those which frequent the waters down to about 1,000 fathoms,
have unusually large eyes, whilst those which go down to the deeper
abysses have very small ones. Many of the animals which have retained
their eyes carry them at the end of processes. Chun, in his brilliant
account of the voyage of the _Valdivia_, has figured a series of fishes
whose eyes stand out from the head like a pair of binoculars; and
similar ‘telescope’ eyes, as he calls them, occur on some of the
eight-armed cuttle-fish. The larva of one of the fishes has eyes at the
end of two stalks, each of which measures quite one-fourth of the total
length of the body.