SHALLOW WATER FAUNA.
Having considered thus briefly the general conditions under which the
animals of the sea must live, we may now consider more in detail the
special conditions of shallow water life.
In water of only a few fathoms in depth, the direct light of the sun
is capable of reaching and influencing all living things that occur,
either at the bottom, at the surface, or in the intermediate waters.
Great as the influence of direct sunlight must be upon animals, it
is even greater upon plants. Nearly all the Sea-weeds are, like the
plants of the dry land, dependent upon carbonic acid gas dissolved in
the water for one of the most important constituents of their food,
but it can only be absorbed by the plant in the presence of sunlight.
It is possible, therefore, for Sea-weeds to flourish in the shallow
waters of the sea, while they are necessarily absent from the deeper
and darker regions to which the rays of the sun cannot penetrate.
Everyone knows that in the shallow waters of our own coast there is
in many places a dense tangle of Wracks and long, flat ribbon-like
Sea-weeds growing on the bottom, and that on and amongst these weeds a
rich harvest of animals awaits the eager shore collector. It is true
that there are vast fields of sand on which the Sea-weeds are few and
far between, but we may say that wherever they can obtain a secure
foothold in the shallow waters of the British coast there they will
grow and multiply in great profusion. We must not, however, jump too
hastily to the conclusion that the same is true for all parts of the
world. The British coasts are particularly rich in Sea-weeds, in fact
a distinguished botanist once said that they probably present us with
the greatest number of genera and species of any coast line of the same
extent in the world. In the Temperate regions of both the Southern and
Northern Hemispheres there is generally a rich Sea-weed flora, but in
the warmer regions it is less luxuriant, and on the Coral-reefs of the
Tropical seas it is remarkably poor.
Turning our attention for the present to the Temperate regions, let us
consider the influence that the Sea-weeds have upon the animals of the
shallow waters. In the first place we find that they afford shelter and
support for a large number of animals which could hardly live without
them. In the roots of the weeds may be found little Crabs and Molluscs,
which occur nowhere else; and clinging to the long waving branches are
many forms of Sea-anemones, Zoophytes, flattened limpet-like Molluscs,
Ascidians, and other forms of animal life. The great forests of weeds
are also the haunts of many queer Fish, Prawns, Crabs, and Sea-slugs,
which hunt their prey or hide from their enemies amidst the shelter of
the stems and branches.
[Illustration: +Fig. 3.+
The common Pipe-fish.]
Now many of these animals, which together make up the Fauna of the
Sea-weed region, have assumed, in the course of the ages of evolution,
not only the colours of the Algæ on which they live, but in some cases
even forms which render them at first sight more like plants than
animals. The slender Pipe-fish, for example, which is not uncommonly
found amongst the bright green Sea-weeds of our coast, is wonderfully
similar, both in form and colour, to the weeds on which it lives. The
Sea-horses, which have such a curious form out of the water, in their
natural surrounding resemble the weeds so closely that they may be
easily overlooked. A still more remarkable example is to be found in
the curious fish Phyllopteryx, in which the body is provided with long
branched processes, making it we may suppose much more difficult to
distinguish in its natural haunts than even the Pipe-fish or Sea-horse.
[Illustration: +Fig. 4.+
Phyllopteryx.]
Many other examples to illustrate this feature of the Sea-weed Fauna
could be quoted, but sufficient has been said at present if we have
indicated to the reader the manner in which, by the indirect influence
of light, the form and colour of animals may be modified for their
life among the marine plants. The presence of light, however, modifies
directly the character of the animals themselves in many respects.
[Illustration: +Fig. 5.+
Diagrams of Eyes of _A_ Whelk, _B_ Lobster, _C_ Scallop, _D_ Fish.]
The statement that many of the animals of the shallow water are
provided with eyes because there is light is, when carefully examined,
found to be strictly true, however anomalous it may seem to be. There
must have been light in the shallow waters of the sea when the first
primordial forms of life made their appearance, and it was this light
which, step by step, led to the evolution of the most complicated and
perfect forms of eye from the simplest pigment spot of the Protozoan
to the eye of the Lobster and the Fish. We may say that all animals
that freely swim in the shallow waters or that crawl and creep on the
rocks and sands at the bottom are provided with eyes. The Fish are
provided with a pair of eyes which present us with the same general
features that we find in the other Vertebrates. Lobsters, Crabs and
Prawns have each a pair of stalked eyes, which are probably as perfect
in their functions as the Vertebrate eye, although built up on an
entirely different model. Whelks, Winkles, and other Gastropods,
although so slow in their movements, have each, upon or close to their
tentacles, a pair of minute eyes, which are much simpler and probably
far less perfect in form than the eyes of their more highly organised
neighbours. Even the little Jelly-fish, Star-fish, Sea-urchins, and
other creeping forms of life, are provided with specialised pigment
spots, which we have good reason to believe enable them to perceive the
rays of light.
