SHALLOW WATER FAUNA OF THE TROPICS.
The shallow waters of the Tropical seas present us with so many
different conditions of tides, of coast lines, of temperature, of
liability to storms, and of other natural phenomena, that we find an
infinite variety in the general character of their Fauna. Just as on
land, we find in one part of the Tropics a dense forest, and in another
a dry desert, so in the Tropical seas we find on one coast a crowded
population of animals and plants, and on another a sandy bottom, which
is, comparatively speaking, lifeless. In order to bring before the
reader some of the principal characters of animal life in the shallow
waters of the Tropics it will be well to confine his attention to
one part of the world which is fairly well-known--namely, the Malay
Archipelago--and refer only in passing to other localities. The most
characteristic feature of Tropical coasts is the Coral-reef, and
nowhere in the world may it be seen in more exquisite variety than in
the archipelagoes of the East. Although, however, these vast structures
are so abundant on some coasts, others seem to be entirely without
them. They are not found at all on the Western coasts of America or of
Africa, and even in some regions of the larger islands in the Pacific
and the Indian Oceans, many miles of coast line may be devoid of
them. These curious and interesting variations in the distribution of
the reefs can be explained, but the explanation will be more easily
understood when their general features have been described.
It is a well-known fact that the great masses of limestone which
compose the reefs are formed by the activity of countless thousands
of minute animals, but the popular idea of the general form of these
animals has been very much misled by the unfortunate term “Coral
insect,” which has crept into many books of travel, and the leading
articles in the newspapers. The word “insect” is used by zoologists
as a general term for certain air-breathing animals that are widely
distributed over the surface of the earth. Many of them are extremely
tiny, and hence the natural mistake has arisen in the untrained mind
that all minute animals are insects. It might clear the ground a little
if the reader would note at once that “insects” are very rarely indeed
marine in habit. If there is a need for a popular word for the animals
that form coral, it should be Coral “polyps” or Coral “anemones.” The
word “coral” has, from the zoologist’s point of view, a very indefinite
meaning, for it is applied to the hard skeleton of carbonate of lime
formed by certain Sea-weeds, Sponges, and Worms, as well as to that of
Coral-anemones and other Polyps. In many places on the British coasts
the sea-bottom is very largely composed of a branching Coral formed
by a true Sea-weed called _Lithothamnion_, and in other places
very large lumps of rock are made by a Worm named _Filograna_. In
the Tropical regions, too, the well-known Nullipores, which in many
places play an important part in the formation of Coral-reefs, are of
vegetable, and not animal origin.
However, the greatest part of the Coral-reef is made by animals closely
related to the Sea-anemones, living together in colonies; and of all
the different kinds of Coral-polyps, by far the most prolific as a
reef-builder is one which will be referred to in these pages as the
Madrepore.
[Illustration: +Fig. 19.+
Polyp of a Madrepore Coral, showing the canals by
which it is connected with its fellows.]
Let us consider now the manner in which the Polyps form the Coral. In
a very old work on the natural history of Corals a statement is made
to the effect, that the Polyps construct the Coral in much the same
way as Bees build their hive, or a Bird its nest. This very erroneous
view coincides closely with ideas which might easily be gained by a
casual observation of corals in a museum. The lime is not, however,
collected as such from the sea by the Coral-polyps and plastered
round their bodies to form a house or shelter, but it is formed as a
secretion by the activity of certain organs of the animal’s body, and
is consequently a true shell or skeleton. In a Coral, which is formed
by a colony of numerous Polyps, the shell secreted by each individual
fuses on to those formed by its neighbours, and thus a communal
shell is formed which may assume a most complicated branching, bushy
form, according to the species of the Coral and the conditions that
are favourable or unfavourable to the nourishment and growth of the
different parts of the colony. In such a Coral as the Madrepore every
individual Polyp is connected with its neighbours by a system of
branching canals; and as spaces are left for these when the shell is
formed, the dried Coral is perforated by numerous tubular pores, and
has a soft, spongy texture which can be easily crushed into a powder if
trodden upon.
