THE SEA A VAST AQUARIUM
No results of investigation in natural history have been more amazing
than those that show the marvelous richness of the sea in plant and
animal life--not merely at its warm margin, but far out in the centre
of what the ancients used to call "the desert of waters"; not only at
its surface, but in its profoundest depths, and under the polar ice as
well as amid the tropics. Sea populations differ somewhat according
to situation, those of the shallow shore lines, which are of the
"littoral" fauna, differ largely from those living in the open sea
and belonging to the "pelagic" fauna, and there are surface swimmers,
and others confined to the abysses; but virtually every class and
subdivision in the animal kingdom is represented in greater or less
variety in the zoölogy of the ocean. The list stretches from the merest
monads to the huge sharks and still bigger whales.
This multitude and diversity of animal life is possible in the sea
because of an even greater plenitude of plants there, which furnish
a never-failing food resource. Bacteria and blue-green algæ are at
the base of this. Bacteria exist in all seas, as in all soils, and
the fertility of nature above ground and under water depends on these
microscopic organisms, whose numbers in the ocean are as incalculable
as the grains of sand on its brink. In equal multitude are the
diatoms, unicellular algæ with flinty cases, by which the waves are
sometimes discolored over broad areas; and millions of other green
plants, living alone, or in chains, minute in size, but each a chemical
laboratory converting the salt water they absorb into meals for the
animals that swallow them--animals in most cases almost as small and
simple as the things they eat, and themselves destined to be sucked
into the mouth of something a little bigger, to be in turn a tidbit for
a third hungry mouth, and so on to the broiled mackerel for our own
breakfast.
THE PLANKTON AND ITS PHOSPHORESCENCE
The assemblage of plants and animals that together float or swim at or
near the surface of the ocean (or other water), say within a layer of
water one hundred fathoms thick, is scientifically called _plankton_
of the sea. In the open ocean, the pelagic plankton is much alike all
round the world of waters, although it varies a little in composition,
and still more in relative abundance, being denser in temperate than
in either tropical or polar latitudes; but nowhere is it absent. The
"waste of waters" teems with life. The plankton of the shallow waters
near continental shores, however, presents a decidedly different
assemblage from the pelagic plankton.
In the pelagic plankton, single-celled animals of the groups called
foraminifers and radiolarians are exceedingly prominent, and play an
enormous part in the economy of the sea, although almost or quite
microscopic in size. They are incased in chambered shells of lime or
flint; and over vast areas in warm latitudes the ocean floor is so
thickly covered with the dead shells of one kind that the mud is
called globigerina ooze. They are the eaters of the microscopic plants,
and themselves are food for a wide variety of hydroids and jellyfish,
large and small, whose silvery forms are often visible to the voyager,
and which are mostly responsible for the pale stars of phosphorescence
that shine about his prow and glorify his wake in dark nights. The
queen of these far swimmers is the radiant Portuguese man-of-war. In
the night a dragging fine-meshed net will capture more than by day of
the plankton, because many little creatures that in daylight sink to
considerable depths come to the surface at night.
Rising a step to the worms, we find them comparatively rare, but one
kind of marine flatworm that abounds in midocean is rose-red and
several inches long. Much more numerous is another flatworm, Sagitta,
"which along with copepoda, salpæ, pteropoda and radiolaria, everywhere
constitute the bulk of the small pelagic organisms" captured by towing
nets. Like almost all of these usually defenseless creatures they
are perfectly transparent, but some of them depart from the rule of
pale blue in tint and shine in bright red. A longer step takes us to
the Crustacea, represented in the pelagic plankton by queer little
shrimplike forms that in countless hosts of individuals play a part in
the ocean comparable to that of insects on land. The copepods are the
most numerous probably--little things only a fraction of an inch in
length, but amazingly abundant, and the principal users of plant food.
