CRUSTACEA OF THE DEEP SEA
It has already been mentioned that the animals living on the sea-bottom
in shallow water do not differ greatly in character from those that may
be found between tide-marks. As we go farther out from land, however,
into the deeper water, the character of the fauna gradually changes. One
by one the species found near the shore become rare and disappear, and
their places are taken by others characteristic of the intermediate
depths. These in their turn give way to others, till in the abysses of
the great oceans we find an assemblage of strange animals adapted to the
conditions of life in the great depths, and differing widely in many
respects from the more familiar inhabitants of the coastal waters. In
this "fauna of the deep sea," which extends to the greatest depths
reached by the dredge or trawl, the Crustacea occupy a prominent place.
Before proceeding to discuss some of these peculiar forms, however, it
is necessary to attempt to form some idea of the conditions under which
they live.
In the first place, the character of the sea-bottom changes very greatly
as we pass away from the coast. Near the shore it is extremely
diversified, consisting in one place of rocks swept bare by the tides or
overgrown with jungles of sea-weed, in another of banks of gravel or
shingle, of sand or of mud, but in all cases derived from the "waste" of
the land, as it is eaten away by the waves or washed down by the rivers.
As the distance from land increases, the deposits become finer and
finer, till they shade off into a soft oozy mud, composed of the finest
particles brought down by the rivers. In the neighbourhood of large
rivers this mud may sometimes extend for hundreds of miles from the
land, but there is a limit to the distance to which even the finest
particles can drift before they settle to the bottom, and beyond this
limit the floor of the ocean is covered by sediments which owe their
origin, not to the land, but to the ocean itself. The surface waters of
the ocean everywhere teem with a vast variety of floating animals and
plants, and, as these die, their remains sink to the bottom "like a
perpetual shower of rain."
Among the most abundant floating organisms--in the warmer seas, at any
rate--are certain minute animals known as _Foraminifera_, which belong
to the lowest class of the animal kingdom, and have shells composed, in
most cases, of carbonate of lime. Over vast areas the bottom of the
ocean is covered with a soft grey ooze, made up almost entirely of the
dead shells of Foraminifera rained down from above. Since the commonest
species of Foraminifera found under these circumstances belong to the
genus _Globigerina_, the deposit is known as "Globigerina ooze."
In certain regions of the ocean the shells of other floating organisms
largely replace those of the Foraminifera in covering the ocean floor,
and in the deepest abysses--so deep that the shells of surface animals
are dissolved before they can sink to the bottom--there is found a
deposit known as the "red clay," which appears to be derived largely
from the impalpable volcanic and cosmic dust that floats in the
atmosphere. It is not necessary for our present purpose to enter more
fully into the interesting questions connected with these deep-sea
deposits, but it is important to remember that, generally speaking, the
floor of the deep sea is everywhere soft ooze, without rocks or stones,
except for an occasional water-logged lump of pumice or a stone dropped
by a melting iceberg. This fact is probably of great importance in the
life of deep-sea animals.
One of the most peculiar and characteristic of the physical conditions
in the deep sea is the enormous pressure under which life has to be
carried on. At the surface of the sea the pressure of the atmosphere is,
roughly speaking, 14-1/2 pounds per square inch. At a depth of only 33
feet of water this pressure is doubled, and at greater depths the
pressure increases in proportion, till at 2,000 fathoms it is more than
2-1/2 tons on the square inch. As a matter of fact, however, the animals
at the bottom of the sea are probably but little affected by this
enormous pressure. Only, when they are brought up by the dredge the
sudden release of pressure causes the fluids of the body to expand and
destroys the tissues, so that the animals are generally dead or dying
when they reach the surface.
More important than the pressure in its influence on life is the
darkness of the depths. The light of the sun only penetrates the water
of the sea to a comparatively small depth. At 200 fathoms there is not
enough light to produce any effect on a photographic plate. Even at a
considerably less depth the absence of light puts an end to all
plant-life, except for the ubiquitous bacteria, and it follows that all
the animals of the deep sea ultimately depend for their food-supply on
the rain of dead bodies of surface animals which, as already mentioned,
is constantly falling on the sea-bottom.
The temperature at the bottom of the deep sea is always very low. Dr.