But we must notice that it is only the animals that are capable of
locomotion from place to place that need these organs of vision,
and that the stationary forms are blind. In the large class of the
Lamellibranchs, for example, to which the Oyster and the Mussel belong,
we find that with a few exceptions there are no eyes. These animals,
after the first few stages of their life are past, settle down into
the sand, or fasten on to a rock and remain there until their life
is done. Their food, consisting of the minutest specks of animal and
vegetable life, is brought to them by the sea-currents; they do not
need nor desire to seek the society of their relations, and when their
enemies approach they resign themselves almost without an effort to the
inevitable. To such animals eyes would be useless, and so nature has
withheld them.
There are, however, a few Lamellibranchs that possess good eyes, eyes
that are almost as complicated in their structure as the Vertebrate
eye; and not a single pair only, but sixty, eighty, a hundred, or even
more, may be found on a single individual. Such a Lamellibranch is the
common Scallop which may be seen in many of our fishmongers’ shops.
Unlike most of the group, this animal is able to make long flights
through the water by the flapping together of its shells, and there
can be no doubt that on the approach of danger it uses this method of
locomotion for escape. One of its most deadly enemies is the Star-fish,
which forces the shells apart and sucks out the flesh by means of its
protrusible stomach. When a Star-fish is placed in an aquarium in which
there are some Scallops, lying, as they do, on their sides, with the
valves slightly open, showing the double row of gleaming metallic eyes
on the margin of the mantle, the Star-fish immediately moves, with what
in such an animal may be considered extraordinary rapidity, straight
in the direction of the Scallop. Before, however, it reaches its prey,
the coveted victim gives four or five vigorous flaps of its shells and
swims away to another part of the tank. This suggests that the eyes
of the Scallop are used in the light as a means of giving warning of
the approach of an enemy, and they are found in the Scallops only,
among common British bivalves, because they alone possess this power
of swimming away. Of course if there were no light in the water the
eyes would be useless, and it is an interesting fact that the Scallops
which live in the darkness of the great depths of the ocean are quite
blind.
[Illustration: +Fig. 6.+
A branch of the Zoophyte Obelia.]
Very interesting examples of the connection between the presence of
eyes and the power of locomotion are found among the sedentary or fixed
forms of life. The great class of Sea-squirts or Tunicates includes a
number of genera, most of which are, in the adult condition, immoveably
fixed to the bottom, and in that stage have no eyes; but the eggs
which they produce give rise to little creatures, like tad-poles in
form, which swim about freely in the water. Each of these tad-poles
has a large eye in its brain, which remains so long as the animal
leads a free life. As soon, however, as it settles down upon the rock
which is to become its permanent resting-place through life, the eye
and the organ of locomotion, the tail, both degenerate and ultimately
disappear. Again, we often find upon rocks, Sea-weed, old shells, and
the like, some curious delicate branching organisms called Zoophytes.
Notwithstanding their general resemblance in form to Sea-weeds, these
Zoophytes are known to be animals. Each tuft or branch is formed by
a number of delicate little Polyps, each with a crown of tentacles
round a terminal mouth. The Polyp cannot move away from the branch it
grows upon, nor the branch from the stem, nor the stem from the rock
on which it rests, and none of these minute creatures are provided
with eyes. There are, however, a few Zoophytes that give rise to buds,
which grow into the form of minute Jelly-fish (or Medusæ, as they are
called); and these, becoming detached from the parents, swim away and
lead an independent existence. These Medusæ are in many cases provided
with simple little eyes. During their short life they are drifted away
by the sea-currents to some distance from their parents, and produce a
number of eggs which are capable of developing into a new fixed colony
of Polyps.
[Illustration: +Fig. 7.+
Medusa produced by Obelia.]
