FLOATING CRUSTACEA OF THE OPEN SEA
It is only rarely that the floating organisms of the surface of the sea
are so large or so abundant as to catch the attention of the casual
observer. Except for an occasional shoal of porpoises or of flying-fish,
the waste of waters seen from the deck of a ship in mid-ocean usually
seems to be barren of life. Nevertheless, there is probably no region of
the ocean where the tow-net will not reveal the existence of a more or
less varied fauna and flora. Sometimes, indeed, these organisms, though
minute, are so numerous as to discolour the water over large areas;
whalers in the Arctic seas know by the appearance of "whale-food" where
whales are likely to be found, and Herring or Mackerel fishermen
recognize the changes in colour of the water among the "signs" which
guide them when and where to shoot their nets.
The organisms which make up this "pelagic" fauna and flora may be
grouped into two classes, which may be termed the "swimmers," or
Necton, and the "drifters," or Plankton. The former include the larger
and more active animals, such as fish, whales, and the like, whose
movements are more or less independent of the movements of the water;
the latter comprise the plant-life and the floating or feebly swimming
animals that drift at the mercy of waves and currents. A great deal of
attention has been given in recent years to the study of the plankton,
and it has come to be recognized as filling a very important place in
the balance of life in the sea. In the sea, as on land, all the animals
are ultimately dependent on plants for their food. The larger and more
conspicuous sea-weeds which grow on the sea-bottom, however, can only
flourish in comparatively shallow water, and the region which they
occupy forms only a narrow fringe round the land-masses of the globe. It
is only necessary to look at a map of the world, showing the depth of
the sea, to realize what an insignificant part of the area of the oceans
contributes in this way to the food-supply of marine animals. The
microscopic plant-life of the plankton, however, makes up for the
individual minuteness of its constituents by their incalculable numbers.
The lowly organisms known as "diatoms," familiar to the microscopist
from the beauty of their flinty skeletons, are among the most numerous
and important of these, and they are associated with a great variety of
other single-celled algae and allied organisms, some of them so minute
that they pass through the finest silk plankton-nets, and have to be
sought for by special methods of collection recently devised for the
purpose. All these organisms possess the green colouring matter
(chlorophyll) that enables them to live, as the higher plants do, on the
carbon dioxide and other substances dissolved in the water. The smaller
animals of the plankton feed on these vegetable organisms, and in their
turn serve as food for larger animals. The Herring, the Mackerel, the
gigantic Basking Shark, and the still more gigantic Greenland Whale, all
feed directly on the animal plankton, and we have already seen that the
animals of the deep sea depend entirely on the same source of
food-supply. Further, very many of the bottom-living animals of shallow
water swim at the surface in the early stages of their life, and feed on
the other plankton animals and plants. Indeed, it is no exaggeration to
say that "all fish is diatom" in the same physiological sense as "all
flesh is grass," and the study of the plankton is thus of practical
importance as well as of scientific interest.
Of all the minute animals that form the intermediate links in the chain
between diatom and fish or whale, the Crustacea are the most important
and the most numerous both in species and in individuals. The Copepoda
are more richly represented than any of the other groups, and it would
be difficult to find a sample of marine plankton from which they were
altogether absent. Associated with them we find one or two species of
Cladocera, a larger number of Ostracoda (chiefly of the family
Halocypridae), a few Mysidacea, the Amphipoda of the suborder Hyperiidea,
the Euphausiacea, and some of the shrimp-like Decapods; while the larval
stages of these and other groups also form an important part of the
plankton.
It is necessary to make a distinction between the "neritic" plankton of
shallow water near the coast and the "oceanic" plankton of the open sea.
In the inshore waters the plankton consists not only of organisms that
pass the whole of their life at or near the surface, but also, and very
largely, of the free-swimming larvae of bottom-living species, and of
others that make occasional and temporary excursions to the surface. For
example, if the tow-net be used a short distance from land--say in some
sheltered bay on our own coasts--the catch will often be found to
consist largely of larval Crustacea. The zoea and megalopa stages of
Crabs, the zoea and schizopod stages of Prawns and Shrimps, are often
conspicuous by their numbers, or we may find swarms of the nauplius and
cypris larvae of Barnacles. Sometimes, and especially at night, numbers
of Cumacea may be found in the tow-net; and it is noteworthy that these
are usually males, which leave the females burrowing in the mud at the
bottom, and swarm to the surface for a brief period of activity.