Alcock states that "in the open part of the Bay of Bengal, where the
mean surface temperature is about 80 deg. F., the temperature at a depth of
100 fathoms is only about 60 deg. F., at a depth of 300 fathoms not quite
50 deg. F.; while at a depth of 2,000 fathoms the temperature all the year
round is only 3 deg. above freezing-point."
Finally, it is important to notice the _uniformity_ of the conditions at
the bottom of the sea; not only are the alternation of night and day and
the progress of the seasons unfelt in the abysses, but the conditions
are practically the same over vast areas in all the oceans.
In the case of deep-sea Crustacea, we are frequently confronted with a
difficulty which does not occur in the case of some other groups of
animals--Corals or Echinoderms, for example--the difficulty, namely, of
deciding whether the animals really lived on or near the bottom, or were
captured by the open mouth of the trawl on its way to the surface. When
the animals are plainly not well adapted for swimming--as, for instance,
most of the Crabs--it may be assumed that they did actually live on the
bottom; but, with the prawn-like forms, the possibility that they may
really be inhabitants of the intermediate depths must always be taken
into consideration.
[Illustration: FIG. 42--A DEEP-SEA LOBSTER (_Nephropsis stewartii_),
FROM THE BAY OF BENGAL. REDUCED. (After Alcock and Anderson.)]
[Illustration: FIG. 43--_Munidopsis regia_, A DEEP-SEA GALATHEID FROM
THE BAY OF BENGAL. REDUCED. (After Alcock and Anderson.)]
In animals that live in perpetual darkness we should expect to find, in
accordance with the principle of adaptation which runs through the whole
of organic nature, that the eyes are wanting or imperfectly developed.
In a great many deep-sea animals this is indeed the case. The deep-sea
Lobsters of the genus _Nephropsis_ (Fig. 42), which are very closely
allied to the Norway Lobster (_Nephrops_) of shallow water, have very
short and slender eye-stalks hidden under the rostrum, and showing at
the tip only the merest traces of what was once an eye. In the
lobster-like Eryonidea (see Fig. 46, p. 133), the reduced eye-stalks are
firmly fixed in notches in the front edge of the carapace. Some of the
deep-sea Crabs and Prawns seem also to be totally blind. In a great many
cases degeneration has not quite gone so far, and the eyes are present,
although much reduced and modified. Thus, the very numerous deep-sea
species of Galatheidae, belonging to the genus _Munidopsis_ (Fig. 43) and
its allies have, as Alcock says, "pallid, milky-yellow, lack-lustre
eyes which, though they may perhaps serve to distinguish between light
and darkness, can never form a definite visual image." It is probable,
indeed, that these pale-coloured eyes are specially adapted for vision
in a dim light, for it has been shown that in certain deep-sea
Euphausiacea the pigment-sheaths between the separate elements of the
compound eyes are greatly reduced, and are fixed in the position
temporarily assumed by those in the eyes of normal Crustacea when kept
in the dark. Be this as it may, there are many deep-sea Crustacea which
have well-developed and darkly-pigmented eyes. Some of these are
swimming forms, which may at times migrate into the upper strata of
water to which some rays of light penetrate; but there are some cases of
Crabs and other bottom-living species that have well-developed eyes,
although they live at great depths. This would seem to suggest that,
although shut off from the light of day, they are not condemned to grope
in perpetual darkness. Many deep-sea animals are known to be
phosphorescent, and it seems probable that the large-eyed species may
profit by the light emitted by the glow-worms and fireflies of the
abyss. Thus, Alcock points out that the deep-sea Hermit Crab
_Parapagurus pilosimanus_ (Plate XVI.), which lives in partnership with
a colony of Sea-anemones which it carries about with it, has large eyes,
although it descends to depths of at least 2,000 fathoms; and he
suggests that the Crab may be able to see its way by the light emitted
by the zoophytes.
[Illustration: _PLATE XVI_
DEEP-SEA HERMIT-CRAB, _Parapagurus pilosimanus_, SHELTERED BY A
COLONY OF _Epizoanthus_. FROM DEEP WATER OFF THE WEST OF IRELAND
(SLIGHTLY REDUCED)]
Some of the Crustacea, however, are themselves luminous. Thus, Alcock
records how specimens of a deep-sea Prawn, _Heterocarpus alphonsi_,
"poured out, apparently from the orifices of the 'green glands' at the
base of the antennae, copious clouds of a ghostly blue light of
sufficient intensity to illuminate a bucket of sea-water so that all its
contents were visible in the clearest detail." Certain other Prawns are
known to possess special light-producing organs on various parts of the
body and limbs. It is in the Euphausiacea, however, that these organs
have been most fully examined, and although the members of this group
(see Fig. 24, p. 56) are by no means all deep-sea animals, some of them
occurring at the surface of the sea, the structure of their luminous
organs, or "photophores," may appropriately be described here. They are
situated on the under-surface of the abdomen, in the basal segments of
some of the thoracic legs, and on the upper surface of the eye-stalks.