We may consider it one of the maxims of Science that in a population
of animals which possess eyes, colour as well as form is of extreme
importance. As well as the alteration in form that takes place in the
animals living among Sea-weeds, we find a modification in colour in
the creatures dwelling among rocks or on the sand, so that they may
resemble the ground on which they live. No more striking example of
this could be found than in the common Shrimps of our coasts. Anyone
who has watched them in the sea-pools must have been struck with their
close resemblance to the sand. In fact it is only by close observation
that they can be distinguished from it. In an aquarium, too, it
may be observed how very much the upper sides of Soles, Flounders,
Turbots and other flat fishes are like the sand in colour, while the
under sides are almost invariably pure white. The black colour of the
Lobsters, speckled and striped with blue, has a close resemblance to
the holes and crannies of the rocks among which they live. The bright
transparent green Prawns are almost invisible as they move about among
the Sea-weeds, and Sea-slugs assume all manner of beautiful colours
according to the ground on which they feed.
On the Coral-reefs of the warmer regions of the world the pools that
are left when the tide goes down are characterised by their brilliancy
of colour. The bright purple, green, and yellow tips of the Coral
branches, the red and bright green Sponges, and the white pieces of
dead and broken Corals make up a scene of beauty which can only be
compared with a bed of variegated flowers. Here the Fish, Prawns, and
other moving animals have assumed the most gorgeous colours in patterns
of spots and stripes which verily astonish the naturalist when he
sees them for the first time. The great Sea-perches, with their sides
covered with bright red or brown blotches, the curious Trigger-fish,
with bright red, blue, or yellow bands crossing their bodies, the
banded Lobsters, and the spotted Cuttlefishes, strange and conspicuous
as they may seem when they are taken out of the water, are in life but
in harmony with their surroundings, and, in reality, less conspicuous
than they would be if less brilliantly arrayed.
I remember on one occasion as I was watching some expanded Polyps in
a little shore pool in the Tropics, I noticed something suddenly move
close to the coral block on which I was standing. As there was no
escape from the little pool, nor any holes in which the creature could
hide itself from my observation, I searched with diligence to find it
again, but for a long time without avail. Suddenly it moved again, and
then I saw, resting on the brilliantly coloured corals, a remarkable
little Shrimp called by zoologists _Stenopus_. Its body, which was
almost transparent, was marked by a number of bands of a bright red
colour, it had enormously long antennæ similarly banded, and its legs
and body were covered with short red-tipped spines. When I succeeded in
safely landing it in my collecting bottle I felt perfectly astonished
that I had been so baffled by such a lovely little jewel, so bright, so
strange, and so generally conspicuous did it then seem to be.
The colours of the animals I have referred to hitherto may be accounted
for by their need to escape the attention of their enemies, or to
avoid detection in the pursuit of their prey; but the animal colour
problem is not yet exhausted. The beautiful colours of the Anemones,
Corals, Sponges and many other sedentary forms of animal life, and
the marvellous patterns on the shells of Molluscs cannot be due to
these causes. Many ingenious theories have been brought forward to
explain away the difficulty, but none of them seem to be perfectly
satisfactory, and consequently it is unnecessary to enunciate them in
detail in this small book.
The character of the bottom of shallow water, especially in the
neighbourhood of the coasts, presents us with so many variations
that it would be a long task to consider in detail all the different
adaptations that the animals exhibit. The Fauna of the sand, shingle,
and mud at the mouths of rivers, of the rocks of iron-bound coasts,
and of the coral-reefs, each present us with many curious kinds of
modification of form and structure. A brief reference to one or two
characteristic regions must be made before passing on.
The sandy bottoms which are so prevalent, not only upon our coasts,
but in nearly all parts of the world, invariably support a Fauna with
many curiously altered forms. In walking across the sands at low water
we may have often noticed many worm-like and twisted columns of sand;
these are the casts of the common Lug-worm, which is a favourite bait
for many kinds of Fish. The Lug-worm lives in a U-shaped tube, which it
forms from a slimy secretion of the body; it feeds by swallowing the
sand in which it burrows, extracting from it as it passes through the
intestine whatever animal or vegetable food it may contain. There can
be little doubt that the sand is a very poor form of diet, and that
an immense quantity must pass through the body of the animal compared
with its weight in order to afford sufficient nourishment. It has been
reckoned that as many as 82,433 casts may be found on an acre of sand
where conditions are favourable for these Worms, and that they would
weigh together nearly 2000 tons. This would mean that the whole mass of
the sand would pass through the bodies of Lug-worms on an average of
once in every twenty-two months.
But the Lug-worm is only one of the many forms of life that burrow
in the sand. A very large number of bivalve Molluscs live with the
greater part of their bodies perpetually buried. Their organisation
is such that their food and the water that is used for respiration
can be brought to them by a tubular prolongation of the body called
the siphon. Some of these animals live much deeper down than others,
and while some have but feeble powers of moving either up or down in
their burrows, others can penetrate to great depths with extraordinary
rapidity.