In other Corals the canals of communication between the Polyps are
entirely at the surface, and the shell that is formed is much harder
and imperforate. In others again colonies are not formed, but each
individual grows to a considerable size and remains independent of its
fellows all its life.
These, then, are three of the more important varieties of Corals found
on the reef, the Perforate, the Imperforate, and the Solitary Corals;
but it must be remembered that all the Corals of the reef cannot be
included in these three groups. The varieties are much more numerous
and in many cases much more complicated and difficult to understand.
Although the Coral-reefs of the Tropical world have a general
resemblance to one another, the differences in detail are so great that
it is impossible to describe any one as typical. In sailing over the
edge of a reef near the coast on a calm day, when the water is so clear
that the bottom can be seen at a depth of 8 or 9 feet, the reef may,
in some regions, be observed to change in character every few minutes.
In one spot there may be clumps of living Corals surrounded by beds of
fine white sand; in another there will be great stretches of branching
Madrepores; in another Madrepores, Mushroom-corals, the Imperforate
Brain-corals, Sponges, and many other forms of life will be clustered
together; while further on the predominant features will be the soft
and slimy Sarcophytums, looking like large green toad-stools, some
lumps of Organ-pipe Corals and a few colonies of the Blue-coral.
On other coasts I have wandered for miles along a reef mainly composed
of endless tangles of Madrepores, with very little variation indeed in
the general form of the Corals, in the character of the Sponges and
Sea-weeds that grow with them, in their colour or in any other detail.
Anyone reading the many accounts of Coral-reefs that have been written
by travellers, must be struck with their inconsistency as regards
many particulars; and in no one point are they more inconsistent than
in the description of the colours--some writing in glowing terms of
the beauties of the sea-gardens, and others complaining that their
charms have been grossly exaggerated. As a matter of fact some reefs
have a prevailing dull green or brown tone, while others exhibit all
the colours of the rainbow in their more brilliant shades and tints.
Another cause of the discrepancy is that some reefs can only rarely be
approached in a small boat owing to the breakers that dash over them;
whilst in the Tropical calms a tiny canoe can with perfect safety be
manœuvred over the reefs during nine months of the year. From my own
experience I can assert that it would be difficult indeed to exaggerate
the glorious beauty of some of the reefs in the Malay Archipelago,
more particularly of those where many different kinds of Corals may be
seen in close proximity to one another. On such a reef, for example,
there may be seen Madrepores with bright violet growing-points to their
branches, orange or red Fan-corals, bright brick-red Sponges, yellow
Sarcophytums, emerald green Organ-pipe Corals and dozens of other
forms of animal life in every imaginable colour. When seen from a boat
through two or three feet of water, these portions of the reef look
more like a beautifully planted flower-bed than a mass of animals; but
the simile is not a complete one, for the branches of the Madrepores,
the great knobs and lumps of the Brain and Organ-pipe corals, the
fronds of the Gorgonias and other forms make a wild mass of organisms
resembling a tangled thicket or a miniature forest. At low-water of
spring-tides the living reef is partly left exposed, and then the scene
changes, for the Polyps retract their tentacles after the manner of the
Sea-anemones and retreat as far as possible into the shelter afforded
by their shells.
[Illustration: +Fig. 20.+
Chætodon.]
The interest of the living reefs is, however, by no means confined to
such organisms as the Corals and Sponges, which are immoveably fixed to
the bottom; for numerous brightly-coloured Star-fishes, Sea-urchins,
Brittle-stars, Sea-slugs, and their allies crawl about among the
branches and the débris of the dead Corals; while Crabs, Lobsters and
Shrimps of many kinds may be seen swimming or crawling in search of
their prey, and the marvellously striped and spotted Coral fishes dart
hither and thither in the thicket, or remain hovering in the water
among the Corals. The whole scene is most fascinating. As the boat
slowly drifts along, new and strange creatures are constantly coming
into sight and disappearing again. Here the writhing arms of a bright
blue Brittle-star may be seen embracing the stem of a Coral-branch;
there a curiously flattened Chætodon, with its body marked by great
diagonal yellow bands, is nibbling at the young, tender branchlets;
in another place four or five Sea-urchins with very long and slender
spines are lying apparently motionless on the bottom; while a little
further on a long black Slug-like creature, the famous “Trepang” of
commerce, is slowly wending its way across the reef. Now and again a
large shoal of little Fish or a small party of Cuttlefishes may be
seen, and these may rapidly be dispersed in all directions by the
sudden dash of a Sea-perch or a small Shark. The interest is so varied,
so many-sided, in these scenes of animal life that the attention of the
naturalist is with difficulty kept to any particular point. The feature
which is perhaps the most striking is, however, the wonderful variety
in the colours of the animals and of the character of their markings.