Each consists of a globular capsule covered by a layer of pigment,
except on the outer side, where there is a transparent biconvex lens. In
the centre of the capsule is a peculiar "striated body" which seems to
be the actual seat of luminescence, and behind it is a concave reflector
composed of concentric lamellae, and having a silvery lustre. Before
their luminosity was observed, these organs were described as "accessory
eyes," but there can be little doubt that they serve rather as
searchlights, although, from the positions that some of them occupy on
the body, it is not easy to see how they can illuminate objects within
range of the eyes. That the function of phosphorescent organs is not
always that of giving light for their possessor to see by is shown by
the fact that many luminous animals are blind. It is important to
notice, however, that these blind animals never have complex
"photophores" like those just described, but only exhibit a diffuse
luminosity or give off luminous secretions; as an example among
Crustacea, the blind Eryonidea (see Fig. 46, p. 133) may be mentioned,
one species of which was observed by Alcock to be "luminous at two
points between the last pair of thoracic legs, where there is a
triangular glandular patch." In a recent discussion of the whole
question of phosphorescence in marine organisms, Dr. Doflein concludes
that the part it plays in the life of the animal probably differs in the
different cases. In some it may serve to attract prey, as moths are
attracted to a candle; in others it may help individuals of the same
species to keep together in a swarm or to find their mates, the varying
arrangement of the photophores producing characteristic light-patterns
that serve as "recognition marks" like the colour-patterns of animals
that live in the light of day. The clouds of luminous secretion thrown
out by _Heterocarpus_ and other Prawns, and by certain Mysidacea and
Ostracods, may serve to baffle pursuers, like the cloud of ink thrown
out by a Cuttlefish, and in some cases the more complex organs may
illuminate objects within the range of vision. That this does not
exhaust the possibilities of speculation on the subject, however, is
shown by the case of certain deep-sea Prawns which have been recently
discovered to possess photophores placed so as to illuminate the
interior of the gill cavities. What function they can discharge in this
position seems beyond conjecture.
The colours of deep-sea Crustacea are very curious. Few of them have the
blanched appearance common, for instance, in animals that live in the
darkness of caves; on the contrary, their colours are often very vivid,
but they are nearly always uniform, without spots or markings, and in a
large proportion of cases are in some shade of red or orange. This red
colour seems to be associated, in some way that we do not understand,
with the darkness of their habitat. The general absence of markings is
very striking. Dr. Alcock remarks that in deep-sea Crustacea we never
see "those freaks of colour, or those labyrinthine mottlings and
dapplings, that excite our curiosity when handling the Crabs and Shrimps
of the reefs." Possibly the explanation of this may be that in these
dwellers in darkness colour is merely, as it were, an accident, a
by-product of physiological processes directed to other ends, not a
character of protective or warning value, as in animals that hunt and
are hunted in the light of day. It is a curious fact, which may have
some bearing on this problem, that in many cases, while the adults are
coloured in some shade of red, the eggs carried by the female are bright
blue or green.
Some of the peculiarities of structure observed in deep-sea Crustacea
seem to be correlated with the difficulties of resting or moving about
with security on the soft ooze of the sea-floor. Among the Crabs we find
a preponderance of long-legged species, not only among the true Spider
Crabs (Oxyrhyncha), but also in other groups (Dromiacea like
_Latreillia_, figured on Plate XIX., and Oxystomata), the members of
which assume the same spider-like form. In some cases the legs are
fringed with long stiff hairs, which may help to prevent the animal from
sinking in the ooze, and the spines on the body and legs of many species
may have the same effect. Among the deep-sea Prawns, the species of the
family Nematocarcinidae (Plate XVII.) have extremely long and slender
legs, which we may assume to be used like stilts for walking over the
soft ooze.