[Illustration: +Fig. 8.+
A Bivalve Mollusc, showing below the foot with which it burrows into
the sand, and above the siphons.]
The shell of a Lamellibranch, called the Razor-shell (_Solen_), is
not an uncommon object of the sea-shore where stretches of sand occur.
When the animal is alive it has the power of burrowing down so quickly
that it is practically impossible for one man to capture a specimen
by digging, when it is thoroughly alarmed. Occasionally, however, the
sea itself, when lashed into fury by a storm, is a better digger than
the Solen is a burrower; and after a heavy storm in the Isle of Man I
have seen the shore littered with Solens scooped out of the sand by the
force of the waves and cast up with lacerated shells to fall an easy
prey to the Seagulls.
The sandy bottoms in shallow waters are also the haunts of many kinds
of Fish that are specially modified in form and colour to resemble
their surroundings. The large family of the Skates have their bodies
compressed from above downwards so that they can lie perfectly flat
upon the sand. Their upper surface is deeply pigmented, giving them
a general resemblance to the ground on which they lie; but to assist
them in escaping observation they have a habit of shaking their fins in
such a manner as to scatter a considerable quantity of sand over their
bodies. Thus it can well be understood that in the dim light of the
sea-bottom the little Fish and Shrimps, which form a large portion of
their food, may approach quite close to them without being in the least
aware of the danger into which they are running. The upper side of the
Skate is also armed with a number of sharp and hard spines, and in some
forms--called the Sting-rays--one of these, situated at the base of
the tail, is much larger than the others, and provided with muscles so
that it can be suddenly erected. In connection with this spine there is
a poison-gland and duct. The wound that is inflicted upon the arms or
feet of the fishermen by this formidable weapon of offence is said to
be of a very serious nature.
Some of the Skates, too, show another very interesting modification
of structure, which, however, is not directly associated with their
mode of life, but may be briefly referred to here whilst dealing with
the family. This is the electric organ. In the younger stages of the
common English Skate a small region of the muscular system on each side
of the base of the tail becomes changed into an electric organ, but
the discharges which it is able to give are so feeble that they can
only be appreciated by a galvanometer. In the Torpedo, however, the
electric organs are very large indeed, and situated one on each side
of the head. They can give a shock which is powerful enough to kill
small animals and to stun larger ones. With such a formidable weapon
of offence and defence, it is clear that the need for active movements
is considerably diminished, and the Torpedoes are described as being
exceedingly slow in progression and incapable of the violent movements
of other Skates.
[Illustration: +Fig. 9.+
The Common Sole, showing both eyes on one side of the head.]
The other Flat Fishes found on sandy bottoms belong to a different
group altogether, and are characterised by their bony skeletons and
other features. The Soles and their allies do not, like the Skates, lie
flat upon their bellies, but are _laterally_ compressed and lie upon
one side. The side which is habitually uppermost is of the colour of
the sand, and the other almost invariably pure white. In the course
of their development the Soles undergo very extraordinary changes in
form to reach the perfectly flattened condition of the adult stage.
These changes affect many of the organs of the body, but perhaps the
most interesting of all is the history of the eyes. In the young Soles,
which swim almost vertically, like the majority of fishes, there is
one eye o each side of the head, but as they grow older and gradually
take to the habit of swimming on one side, the eye of that side sinks
down into the head, and rotating as it goes passes through to the other
side. This process naturally leads to a considerable distortion of the
skull, so that the bones of the adult Sole show as complete a want of
symmetry as can be found in any Vertebrate.
[Illustration: +Fig. 10.+
The Angler.]
Some of the Bony Fish, however, that live on the sand are flattened
dorso-ventrally like the Skates. The Angler or Fishing-frog, for
example, is a Flat Fish, which is perfectly symmetrical. Like the other
Flat Fish its upper surface is coloured in such a manner as to resemble
the ground on which it lives. Its great mouth, armed with formidable
rows of sharp-pointed teeth, is directed upwards, and it receives its
name from a curious tentacle terminating in a brightly-coloured knob
which dangles over its mouth. The brightly-coloured knob looks no doubt
a tempting morsel to the little Fish upon which the Angler preys, but
the greed or curiosity, whichever it may be, that induces them to
inspect the bait leads them to the fate which follows one snap of the
great tooth-armed jaws.