Besides all these more or less temporary visitors, however, there are
numerous species, even in the inshore waters, which are adapted to a
floating life, and pass their whole existence as members of the
plankton. Copepoda of many kinds, some Mysidae, Amphipods like
_Hyperia_--which is commonly found sheltering under large
jellyfish--some species of actively swimming Isopods, and many other
forms, are only to be captured by the tow-net; and now and then, in
certain localities, winds and currents may drive into coastal waters
shoals of species whose proper home is the open ocean.
In a similar way the strictly neritic forms may sometimes be carried far
out to sea, so that it is nowhere possible to draw a hard-and-fast line
between the regions occupied by the neritic and the oceanic plankton.
With increasing distance from land, however, the larval stages of
bottom-living species become fewer, and finally disappear altogether,
and there is left an assemblage of animals whose whole existence is
passed floating at the surface or at the intermediate depths. How far
down from the surface this floating fauna actually descends is a
question which has been much debated. It appears now to be certain that
there is no stratum of water between the surface and the bottom of the
ocean which is devoid of life, although the upper layers (not at, but
some distance below, the surface) are probably much more densely
populated than those of the abyss. Many of the species appear to
undertake more or less extensive migrations in a vertical direction,
coming nearer the surface at certain stages of their life-history, and
sinking into deeper water at others. Further, some species at least seem
to rise to the surface at night, and to sink again during the day. Apart
from these vertical movements, which are as yet only imperfectly
understood, it is desirable to distinguish between the "epiplankton,"
comprising the organisms which inhabit the superficial strata of the
ocean down to about 100 fathoms, and the "mesoplankton," found at
greater depths. The plant-life which is dependent on sunlight belongs to
the epiplankton, while the animals of the mesoplankton are dependent,
like the bottom animals of the deep sea, on the supply of dead food
material falling from above. A third division, the "hypoplankton," has
been established for those animals which live immediately above the
bottom, but its distinctness from the mesoplankton has not yet been
satisfactorily established. Indeed, many of the swimming forms which
have already been mentioned in dealing with the Crustacea of the deep
sea are probably rather to be considered as belonging to the deep
mesoplankton--at least, where their size and swimming powers do not
entitle them to be ranked with the "necton."
[Illustration: FIG. 48--_Conchoecia curta_, AN OSTRACOD OF THE PLANKTON.
x 40. (Partly after G. W. Mueller.)]
Many of the modifications in structure characteristic of pelagic
animals may be traced to the necessity for keeping continuously afloat
with a minimum of exertion. The Crustacea of the plankton never carry
the heavy armour found in bottom-living species. Thus, the thick-shelled
Ostracoda of the bottom are represented in the plankton chiefly by the
family Halocypridae (Fig. 48), in which the shell is thin, uncalcified,
and almost membranous. Many species, particularly of the Copepoda, are
seen, under the microscope, to have large globules of oil distributed
through the tissues of the body, and these no doubt serve as floats,
increasing the buoyancy of the animal. The same purpose is probably
served, in many cases, by having large spaces, filled with fluid, within
the body. This is characteristic of pelagic animals, and is well seen in
many of the Crustacea in which the viscera and muscles occupy a
relatively small part of the interior of the animals, the intervening
spaces being filled with colourless transparent fluid. Many of the
Hyperid Amphipoda show this peculiarity--for example, the relatively
gigantic _Cystisoma_, which is mesoplanktonic in deep water; and it
reaches its extreme in _Mimonectes_ (Fig. 49), in which the anterior
part of the body is, as it were, blown out into a balloon, giving the
animal the aspect of a small jellyfish rather than an Amphipod.
[Illustration: FIG. 49--_Mimonectes loveni._ A FEMALE SPECIMEN SEEN FROM
THE SIDE AND FROM BELOW, SHOWING THE DISTENDED-BALLOON-LIKE FORM OF THE
ANTERIOR PART OF THE BODY. x 3. (After Bovallius.)]
If, as seems probable, the body-fluid of these animals is of a lower
specific gravity than the sea-water, it will act like the oil-globules
of the Copepoda in keeping the animals afloat. Even if the specific
gravity be the same, however, the distension of the body with fluid
acts in another way, by increasing the surface exposed to friction with
the surrounding water, and so retarding sinking. The principle involved
is illustrated by the fact that a soap-bubble sinks much more slowly
through the air than the drop of water into which it collapses. The
same result is produced if the surface is increased by outstanding
spines or hairs, just as, for instance, a downy feather sinks slowly
through the air, but drops rapidly if it is rolled into a ball between
the fingers. This is, no doubt, one function of the spines with which
plankton Crustacea, and particularly larvae, are frequently provided,
though they may also serve in some cases as a protection against
enemies. The spines have been already alluded to in describing the
various larvae, but it may be noted here that they are most strongly
developed in larvae which live in the open ocean; for example, the most
elaborately armed of all Decapod larvae are the zoea stages of
_Sergestes_ (Fig. 50), which, like the adults, belong to the oceanic
plankton. The nauplius larvae of Cirripedes are all more or less spiny,
and the spines reach an exaggerated development in the larvae of the
genus _Lepas_ (Fig. 51), of which the adults are attached to floating
drift-wood or the like, and belong to the oceanic fauna, although hardly
to be classed with the plankton.