If we consider the Fishes alone, we find some of them have broad
yellow bands running diagonally across their bodies, others have thin
longitudinal stripes of blue and yellow, some have a uniform bright
red colour, and others again have their red skins speckled with blue
spots. It would take more than a whole chapter of this book to describe
even the principal varieties of pattern found on one such Coral-reef,
but the main fact that has to be related is that where the reefs are
built by brightly-coloured Polyps, there we find these curiously marked
Fishes. There can be little doubt that the marking and colouring does
give a certain amount of protection to them. Numerous individual cases
have been mentioned of Fishes which resemble some particular Sea-weed
or Coral; but this general statement is the important one, that on
a parti-coloured background the striped and speckled Fish are less
conspicuous than those that are modestly attired.
The Fishes of the reefs, however, have other means of protection than
that afforded by their colours. The Trigger-fish and Coffer-fish, for
instance, have a body encased in closely fitting hard, thick scales,
so that they might almost be called “armour-plated fish”; and the
Globe-fish bristle all over with long and extremely sharp spines. In
the Trigger-fish there is a curious modification of the three front
spines on the back, to which their name is due. It is not known
precisely how they act, but they probably form an effective weapon of
defence. In the same family of Fish we frequently find on each side of
the tail two or three rows of sharp spines, which may also be regarded
as defensive. In the family of the Surgeons there is only one of these
spines on each side of the tail, and it is much larger than any of
those in the Trigger-fish, and capable of being folded down into a
case in the skin like a clasp-knife. It is said that these spines are
connected with a poison-gland, and can give very severe and painful
wounds, like the poison spines of our European Sting-rays and Weavers.
[Illustration: +Fig. 21.+
Globe-fish.]
A few days’ hunting on a Coral-reef will reveal the fact that it
supports a very considerable population of Crustaceans. They are not
at all obvious at first to one who has had no previous experience of
reef-work, partly because they resemble the general appearance of
their surroundings very closely, and partly because of their habit
of remaining perfectly motionless when first alarmed. In form, many
of them are, like the Lobsters, Crabs and Prawns of our own coasts,
but their colours and markings are, like those of the reef-fishes,
characterised by their brilliancy and their arrangement in bands and
stripes. The smaller ones can be caught after a little practice with
a simple hand-net, but the larger ones are more easily captured by a
rattan noose in the pools, after the blocks of Coral are loosened by a
pick-axe and slowly turned over.
The Coral-reef is a favourite hunting-ground for the conchologist,
some of the largest and most beautiful shells in the world being found
amongst the Corals. In Celebes, the giant bivalves, the huge Tridacnas,
which are sometimes two feet across, and whose shells have been known
to weigh as much as 500 lbs., may be seen wedged in among the Corals.
The mantle of the living animal presents to the observer a wonderful
display of colour as it lies in the shallow water with its shells open.
The animal is eaten by the Malays, who roast it on a tripod spit over
a fire, and cut it into steaks. A fair-sized Tridacna will afford a
good meal for four or five men. The great Cowries, Helmet-shells, and
many other species may be found in hunting over the reefs, but their
beauties are frequently hidden, when alive, by the coal-black mantle
which folds back over the shell as they crawl along. It must not be
supposed, however, that all the shells of the reefs reach to such
enormous size as those we have hitherto mentioned, for a rich harvest
of species awaits the eager conchologist who hunts for the smallest
shells he can find in the pools. Within the last few years a large
number of new species of small Molluscs have been described from the
coral seas, many of which do not attain to a total length of more than
⅛ of an inch when perfectly adult, so that the range in size of this
class of animals is very great indeed.