The Fauna of the shallow waters where rocks abound also possesses many
peculiarities. In the first place we must remember that the rocks,
being firm and hard, present a basis upon which many of the sedentary
forms of life that would be swept away or smothered if they attempted
to live on the ever-shifting sands, can fix themselves. Consequently
the rocky bottom is characterised by a rich Fauna of those groups of
animals, which, in the adult condition, are immoveably fixed. If a
large stone or a water-logged piece of timber that has been at a depth
of a few fathoms for some months or years be captured in the dredge and
brought on board for examination, it is sure to present the observer
with a multitude of firmly-fixed creatures. Among them there is almost
certain to be found a number of small conical shells, made up of a
series of triangular plates, fixed to the rock by their bases. These
are commonly spoken of as Barnacles (_Balanus_), and they pass
through an interesting history. For many years they were considered,
from the character of their shells, to be allied to the Molluscs, but
an examination of the soft parts of the animal shows that, unlike any
Molluscs, they are provided with six pairs of jointed legs; and a still
further study of their anatomy proves beyond a doubt that they can no
longer be classified, with any pretence of scientific accuracy, with
that group.
[Illustration: +Fig. 11.+
Vertical section of a Balanus, showing animal
_in situ_ in its shell.]
The secret of their true relationship was not discovered until the
story of the development was worked out, when it was found that the
eggs they discharged each gave rise to a little larva called Nauplius,
which is provided with three pairs of legs like the larvæ of some of
the Prawns and their allies. The result of these observations then was
to prove that the Barnacles are really Crustaceans, notwithstanding the
fact that, unlike most of that group, they are, in the adult stage,
permanently fixed to the rocks.
On the same piece of stone there will most probably be found several
twisted or coiled tubes of lime, formed by a little Sea-worm called
Serpula. When living in the water the head of this worm projects from
its shell and expands a circlet of delicate tentacles, by means of
which the food is brought to the mouth. One of these tentacles is
specially modified and enlarged at the extremity to form a conical
knob, which, when the animal is retracted into the shell, closes the
aperture like a stopper.
[Illustration: +Fig. 12.+
Nauplius larva of a Balanus, enormously magnified.]
Then there may be found some spherical or lobate masses of a fleshy
consistency, white, pale pink or yellow in colour, and studded
with numerous star-shaped apertures. When these are allowed to
remain in a basin of fresh sea-water for some time, each one of
the star-like apertures opens, and a beautiful transparent little
Polyp with eight-feathered tentacles gradually unfolds itself, only
to be slowly withdrawn into the mass when the vessel is shaken or
otherwise disturbed. These Polyps form colonies, known as _Alcyonium
digitatum_.
Now it is to be noticed that none of these three examples of the
sedentary Fauna can move in the least degree from the rock or shell
to which they are fixed. When once the young larva has taken up its
position there it must remain until old age or some disaster brings
its life to an end. When they are first hatched from the egg that
is thrust into the water by the parent, they pass through a larval
stage that, like the Nauplius of the Barnacle, is active and free.
Then they are carried away from the parent stock partly by their own
active movements, but more particularly by the tides and currents of
the sea-water. At last a change in their structure occurs. They sink
to the bottom, become attached to a rock or stone, complete their
metamorphosis, and remain anchored to the spot for the rest of their
lives.
The number of different forms of animal life which constitutes this
sedentary Fauna of rocky coasts is very great indeed. In addition to
the Barnacles, Worms and Alcyoniums, there are numerous species of
Sea-anemones, Sponges, Corals, Zoophytes, Sea-squirts, and other forms,
and in _all_ these cases the eggs give rise to free-swimming
larvæ, by which the distribution of the species is effected.
Another group of animals, which forms an important feature of some
rocky coasts, are the Boring-molluscs. The Rock-borers belong to
many different species. Some of them, such as the Pholases, make long
cylindrical holes in chalk, and even in harder rocks. The Teredo is the
borer, commonly known as the Ship-worm, on account of its powers of
penetrating into timber. The long calcareous tube which it forms as it
works its way into the wood, gives it a superficial resemblance to a
large sedentary worm, but it is in reality a bivalve Mollusc, specially
modified in structure for its peculiar habits.
[Illustration: +Fig. 13.+
Sea-urchin with large thick spines.]