[Illustration: FIG. 50--THE ZOEA LARVA OF A SPECIES OF _Sergestes_,
TAKEN BY THE "CHALLENGER" EXPEDITION. x 25. (After Spence Bate.)]
[Illustration: FIG. 51--THE NAUPLIUS LARVA OF A SPECIES OF BARNACLE OF
THE FAMILY LEPADIDAE, SHOWING GREATLY-DEVELOPED SPINES. FROM A SPECIMEN
TAKEN IN THE ATLANTIC OCEAN, NEAR MADEIRA. x 11. (After Chun.)]
The large feathered bristles that decorate the limbs or tail of many
plankton Copepoda have no doubt the same function in assisting
flotation. In the genus _Calocalanus_ (Fig. 52), for example, the tail
setae are large and brilliantly coloured feathery plumes, and in one
species, _C. plumulosus_, one of these setae is of relatively enormous
size, five or six times as long as the body of the animal itself.
[Illustration: FIG. 52--_Calocalanus pavo_, ONE OF THE FREE-SWIMMING
COPEPODA OF THE PLANKTON. ENLARGED. (From Lankester's "Treatise on
Zoology," after Giesbrecht.)]
Among the most singular of plankton Crustacea are the _Phyllosoma_ larvae
(see Fig. 28, p. 72) of the Spiny Lobsters and their allies
(Scyllaridea), which have been already described. These larvae are
sometimes found far out at sea, and it seems likely that their larval
life is unusually prolonged, and that they may be drifted to great
distances by ocean currents. At all events, they are well adapted for
pelagic life, since the broad flat body, hardly thicker than a sheet of
paper, can be sustained in the water like a "hydroplane" by
comparatively slight efforts of the swimming legs.
The watery character of the body, together with the thinness of the
exoskeleton, helps to explain the glassy transparency which is a feature
of most plankton Crustacea. This transparency has been regarded as a
protective adaptation rendering the animals inconspicuous in the water,
and it has indeed that effect to human eyes, but it is very doubtful
whether the animals derive much benefit from this. Many of the
animals--such as Herring and other pelagic fishes--that prey upon
plankton Crustacea appear to swallow them in bulk, without much
selection; and the Greenland Whale, as it swims open-mouthed through the
sea, is not likely to be guided by the greater or less visibility of the
Copepods that it sifts out on its baleen plates. Further, this
glass-like transparency is by no means universal, for many plankton
Copepoda are brightly coloured. In some, as in the beautiful blue
_Anomalocera_, common in British waters, the colour is due to pigment in
the fluids and tissues of the body; in others the feathery hairs on the
body and limbs show brilliant metallic colours, produced, like the
colours of a peacock's feather, not by pigments, but by the diffraction
of light in the texture of the organ. The most beautiful of all Copepoda
is _Sapphirina_, in which the surface of the body absolutely sparkles
with iridescent colours.
The striking phenomenon known as the "phosphorescence of the sea" is
familiar to every ocean voyager, and is seen from time to time on our
own coast. On a dark night the crest of every wave often seems to break
in a pale glow, the wake of the vessel is a trail of light, and an oar
dipped in the water seems on fire. This luminosity is due to the animals
of the plankton, largely to the lowly Protozoa and the jellyfishes, but
in part also to certain Crustacea. A number of pelagic Copepoda have
been shown by Giesbrecht to secrete, from special glands on the surface
of the body, a substance which becomes luminous on coming in contact
with the water. Even specimens which had been dried were found to give
out light on being wetted. Some pelagic Ostracods of the family
Halocypridae have been observed to emit clouds of a luminous secretion
from a gland in the neighbourhood of the mouth. A similar habit has been
seen, as already mentioned, in certain deep-sea Prawns and Mysidacea,
which may perhaps belong to the deeper part of the mesoplankton rather
than to the bottom fauna. The complex light-producing organs of the
Euphausiacea have already been described in dealing with deep-sea
Crustacea. A great many species of this group, however, are members of
the epiplankton, and in these the phosphorescent apparatus is quite as
fully developed as in species coming from greater depths.