[Illustration: _PLATE XVII_
A DEEP-SEA PRAWN, _Nematocarcinus undulatipes_. (SLIGHTLY REDUCED)
(_From Brit. Mus. Guide_)]
Not much is known regarding the food of deep-sea animals. In the absence
of plant-life they must of necessity be all carnivorous, and all
ultimately dependent on the food-supply falling from above. Some
species have been found to have the food-canal filled with Globigerina
ooze, which they no doubt swallow, as earth-worms do the soil in which
they burrow, for the purpose of extracting the nutriment that it
contains. In one species of deep-sea Cumacea (_Platycuma holti_), which
appears to feed in this manner, the food-canal is coiled, a condition
very rare in Crustacea; in all probability this is due to the necessity
for an increase of the absorptive surface, since it is common to find
such an increase, either by lengthening and consequent coiling of the
gut, or by infolding of its walls, in animals that have to swallow large
quantities of relatively innutritious food material. Many species,
however, no doubt have more selective habits of feeding. The
lobster-like _Thaumastocheles_ (Fig. 44), which was dredged by the
_Challenger_ expedition in the West Indies at a depth of 450 fathoms,
and has since been got from deep water off the Japanese coast, has one
of the chelae enormously enlarged, with long and slender fingers set with
spines like the teeth of a rake. It has been suggested that this
remarkable claw may be used for raking or sifting the ooze for small
animals on which the _Thaumastocheles_ feeds. A similar function may be
suggested for the long and spiny first pair of walking legs in the
Spider Crab _Platymaia_ (Fig. 45).
[Illustration: FIG. 44--_Thaumastocheles zaleucus._ REDUCED. (After
Spence Bate.)]
[Illustration: FIG. 45--A DEEP-SEA CRAB (_Platymaia wyville-thomsoni_).
REDUCED. (After Miers.)]
In many deep-sea Crustacea the eggs are of very large size, indicating
that the young are hatched in an advanced stage of development. For
example, in the numerous species of the genus _Munidopsis_ the eggs are
always large and correspondingly few in number, in striking contrast to
the closely allied genus _Galathea_, from shallow water, in which the
eggs are small and very numerous. Alcock mentions that a deep-sea Prawn
of the genus _Psathyrocaris_, although only about 3-1/2 inches long, has
eggs nearly a quarter of an inch in length. It would seem that, in some
way or other, the conditions are unfavourable for a free-swimming larval
life; but they cannot be altogether prohibitive, for there are a good
many characteristically deep-sea Crustacea, such as the Eryonidea, that
have small eggs and presumably a larval metamorphosis.
[Illustration: _PLATE XVIII_
_Bathynomus giganteus_, ABOUT ONE-HALF NATURAL SIZE.
(_From Lankester's "Treatise on Zoology," after
Milne-Edwards and Bouvier_)]
The uniformity of the physical conditions over vast areas in the deep
sea is no doubt the cause of the enormously wide geographical range of
many species of deep-sea animals. There are many examples of this among
Crustacea, and they are added to by every deep-sea dredging expedition.
For example, the giant Isopod _Bathynomus_ (Plate XVIII.) was first
discovered in West Indian seas, and the same species has since been
dredged near Ceylon, while a second species has been found off the
Japanese coast. Of the strange lobster-like _Thaumastocheles_ (Fig.
44), mentioned above, only four specimens are known--one dredged by the
_Challenger_ in the West Indies, and three others more recently brought
from Japan.
The low temperatures prevailing in deep water, even in tropical seas,
render it possible for many Crustacea to live there which are closely
allied to, or identical with, species occurring in shallow water in the
colder seas of the North and South. Many examples of this are mentioned
by Dr. Alcock in his discussion of the deep-sea fauna of Indian seas;
for example, the Lobster _Nephrops andamanicus_, found at depths of 150
to 400 fathoms in the Indian seas, is very closely allied to the Norway
Lobster (_Nephrops norvegicus_) of our own coasts. To some extent this
fact affords an explanation of the phenomenon that has been called
"bipolarity" in the distribution of marine animals. It has been observed
that certain families, genera, and even species, are found in the Arctic
and Antarctic seas, although they seem to be entirely absent from the
intervening tropical zones. In some cases, however, it has been found
that these forms occur in the deep sea in the warmer regions where the
cold water offers them a connection between North and South without any
great difference of temperature.