The next group of animals we have to consider in the Fauna of the rocks
includes all those that slowly creep or crawl, without possessing
any powers of rapid locomotion. Amongst these may be mentioned
the Sea-urchins and Star-fishes. The former possess spherical or
heart-shaped bodies covered with a formidable array of spines, among
which there protrude several rows of soft transparent tubes terminating
in little cup-like discs. Some of these tubes--or tube-feet as they are
called--are fixed to a rock, and the heavy body is slowly dragged after
them; another set is then attached, whilst the former is released to
obtain another hold a little further on. The progress is slow, but the
Urchin is able to climb the smooth face of an absolutely perpendicular
rock with perfect ease or to get over any other obstacles that may be
in its path. The Star-fishes are similarly provided with tube-feet,
but in their case these are confined to the lower surface of the body,
the upper side being entirely devoid of them. Star-fishes have a very
wide distribution in the sea, and occur on sandy shores as well as
among rocks and shingles. If a specimen be watched gliding slowly and
smoothly over the sea-bottom and then the mouth be examined with a
pocket lens, a doubt might arise in the mind of the young naturalist
as to the justice of the charge that is made against these animals of
their being the principal enemies of the hard-shelled Oysters. But the
charge is well founded, for if a Star-fish be placed in an aquarium
with an Oyster or a Cockle, or, in fact, almost any bivalve Mollusc,
it may be seen to clasp its prey in its arms and slowly but firmly and
surely force open the shells, and then protrude on to the soft parts
a long tubular stomach which gradually digests and absorbs them. The
Star-fishes, then, are undoubtedly to be reckoned among the most
voracious and destructive inhabitants of the shallow waters, and it is
probable that the covering of spines which we find so commonly among
shallow water animals is an adaptation to prevent or render difficult
the operations of these creatures.
The Gastropod Molluscs form another large and important group of
creeping animals of rocky coasts. On nearly all our own coasts numerous
Periwinkles may be seen clinging to the rocks at low tides, and if a
search be made in the deeper pools and on the rocks nearest to the
low-water marks many other species will be found of animals with
spirally-coiled shells which are included in this group of Gastropods.
The Periwinkles on the rocks might at first sight be thought to
belong to the sedentary group of animals, but when the water covers
them again, or when they are put into an aquarium, they may be seen
to protrude a head and an elongated slimy foot, which, gliding over
the surface of the rock, drags the great shell and its contents with
it. On the approach of danger the foot and head are withdrawn into
the shell, and the animal rests secure from many enemies that might
otherwise have found it a dainty morsel. Some of the Gastropods are
purely vegetable-feeders, but most of those living in shallow sea-water
feed upon Molluscs and other animals. It might well be a matter for
wonderment when the soft head and little mouth of a Gastropod, such as
a Whelk, are examined, that it is carnivorous and attacks and devours
animals as large as itself. But the anatomist shows us that hidden in
the recesses of that mouth there is a ribbon beset with numerous sharp
little teeth, which by a complicated mechanism can be worked backwards
and forwards in such a manner that it can bore a hole through very
thick and dense shells; and, the soft parts being reached, a tube is
protruded which dissolves and sucks them up into the animal’s stomach.
Many people must have noticed that numbers of the bivalve shells
that are cast up on the sand at low tide are perforated close to the
hinge by a neat little round hole. This is the hole made by some
predaceous Gastropod which, having killed its prey and devoured all
that is digestible of it, leaves its empty shells at the mercy of the
waves. Amongst the rocks numerous species of Gastropods are found,
some undoubtedly carnivorous, others herbivorous. The many beautiful
forms and colours that their shells assume may be seen in any good
museum or conchological cabinet. Some of them are very minute, others
are provided with a shell more than a foot in length; some are marked
with numerous coloured spots, others with bands or lines; some have
perfectly smooth shells, others are ribbed or spiny. It is extremely
difficult to account for all these modifications, partly because it
is impossible to study the animals alive in their natural habitats a
few fathoms below the surface of the sea, and partly because life in
the shallow waters must be so complicated that we are at a loss to
understand the value to a species of slight modifications in structure
such as these. The difficulty that has been found in explaining these
various forms and colours has led some naturalists to the belief that
they are of no importance to the species in the struggle for existence,
that they are, as it were, the accidental result of some process of
excretion, and not the outcome of a long series of slight changes,
bringing about at length an adaptation of form most suitable to the
habits of the animal. Such views are, however, to be accepted with
great caution, and most zoologists will be contented to wait until our
knowledge is much greater than it is at present, before wholly agreeing
with them.
[Illustration: +Fig. 14.+
Smooth-shelled Gastropod.]
[Illustration: +Fig. 15.+
Spiny Gastropod.]