_Meganyctiphanes norvegica_ (Fig. 24, p. 56), which is one of the
largest of the Euphausiacea, is common at no great depths in many places
in British seas. If a jar of sea-water in which specimens of this
species are swimming be brought into a dark room, a tap on the glass
will cause the photophores to flash out like a row of tiny lamps along
the side of the body. After shining for a few seconds the light dies
out, to appear again if the tapping be repeated.
There are certain peculiarities in the structure of the eyes in some
plankton Crustacea which suggest that the sense of sight is of special
importance to their possessors, although we can hardly do more than
guess at their special significance. Most Copepoda have only a single
eye in the middle of the head, corresponding to the single eye of the
nauplius larva, and of far simpler structure than the paired compound
eyes of most other Crustacea. In many plankton species, however, this
simple eye becomes much enlarged and complicated in various ways. The
three parts of which it is normally made up may become separated from
each other, and are sometimes increased in number to five, while lenses
serving to concentrate the light are often developed by thickening of
the overlying cuticle. The most elaborately constructed eyes are found
in the family Corycaeidae. In _Copilia_ (Fig. 53) a pair of eyes of
relatively enormous size are present. Each has in front a large biconvex
lens set at the end of a conical tube which extends backwards to a
smaller lens (like a telescope with object-glass and eyepiece), behind
which, again, are the sensory cells, corresponding to the retina,
enclosed in a tube of dark pigment, the whole apparatus being more than
half the length of the body. These eyes, although paired, do not
correspond to the paired compound eyes of other Crustacea, but have
arisen by the separation and enlargement of two of the three divisions
of the typical median Copepod eye.
[Illustration: FIG. 53--_Copilia quadrata_ (FEMALE), A COPEPOD OF THE
FAMILY CORYCAEIDAE, SHOWING THE PAIR OF LARGE "TELESCOPIC" EYES. x 20.
(After Giesbrecht.)]
A peculiarity of the paired compound eyes found in plankton Crustacea of
several different orders consists in the division of each eye into two
parts, which differ in structure. In many Euphausiacea and Mysidacea,
especially in those haunting the deeper strata (mesoplankton), this
division of the eyes is well marked, a frontal or dorsal part having the
separate elements of the eye (ommatidia) greatly lengthened and with
reduced pigment, while the lateral part is of more normal structure. It
seems probable, from the researches of Professor Chun, that the
fronto-dorsal division is adapted for the perception of very faint
light, while the lateral division will give a more accurate image of
brightly illuminated objects.
[Illustration: FIG. 54--_Phronima colletti_, MALE. FROM A SPECIMEN TAKEN
IN DEEP WATER NEAR THE CANARY ISLANDS. x 12. (After Chun.)]
In the pelagic Amphipoda, forming the suborder Hyperiidea, the eyes are
of very large size, generally occupying almost the whole surface of the
head, and giving the animals a very characteristic appearance, in
contrast to the small-eyed, bottom-living Gammaridea. In the family
Phronimidae (Fig. 54) the eyes are each divided into two parts, differing
in structure in the way just described.
There are a few Crustacea living habitually on the high seas which
cannot be reckoned as belonging either to the true plankton or to the
necton, since they depend on outside help for keeping themselves afloat.
Among these are the Barnacles which cluster on logs of drift-wood, and
are among the most important causes of the "fouling" of ships' hulls on
long voyages. The stalked Barnacles of the genus _Lepas_ are especially
common in such situations, and the characters of their larvae have been
already alluded to. Certain species of sessile Barnacles are constantly
found attached to large marine animals. For example, _Chelonobia_
adheres to the shell of Turtles, while _Coronula_ and some allied genera
are found on Whales.
[Illustration: _PLATE XIX_
_Latreillia elegans_, ONE OF THE DROMIACEA WHICH RESEMBLES A
SPIDER-CRAB. FROM THE MEDITERRANEAN. (NATURAL SIZE)
THE GULF-WEED CRAB, _Planes minutus_. (SLIGHTLY ENLARGED)]
The little "Gulf-weed Crab" (_Planes minutus_--Plate XIX.) is found
clinging to floating drift-weed nearly everywhere throughout the
temperate and tropical seas of the globe, and is especially common in
the area known as the Sargasso Sea, in mid-Atlantic. It is occasionally
drifted to the south coasts of the British Islands. In Sloane's "Natural
History of Jamaica," published in 1707-1725, it is stated of the
Gulf-weed Crab that "Columbus, finding this alive on the Sargasso
floating in the sea, conceived himself not far from some land, on the
first voyage he made on the discovery of the West Indies."
A few other Crustacea also form part of the peculiar fauna which is
associated with the Sargasso weed, notably a swimming Crab, _Neptunus
sayi_, and two or three species of Prawns. All of these are coloured
olive-green, like the weed among which they live.