So much has been said about the Madrepores, the Imperforate Corals and
the solitary Corals of the reef, that the impression might be left
that all the Polyps of the Tropics differ from those of the Temperate
regions in the fact that they form shells or skeletal structures.
This is by no means the case, for there are many species of true
Sea-anemones and other Polyps to be found on Coral-reefs which make
no shell at all, and others in which the body-wall is strengthened by
numerous, but very minute spicules or grains of lime which, on the
decomposition of the animal’s body, fall down into a shapeless powder
or sand.
True Sea-anemones are not very abundant on the reefs of North Celebes,
but many species have been found on the Barrier-reef of the Australian
coast, and among them specimens reaching the gigantic size of two
feet in diameter--the largest size attained by single individuals of
the class of animals to which the Anemones belong. Another family of
Polyps called the Clavulariidæ belonging to the Alcyonarians has some
species which make no skeleton of calcium carbonate. An illustration
of one of these is given below (Fig. 22). Another species, called
_Clavularia viridis_, which forms a few spicules in its body-wall,
has a very wide distribution in the East Indies. In some places patches
of it may be seen several square yards in extent, and the crowds of
little Polyp heads with their eight-feathered tentacles waving to and
fro with the pulsations of the tide, is a sight that excites immense
interest and admiration in the mind of the observer.
[Illustration: +Fig. 22.+
Stereosoma, one of the Clavulariidæ.]
One word more about the Corals. Where they are so abundant in number
and species, where rocks hundreds of miles in extent are mainly
composed of their shells and skeletons, it might be thought that a
rich profit could be gained by collecting ship-loads of the Coral that
is used for making beads and brooches by our jewellers. Any expedition,
however, fitted out for this purpose would end in disastrous
failure, for the Precious-coral is not known to occur anywhere in
the neighbourhood of Coral-reefs, but the fishery is confined to
certain parts of the Mediterranean Sea. Species closely allied to the
Precious-coral, but of an inferior colour, have been found in the
Japanese waters. None of the Coral structures found on the reefs have
the same delicate salmon-pink colour and probably none of them are hard
enough to take a good polish.
The Coral-reefs which occur in different parts of the Tropical world
were considered by Darwin under three heads,--Barrier-reefs, Atolls,
and Fringing-reefs. The distinction between these three kinds of
reef is not one that can be insisted upon scientifically, but the
arrangement is convenient for purposes of description.
The Barrier-reefs (Fig. 23, _B_) are situated at a distance of one
to eight miles from the coast, and are separated from it by a lagoon
of moderately deep water. The Barrier-reef of New Caledonia is said
to be 400 miles in length and it follows the general contour of the
coast line.
The Atolls (Fig. 23, _C_) are ring-shaped islands composed of
coral limestone with a lagoon of salt water within them, situated in
the sea without any definite relation to other existing land.
The Fringing or shore-reefs (Fig. 23, _A_) are situated at a
distance of 100 yards or less from the beach and separated from it by
a shallow lagoon which is frequently left as a dry sand-bank at low
tides.
[Illustration: +Fig. 23.+
_A_, Fringing-reef; _B_, Barrier-reef; _C_, Atoll.
_a_, sea; _b_, reef; _c_, rocks of the coast.]
Many forms of reefs are found in different parts of the world, but
they may all be looked upon as special modifications of one of these
three groups. The facts which Darwin collected about Coral-reefs in
his memorable voyage round the world in the _Beagle_, suggested
to this great observer that all the different forms of reef must be
related to one another, and he formulated an ingenious theory to show
how, by the gradual sinking of the crust of the earth, Fringing-reefs
may have become, in the course of a long period of time, either Atolls
or Barrier-reefs. Some doubts have recently been expressed as to the
truth of Darwin’s famous “subsidence theory.” But, whether it is true
or not, to Darwin is due the credit of bringing home forcibly to our
minds the fact that Coral-reefs are slowly undergoing changes of growth
and of destruction, which must lead to most important and far-reaching
alteration in the character of the Tropical seas. It is not intended
in this work to enter into a discussion of the various alternative
theories of Coral-reef formation, but a few words may be added on the
method of formation of Coral limestone.