Another great class of animals which has many representative forms
among the rocks is the group of Crustaceans. The Lobster, the Prawn
and the Crab are all familiar examples of this class. They may be
found by searching rock pools at low-water, or can be captured in
basket-work traps in places beyond low-water mark. When undisturbed
they crawl slowly over the rocks and weeds by their long jointed
legs, searching for their prey, but when alarmed the Lobster and
the Prawn can, by violent flapping movements of their tails, dart
rapidly backwards through the water, while the Crab beats a hasty
retreat sideways into some shelter among the rocks. Like many of the
Molluscs, the Crustaceans have a hard covering or shell to protect
them from many of the dangers to which soft-bodied animals would
be exposed, but a momentary glance at them would be sufficient to
satisfy the most inexperienced eye that there are many and important
differences in the character of the shells of these two great groups
of animals. One important distinction between them, however, might
well escape observation, and that is, that whilst in the Mollusc the
shell increases gradually in size during the life of the animal, in
the Crustaceans it cannot do so. In the Lobsters and Crabs the shell
is periodically cast off entirely, and for a day or two at each period
the skin of the animal is quite unprotected. A new shell is gradually
formed, and this is hardened and thickened until it assumes a form
similar to that of the one that has been lost, but larger. During the
moult the Crustacean usually hides itself in a hole in the rocks and
waits patiently until the new shell has grown.
[Illustration: +Fig. 16.+
A Cuttle-fish.]
The animals included under the popular names of Cuttlefishes, Squids
and Octopuses are also capable of crawling about among the rocks by
their long feeler-like arms; but they are in the habit, as well, of
making prolonged journeys through the water, by pumping the sea-water
through a tubular siphon situated on the under side of their bodies.
These animals possess in such a remarkable degree the power of changing
colour that they might be called the Chamæleons of the sea. As they
pass slowly through the water from one part of the coast to another
the colour of the skin changes so as to resemble the colour of the
rocks or weeds which are below them. These changes are brought about by
numerous little bladders in the skin which are filled with different
coloured fluids, and are worked by a complicated system of muscles
under the control of special nerves from the brain. When the colour
blue is predominant, it is found that all the bladders containing blue
fluids are dilated, the others being constricted; when the colour is
red the red bladders only are dilated, and so on; and as the nervous
response to the colour of the rocks perceived by the eye is practically
instantaneous, the change in the general colour of the body brought
about by the dilatations of these vesicles is extremely rapid. Many
other animals have the power of changing colour, but in no group is the
alteration more rapid and remarkable than in this order of Cuttlefishes
and Squids. Another very interesting feature presented by these animals
is their ability to discharge suddenly a cloud of inky substance into
the water. Their principal enemies are the Whales, Porpoises and some
of the larger Sharks and other Fish. When these animals approach, or
any other danger is feared, the Cuttlefishes discharge into the water
from a special bag, called the ink-sac, a quantity of black or brown
pigment which, diffusing rapidly, forms a cloud round their bodies, in
the obscurity of which they frequently escape pursuit. The well-known
Sepia of painters is obtained from these ink-sacs.
The last group of animals occurring among the rocks are those capable
of vigorous swimming movements. Many Crustaceans, such as the Lobsters
and Prawns, are capable, as has just been pointed out, of swimming
rapidly through the water by means of their powerful tails. But this
swimming power is only accessory to that of crawling or creeping, and
is used merely when the animals are disturbed. Cuttlefishes and their
allies seem to spend a considerable portion of their time in floating
or swimming in the water, but still they do crawl about among the
rocks, and very probably attack and feed upon their prey entirely upon
the sea-bottom.
The members of the Rock-fauna which belong to the class of Fishes
very rarely rest upon the sea-bottom at all. They are not, as a rule,
provided with limbs which are capable of crawling or creeping; and
their mouths are adapted for catching food that is swimming, or of
browsing upon or nibbling at fixed forms of life while their bodies
are still floating in the water. Nearly all the animals living among
the rocks that we have hitherto spoken of, have some organs or some
specialised portion of the body-wall for resting upon or for attaching
themselves to the bottom.
[Illustration: +Fig. 17.+
The Wrasse.]