Their relatives, the little ostracods, have similar habits, and are
noted for their intense phosphorescence. Haeckel relates that on his
way to Ceylon he saw the entire sea like a twinkling ocean of light,
and his microscope showed him that it was made by throngs of ostracods,
with some jellyfishes, salpæ and worms. Crustaceans of higher rank
abound also. In northern waters species of Schizopoda, small,
transparent prawns with red spots around the mouth and big, black eyes,
swarm in enormous numbers, and are known to the fishermen as "kril."
An important part of the pelagic plankton consists of certain small
mollusks; and "as regards abundance of individuals few groups of
pelagic animals can compare with the winged snails, or Pteropoda."
These are minute, rapidly swimming creatures with thin, glassy shells,
and in some parts of the warmer oceans these discarded shells are so
numerous on the bottom that they give the name pteropod ooze to the
mud. One kind (Limacina), with a coiled shell about the size of a
pinhead, which abounds in the north Atlantic, is much feared by the
Norwegian fishermen because they very often spoil the herring that
feed on them. Another kind (Clione), looking somewhat like a reddish
butterfly an inch or so long, swims in shoals in the icy seas of the
far North, and is known as "whales' food." Some larger mollusks, of
which the beautiful purple Ianthina is most conspicuous, live among the
vast patches of floating seaweed in the Sargasso Sea.
Great numbers and variety of tunicates or ascidians and their larvæ
are taken in the surface nets of the sea naturalists, among them the
salpæ--free-swimming, barrel-shaped, transparent animals well known
to all seafaring people, and often seen crowding the surface of the
ocean. One genus of them is Pyrosoma, which has from the earliest
days excited the interest of mankind, mainly on account of the strong
phosphorescent light emitted, the name, indeed, meaning "fire animal."
These salpæ aggregate into colonies often several yards in length which
glow like fiery serpents as they move sinuously on their way.
This property of luminosity, so widely possessed by marine animals, is
one of the unsolved mysteries. It is called "phosphorescence," because
it resembles the cold light given by phosphorus when undergoing slow
oxidation, but phosphorus has nothing to do with the manifestation
here, or in such insects as the firefly; nor is it owing to bacteria,
as in the case of shining wood or decaying fish. What it really is no
one knows, but it has, at least, been learned that in animals the power
of emitting light is always attributable to certain structures of a
glandular nature that secrete a slimy, luminous substance, or, rather,
two substances, one luciferin and the other luciferase. When both
together are exposed to seawater phosphorescent light results.
As a rule, the light organ is surrounded by a layer of black pigment
that acts as a reflector, and often the light is projected through a
transparent lens; and there is reason to believe that in the case of
the higher animals, such as deep-sea fishes and squids, the rays may
be thrown when and where the creature desires, as a man handles an
electric flashlight. But for what purpose? Is it to illuminate the
surrounding water so as to perceive, or to attract prey, or is it to
avoid foes? A learned oceanographer replies that no one certainly
knows. "At all events," he concludes, "the answers would probably tend
to show that the many different kinds of light organs serve different
purposes."
PRISONERS IN THE DARK AND ICY DEPTHS
So much for the surface population of the ocean--the plankton layer is
regarded as a hundred fathoms thick. We have considered only that over
the mid-oceanic depths, but that of the shallow margins is different
simply in the absence of some purely pelagic creatures, and in the
presence of vast hordes of eggs and larvæ of the animals rooted in the
sand or attached to the rocks and weeds from high-water mark down to a
comparatively short distance below low-water mark. These I shall speak
of more completely hereafter.
Before that, however, I want to say a few words in regard to the
extraordinary inhabitants of the ocean's depths--depths which in some
places exceed the elevation of the highest mountains on the land.
The conditions under which animal life exists there are vastly
different from those at the surface, and it is not surprising to find
these creatures of an extraordinary character. The pressure exerted
by water on anything lowered into it increases at a rapid rate as the
object sinks, so that at a depth of only 500 fathoms it equals about
100 times the pressure at the surface. This contributes to the density
of underlying waters; the saltiness of the sea also adds to the water's
density, but this decreases slightly from the surface downward. More
important than density in its effect on living things is temperature.