In studying any one particular form of reef-building Coral we can find
a long series of specimens from one inch in length up to a certain
maximum, which varies with the species and the reef, but may reach
over five feet in diameter as can be seen in the specimens now in the
British Museum at South Kensington. Beyond this maximum size--let
us say four feet in diameter--the Coral rarely grows, because at a
certain age, probably when the vitality of the Coral is on the wane,
the stalk of attachment becomes so bored with parasitic Sponges, Worms,
Fungi and other organisms, that it is thoroughly rotten. If a large
block be picked up from the reef, and with the help of two or three
strong natives carried ashore and broken up with a hammer, a most
interesting migration of Crabs, Worms and other creatures occurs, and
the collecting bottles may be filled with a rich variety of animals
parasitic on the Coral. Now the time comes when the stalk becomes so
brittle that a heavy wave breaks it in two, and the Coral topples
over and dies. If it falls into the sand, either on the inside or the
outside of the reef and becomes buried, it may be preserved fairly
complete, but if it lodges between other Corals the waves and the
parasites between them dissolve it and break it up into thousands of
pieces. This constant disintegration leads to the formation of great
quantities of coralline sand which fills up the interstices between
the living Corals, or gets washed over into the lagoon, or falls as a
talus over the seaward slope of the reefs. In the latter case lumps of
Coral torn off from the reef become embedded in it and form with it a
bank which gives support to more living Polyps on the seaward slope.
Consequently, in the course of many years, a reef which was at one
time only fifty yards from the beach may have extended to a distance
of a hundred yards or more, growing, as it were, on the skeletons or
shells of the Corals that have died. There can be little doubt that
Coral-reefs do grow seawards in this manner in some places, but they
may also be either beaten back or kept for a long time perfectly
stationary if the tides are too strong or too slack.
What the precise conditions are which favour the growth seawards of
Coral-reefs has not yet been systematically investigated; but we may
suppose that if the tides are too strong the sand has no opportunity
of settling between the Coral blocks and forming a solid limestone
rock, and if they are too slow the Coral-polyps do not get sufficient
nourishment to allow them to build fast enough to counteract the
solvent action of the water.
An interesting point connected with the Coral-reefs is the manner in
which they are formed at first. A volcanic upheaval gives rise to a new
island which, in the course of time, is surrounded by a Fringing-reef.
How does this reef begin?
The answer to this question has been recently given by the observation
of the formation of new Corals on the shores of the island of Krakatoa
which is situated in the Sunda straits, and was the seat in 1884 of
one of the most violent volcanic eruptions of historical times. After
the eruption the sea-bottom round the island was found to consist of a
fine volcanic mud, in which it may be believed no Coral embryos could
find a secure foothold. Now it is known that living Corals give rise to
a number of very minute larvæ which for a period of time swim freely
in the water, eventually settling down on some solid stone or shell
to give rise by growth and budding to the Coral blocks. These larvæ
frequently settle down on a piece of floating pumice-stone and after
a time grow to such a size that they sink it. If, in sinking, they
fall upon the bottom in shallow water they form together a substratum
on which other larvæ can settle and flourish. This is apparently the
manner in which Coral clumps are beginning on the slopes of Krakatoa
and these will undoubtedly give rise in time to a more or less complete
Fringing-reef.
Any further discussion on this point would lead us into subjects
beyond the scope of this book, but enough has been said to indicate
to the reader the manner in which the countless Coral-polyps may, in
the course of time, change the position of the reefs on Coral shores,
thereby altering the set of the tides there, changing the position of
the sand-banks, affecting the rate of erosion of the cliffs, and in
other ways causing important modifications of the coast line.