The Anemones are attached by their bases; the Sea-urchins and
Star-fishes crawl by means of their tube-feet; the Gastropod creeps
over the rocks by its broad flat foot, and the Octopus stretches out
its muscular arms and drags its body along by the numerous suckers they
bear. In the Flat Fishes of the sandy and gravelly shores we usually
find a white under surface on which they rest when waiting for their
prey. Among the Fish which frequent the rocks, however, such as the
Cods, the Whitings, and the Wrasse, there are no such surfaces. The
body of the Fish is usually more rounded in form, and no well-marked
limit can be assigned to the coloured upper surface and the pale
silvery under side. These Fish are in nearly all cases rapid and
powerful swimmers, rushing through the water after their prey, or away
from their enemies, by vigorous lateral movements of their tails.
A curious exception to this general rule among the Fish occurs in the
family of the Lump-suckers. These Fish are found on the English, but
more commonly on the Scottish coast, and are distinguished by the
presence of a sucker, formed by the throat fins, on the under side of
the head. By means of this the Lump-sucker is able to attach itself
so firmly to rocks and stones that it can only with considerable
difficulty be removed from the object to which it is attached.
Of the Fish that are commonly found among the rocks, a very
considerable number migrate from time to time to other parts of the
sea, and may be caught in the trawl on sandy or shingly bottom, or even
in the drift nets at the surface of the sea. A large number of Fish
belonging to the family of the Codfishes frequent the rocks during a
part of their lives. The Pollack is distinguished from most of the
others by the absence of a barbel on the lower jaw, and is one of the
persistent rock frequenters. In the adult condition it feeds almost
entirely upon other Fish, although in the younger stages of its life
Crustaceans, Worms, and other Invertebrates seem to form the bulk of
its food.
The true Cod and the Haddock seem to have a much wider range,
occurring on shingly bottoms, where they are frequently caught in the
fishermen’s trawls, as well as in the neighbourhood of rocks. The Hake
feeds principally upon Pilchard, Herrings and Sprats near the surface
of the sea.
It is an interesting fact that the Fish belonging to this one family
have very different methods of feeding. The Cod and the Pollack both
hunt their prey principally by day-light. The Pollack is guided by
its sight alone, the Cod-fish is assisted by its barbel, which acts
as a delicate feeler or organ of touch. The Hake, on the other hand,
retires into deep water during the day-time, and only comes to the
surface at night to feed. Similarly the Rockling hides away in holes or
under stones during the day-time, and only comes out to hunt for the
Crustaceans and little Fish upon which it feeds at night.
The development of these Fish presents some features of interest, as
showing us the changes which occur in habit during their life-history.
The eggs of the Cod are buoyant, rising to the surface of the sea as
soon as they are spawned. In twelve or fourteen days, according to
the temperature of the water, the larvæ are hatched and swim about in
large numbers just below the surface, feeding upon minute Crustaceans
and other animals. A little later the young Cod frequently shelter
themselves under large Jelly-fishes, feeding upon the numerous
parasites which infest those creatures. When they are about a year old
they are found feeding among the sea-weeds on rocky coasts, and they
migrate into deeper water when they reach their full size.
[Illustration: +Fig. 18.+
The John Dory.]
The John Dory is a remarkable Fish, by no means confined to the rocks,
as its food often consists largely of Pilchards and Herrings. It
differs from most of the Fish of similar habits in being remarkably
flattened from side to side. This feature seems to be of service to
it in the peculiar manner it has of securing its prey. Mr Cunningham,
to whom we are indebted for this interesting observation, says:--“It
does not overtake (its prey) by superior speed like the mackerel, nor
lie in wait for it like the angler, but stalks it and approaches it by
stealth. It is able to do this in consequence of the extreme thinness
of its body, and the peculiar movement of its hinder dorsal and ventral
fins. The dory places itself end on towards the fish it desires to
devour, and in this position it is evident that it excites no alarm on
the part of its prey. The appearance of the dory seen in this way is
a mere line in the water, to which no particular significance can be
attached. I have not particularly noticed the effect of the ribbons of
membrane, which project from the dorsal fin. But I have observed that
the movements of the dory are very gradual except in turning; it alters
the position of its body by a turn of the tail or side fins, and then
slowly swims forward by vibrating the second dorsal and ventral, a
movement which causes very slight disturbance of the water. The whole
appearance of the dory in these actions is suggestive of suppressed
excitement, his eyes being fixed on his prey.”
* * * * *
We have now considered some of the chief features of animal life in
the shallow seas, the illustrations being taken principally from the
regions of our home shores. The shallow waters of the tropics present
us with so many phenomena of striking interest and importance, that
the subject would be most incompletely treated if they were left out
of consideration altogether, and, therefore, our next chapter will be
devoted to them.