In the early days of deep-sea exploration, when naturalists were
becoming aware of the rich fauna inhabiting the abysses of the ocean,
which till then had been supposed to be barren of all life, it was
confidently expected that representatives would be discovered of some of
the animals known as fossils from the earlier geological periods. It was
believed that the great ocean basins had remained unchanged for vast
periods of geological time, and that numerous "living fossils" would be
found surviving in the depths. These hopes have not been fully realized,
for the deep-sea fauna as a whole has proved to be of a comparatively
modern type; nevertheless, it does include a considerable number of
primitive and old-fashioned forms of life, some of which belong to
groups elsewhere extinct. This is conspicuously the case among the
Crustacea. The lobster-like Eryonidea, which at the present day are only
found in the deep sea, were long known as fossils before they were
discovered to survive as living animals. The existing species (Fig. 46)
are all blind, with only vestiges of eye-stalks, and they may be readily
distinguished by the fact that the first four, and sometimes all five,
pairs of legs end in chelae, no other Decapods having more than three
pairs of chelate legs. The fossils occur in rocks of the Secondary
Period, from the Trias to the early Cretaceous. Some of them, at least,
had well-developed eyes, and probably lived in shallow water. This was
almost certainly the habitat of those (Fig. 47) that are found preserved
in a marvellously perfect state in the lithographic limestone of
Solenhofen (famous for the discovery of _Archaeopteryx_ and many other
remarkable fossils), which is believed to have been deposited in a
lagoon. After the early part of the Cretaceous epoch, the Eryonidea are
no longer found as fossils, and it is, at all events, a probable
conjecture that about that period they forsook the shallow waters for
the deeper recesses of the ocean, where their descendants have held
their own till the present day.
[Illustration: FIG. 46--_Polycheles phosphorus_, ONE OF THE ERYONIDEA,
FEMALE, FROM THE INDIAN SEAS. (From British Museum Guide, after Alcock.)]
[Illustration: FIG. 47--_Eryon propinquus_, ONE OF THE FOSSIL ERYONIDEA,
FROM THE JURASSIC ROCKS OF SOLENHOFEN. (From Lankester's "Treatise on
Zoology," after Oppel.)]
Another group of deep-sea Crustacea which has affinities with certain
fossil forms is the little family Homolodromiidae among the Crabs. It has
already been mentioned that the Dromiacea are the most primitive tribe
of the Brachyura, and Professor Bouvier has shown that among these the
Homolodromiidae approach most nearly to the lobster-like forms from which
the Crabs have been derived. He has further shown that the members of
this family closely resemble in the arrangement of the grooves upon the
carapace the extinct Prosoponidae, which are known as fossils from
Jurassic and Cretaceous rocks.
It is in the deep sea also that we find the curious Hermit Crabs of the
family _Pylochelidae_ (Fig. 37, p. 94), which are perfectly symmetrical
and show no trace of having ever adopted the habit of living in
Gastropod shells; so primitive, indeed, are these forms that it is not
easy to find characters by which to define them from the lobster-like
Thalassinidea or from the true Lobsters themselves, and, although no
fossil representatives are yet known, there seems no reason to doubt
that the Pylochelidae are nearly related to the primitive stock from
which the other Hermit Crabs have been evolved. Among the deep-sea
Prawns there are many forms, both of Penaeidea and of Caridea, which are
more primitive than most of their relatives from shallow water; and
although in these cases also the geological records are faulty, we may
assume, if we cannot prove in detail, a general similarity to the fossil
Prawns of Mesozoic rocks.
When all has been said, however, perhaps the most surprising thing about
the deep-sea fauna is, not that the animals are unlike those living in
shallow water, but that they differ from them so little. When we
consider the physical conditions of the oceanic abysses--the absolute
darkness, the freezing cold, the pressure measured in tons on the square
inch--it would seem inevitable that the physiological processes of
deep-sea animals must differ greatly from those of animals living in
shallow water; yet in very many cases these differences of function are
accompanied only by the most trivial differences in structure. To take
one example, the "Pink Shrimp" (_Pandalus montagui_), which we may find
commonly between tide-marks on our own coasts, differs only in
inconspicuous details from species of the same genus living at a depth
of 600 fathoms; while other genera of the family Pandalidae range
downwards to 2,000 fathoms or more, without any important divergences in
structure.