I have mentioned that the ground on the growing edge of the Coral-reef
is carpeted with Corals, Sponges and many other forms of animal life;
in the water swim countless Fish, and the branches of the Corals yield
to the naturalist innumerable forms of creeping and crawling creatures.
The shallow waters of the Tropics, as a whole, however, do not possess
a particularly rich Fauna,--in fact, the distinguished Dr Kükenthal,
who has had great experience of marine work, says that, in his opinion,
the Tropical seas are not richer in littoral animals than the Arctic
seas. Between the reef and the sea-beach there is a lagoon, of varying
breadth, with a sandy bottom, which is almost as barren of animal life
as a desert. A few Worms and Crabs, here and there a Star-fish and some
shells of Foraminifers, are all the spoils which fall to the bag of
the naturalist after many hours’ wading on this unprofitable ground.
The reason for this is, perhaps, not far to seek. When the tide goes
down many stretches of sand are left dry, and others retain only a
few inches of water. The exposure to the heat of a Tropical sun soon
kills and dries up any living animal that is unable to burrow deeply in
the sand, and the water in the shallow pools rises in temperature to
a degree that the human hand or foot can only just bear, so that the
little Fish that escape into them run the risk of being slowly cooked
alive.
On the inside of the lagoon there is, in many places, a broad belt
of Mangrove trees, forming the “Mangrove-swamp,” which contains
some interesting and important animals. These trees have a peculiar
spreading and branching system of roots which are left exposed when
the tide goes down, and form with one another a kind of network or web
a foot or more above the ground, upon which it is possible, with care,
to walk from place to place, at low tide. Between the roots there is a
slimy black sand or ooze, sometimes hard enough to walk upon, but more
commonly soft and treacherous. At high tide the water rises to a height
of two or three feet, completely covering the roots and giving the
swamp the appearance of a forest growing in the sea. Of the animals,
aërial and terrestrial, that haunt the swamps it is not necessary to
say more than a few words, although they too form a study of great
interest to the enthusiastic naturalist. But the marine zoologist who
visits the swamps cannot fail to take note of the millions of Ants,
Flies and Mosquitoes which torment him at every step, and make a
prolonged stay an impossibility.
[Illustration: +Fig. 24.+
Periophthalmus.]
One of the first creatures to be seen on entering a Mangrove-swamp
at low tide is a curious little Fish called _Periophthalmus_.
In some places hundreds may be seen at one time resting on the roots
of the Mangroves, or skipping over the pools of water from one root
to another. There are many varieties of _Periophthalmus_ in
different parts of the world, and their habits are not exactly the
same, so, to give an accurate description, our attention will be
confined to the form occurring in N. Celebes. This little Fish is about
three inches in length and remarkable for its very peculiar eyes, which
are of a bright yellow colour, situated quite close together on the top
of the head, and projecting so much from their sockets that the outline
of more than two-thirds of the eye-ball can be seen. These eyes are
extraordinarily moveable, and frequently revolve quite independently
of one another, like the eyes of a Chamæleon, giving the animal a most
grotesque and even ludicrous expression. These Fish seem to swim in
the water very seldom; when undisturbed they may be seen clinging to
rocks or trees by their fore-fins with their tails only in the water,
but from time to time they spring into the air to catch a Fly on the
wing, or a small Crab which has come unwarily within their range. Their
fore-fins are peculiarly adapted to their habits, in that they have a
very muscular base and a distinct elbow joint.
These creatures are not easy to capture, as the ground on which they
live is not adapted for rapid pursuit, and it is impossible to get
close enough to them to catch them in a hand-net with a long handle.
When kept in an aquarium it is seen that although they are rapid
swimmers when they do go below the surface, they seem to prefer to
live with their head and shoulders out of the water; and when chased
in their natural haunts they very rarely, if ever, seek to escape by
plunging into the water, but they execute a series of rapid jumps with
extraordinary rapidity from root to root or rock to rock, and so avoid
their pursuers. Their existence is really an amphibious one, and their
food consists partly of Insects on the wing. Their gills are very much
reduced in size, and it seems probable, from observations that have
been recently made, that their respiration is partly carried on by the
thin skin between the rays of the tail-fin.