In the Sargasso Sea in summer the water at the surface will indicate
about 52 degrees F., and at 100 fathoms of depth 48 degrees, below
which it diminishes slowly to a little below the freezing point--32
degrees F. The water below a few hundred fathoms may therefore be
regarded as a series of layers measured by degrees of density,
temperature, etc., and this means a series of biological strata in
each of which the denizens are more or less limited by unfavorable
conditions above and below them.
A fourth factor conditioning deep-sea life is that of light. The
sunlight penetrates to a much greater distance than was formerly
believed; and experiments with photographic plates show that the blue
rays may sink as far as 800 fathoms, but the red rays go much less
down. Below that glimmer is absolute darkness, illuminated only by the
phosphorescent glow of the lanterns carried by the animals moving about
in that Stygian and icy abode--which would seem to us the most dreadful
fate to which any creature on the globe is born.
It has been said that the ocean depths seem to be divided into
horizontal zones, certain groups of animals being confined, when
adults, within limits of depth determined by conditions suitable
to them, one zone above the other. Practically, however, these
intermediate life-zones can hardly be defined, and vary in different
seas, and under changing conditions, as of season, and so forth.
Animals taken only by deep hauls of the nets within the tropics,
for instance, may be captured in cooler latitudes near the surface;
furthermore, the vertical distribution of fishes, as a class, may
differ from that of crustaceans as a class. Nevertheless it is true
in general that many sorts of pelagic animals dwell at intermediate
depths, from which, when they have become mature, they cannot either
rise or descend any great distance. Among them are representatives of
all the classes of marine life.
Let us now consider the creatures of the lowest level--those abysmal
depths where eternal cold, stillness, darkness, and equability unite
to make an environment so forbidding that human imagination would
refuse to people it with living beings; yet where life and strife do
actually exist, although by no means uniformly distributed. We know
most about it as it exists in the bed of the north Atlantic.
The real bottom animals are mainly fixed--sponges, hydroids,
sea anemones, bryozoans, brittle-stars, crinoids, brachiopods,
holothurians, worms and mollusks. They are nowhere numerous remote
from a shore, and below 2,500 fathoms are very scarce, to judge by
the results of dredging. Their food comes wholly from the surface,
apparently, some catching it as it falls and others sucking it out of
the ooze. Moving about among these, and feeding on them, is a scanty
population of snails, squids, crabs, and fishes, making their living
upon or close to the bottom; and a larger and more varied company of
relatives swim in the water above them up to, say, the 2,000-fathoms
line. All these are of forms different in many respects from kindred
species at or near the surface; and some brought up by the deep-sea
dredge can hardly be distinguished from fossils entombed in the oldest
fossiliferous rocks--so unchangeable is the environment in which their
race has been propagated for perhaps fifty millions of years.
Through these dark abysses swim fishes with extraordinary and grotesque
adaptations to their conditions. All are small, rarely six inches long,
often less than an inch, yet armed to the teeth. This is especially
true of the families Stomiatidæ and Sternoptychidæ, in which one finds
fishes of the queerest shape, with big heads and a savage array of long
sharp teeth. All are voracious, for food is scant and must be fought
for; and some, as Chiasmodus, have mouths so capacious that they often
swallow fishes larger than themselves, when their stretched stomachs
hang beneath their slender bodies like the yolk sacs of newly born
trout. All are dark in color, brown, blue or violet marking the abyssal
species. Some of them have light-giving organs; and this was formerly
regarded as a peculiar possession of deep-sea fishes, enabling them to
see their prey in the gloom of their habitat, but it is now known that
light-giving organs are especially characteristic of pelagic fishes of
the region between the surface and 250 fathoms of depth. It must be
remembered, however, that the sedentary invertebrates of the bottom
glow with phosphorescence.
This outline of a vast body of information shows that the waters of the
oceans are everywhere inhabited, to their uttermost deeps, by living
beings; that these are adapted to various circumstances, and so form
faunas of local extent and character; and that probably the sea derived
its wealth of population--at least all that part superior to the
monads--from the land, beginning with the earliest dawn of life on the
globe.