Another animal extremely abundant in the Mangrove-swamps of Celebes,
and, like _Periophthalmus_, having a very wide distribution in
similar places in other tropical countries, is the “Caller-crab”
_Gelasimus_. These Crabs are about an inch in breadth across the
back, and are remarkable for possessing one very much enlarged and
brightly-coloured claw, the others being normal in size and dull in
colour like the rest of the body. On first entering the swamp at low
tide there may be seen on the mud between the roots of the trees a
number of bright yellow, blue or green objects, which, as the traveller
approaches, disappear one by one into holes in the ground. When the
eyes are accustomed to the gloom of the swamp these bright objects are
seen to be the great claws of the “Caller-crabs,” the rest of the body
being inconspicuous owing to its close resemblance in colour to the
slimy ground.
These are the principal and most abundant marine inhabitants of the
swamps, and as has been pointed out, all of them are more or less
amphibious in habit. More locally distributed, Oysters and other
bivalves may be found attached to the roots of the trees. Several
species of marsh Gastropods occur, some of them in great abundance in a
few localities. Occasionally a Sea-anemone, with remarkable powers of
burrowing rapidly in the sand when disturbed, may be found, and to the
microscopist a harvest of Foraminifers and other minute forms of life
awaits investigation and description in the Mangrove-swamp.
Whenever the tide is high a considerable number of Fish-fry,
Jelly-fish, and other forms of floating and swimming life
characteristic of these waters, drift into the swamps; and some being
caught by the tangle of roots are left behind, either in the pools, or
high and moderately dry upon the sand when the tide ebbs. Upon these
victims of the retreating tide swarms of Ants and Flies descend from
the trees, Crabs from the shore and from their holes in the sand are on
the watch for them, Kingfishers and Sandpipers are ready to pounce upon
those which are most to their taste, so that before the friendly waters
of the ocean return to the swamp, scarcely one of them is left.
These constitute what may be called in the swamp the _extraneous_
Fauna, which if it is not truly indigenous is nevertheless necessary
for the continued existence and well-being of the true inhabitants.
The character of the sea-bottom on the outer side of the living
Coral-reefs varies so much in different parts of the world that an
adequate treatment of the Fauna in that region would have to be one of
greater length than is possible in this book.
The living edge of the Coral-reef is in some cases situated on the
top of a submarine precipice of very considerable height, and in
many places the sounding-line goes down to a depth of five or six
hundred feet a few yards beyond the limits of the reef. The practical
difficulties in the way of determining the character of the Fauna of
any sea-bottom that shelves in this manner are very great, but where it
is partly composed of massive lumps of solid Coral they are at present
insurmountable. Every time the dredge or trawl reaches the bottom it
becomes entangled in the Coral branches, and is liable to be seriously
torn, or even lost. Swabs and iron hooks and fish traps may yield some
scraps of information, but speaking generally, the Fauna of these steep
slopes is scarcely known at all.
The most important question, from the geological point of view, that
has to be determined is the depth of water in which reef-building
Corals can live and thrive. This is still a matter of uncertainty owing
to the practical difficulties met with in the attempts to investigate
it. Darwin estimated that the limit of vigorous coral growth was
between 20 and 30 fathoms, but in recent years, owing to the discovery
of luxurious Coral patches in 44 fathoms on the Tizard and Macclesfield
banks, there is a pretty general opinion that his estimate is too low.
Whatever the exact limit may be, it is quite clear that in many parts
of the world the sea-bottom quite close to the outer edge of the
reef cannot support a vigorous Coral fauna. Here and there patches
of peculiar deep-sea species of Corals occur, but they do not form
in such depths anything of the nature of a reef. They are usually
isolated specimens, similar to those that are found in deep water on
the Norwegian coast and other parts of the world outside the limits of
the Tropics; but these specimens really belong to the deep-sea Fauna,
of which we shall learn more in another chapter.
In many places, however, the water at the base of the outer edge of
the reef is not very deep, and may slope away gradually towards the
bed of the ocean. The Fauna of such slopes in the Tropics is not
characteristically rich, as my own experiences of dredging in such
waters have proved. Time after time the dredge that was used in 15 to
20 fathoms off the coast of Talisse, came up with nothing but sand
or gravel. Occasionally a Brittle-star or a branch of dead Coral,
with a few Zoophytes growing on it, came up; and in some places a
few beautiful Lily-stars or Crinoids relieved the monotony of the
investigation. But, on the whole, the animals found in this region were
not numerous, nor of a character to excite any particular interest.
Before bringing this chapter to a close, a brief reference must be made
to one of the most remarkable phenomena in the animal kingdom,--this
is the history of the Palolo worm. On certain parts of the coast of
the Samoan islands the Palolo worm appears in great abundance in the
early morning hours of one or two days at the beginning of the third
quarter of the moon in the months of October and November. As the
worm is regarded as a very great delicacy by the natives, the day of
its appearance is looked upon as one of the most important red-letter
days of the year. Weeks before the worms are expected the advent of
the Palolo is discussed, stories are told of the fisheries of bye-gone
years, anecdotes of the last year are remembered and rehearsed, and the
whole population is prepared for the great event as for a feast.
When the grand day arrives the boats are decorated, the girls put on
all their finery, and everyone who can find a seat in a boat goes off
to the fishery amid a merry chorus of song and laughter. It must,
indeed, be a strange sight to see the flotilla of canoes with their
eagerly expectant and gaily-bedecked crews, waiting in the dim light
of the half moon for the day to break and the exciting fishery to
begin. As soon as it is light enough to see into the water, a few
writhing Worms may be distinguished at the surface, which increase in
number with such extraordinary rapidity, that in a little while it is
impossible to see anything below three or four inches owing to the
multitude of Palolos. As quickly as possible the fishery proceeds,
every man, woman, and child gathering the harvest of Worms during the
precious moments of the sunrise. When at last the sun rises well above
the horizon the Worm disappears again, and the boats hasten to the
shores with their booty.
This remarkable swarming process of the Palolo, occurring as it does,
only once or twice a year, in constant relation with a particular
phase of the moon, and lasting on each occasion only a few minutes in
time, is not the only noteworthy feature of the animal.
The Palolo worm, as it is caught, varies in length from an inch to a
foot or more, and is about a quarter of an inch in breadth, but it
readily breaks up into pieces when handled. It is composed of numerous
rings or segments, each provided with a pair of processes bearing
bristles, but there is no head. Astonishing as it may seem to those
unacquainted with the natural history of Worms, it is nevertheless a
fact that when the Palolo swarms it leaves its head behind among the
Corals, where, in all probability, it regenerates a new body. This
accounts for the fact that while the body of the Palolo is frequently
brought home to our Museums in England, its head is a rarity. The
colour of the Worm varies very considerably. The pieces bearing eggs
are usually of some shade of green, hence the specific name _Palolo
viridis_ that is given to it by scientists, but the males are
usually white. In connection with the appearance of the Worm there is a
curious statement that once in every four years it is exactly one lunar
month late, so that the time of year of its occurrence is constant. The
natives are also forewarned of the advent of the Palolo worm by the
movements of the land Crabs, which, it is reported, come down from the
fields and forest a few days before the Palolo feast and plunge into
the sea.
The precise habitat of the Worm when it is not swarming is still a
matter of some doubt. A few rare specimens have been found in the
Coral blocks in shallow water, but it is generally supposed that the
majority of them live in deep water on the outer side of the reefs. It
is not confined by any means to Samoa. It occurs also in Fiji, Tonga
and other Pacific islands. A Worm similar to the Palolo in habits was
described years ago by Rumphius in the Malay Archipelago, and Saville
Kent mentions a little Nereid worm with similar spawning habits on the
great Barrier-reef of Queensland.