CRUSTACEA OF FRESH WATERS
The Crustacean fauna of fresh water is much less rich and varied than
that of the sea. Although the number of individuals in a pond or lake
may be enormous, they will be found to belong to a comparatively small
number of species. All the subclasses of Crustacea with the exception of
the Cirripedia have representatives in fresh water, but in most of them
only a very few of the families and genera comprise truly fresh-water
species. In spite of the comparative poverty of the fauna, however, it
is of very great interest, more especially with regard to the problems
of geographical distribution; and the ease with which specimens may be
collected everywhere, and kept in small aquaria, renders it a
particularly attractive subject of study for the amateur naturalist.
The general uniformity of the fresh-water fauna throughout the world has
often been remarked. Darwin says: "When first collecting in the fresh
waters of Brazil, I well remember feeling much surprise at the
similarity of the fresh-water insects, shells, etc., and at the
dissimilarity of the surrounding terrestrial beings, compared with those
of Britain." This uniformity is well illustrated by many of the smaller
Crustacea. In a gathering of Cladocera, Copepoda, and Ostracoda, from
Central Africa or from Australia, we find that most of the genera, and
even some of the species, are identical with those found in similar
situations in this country. It is by no means the case that all the
species and genera are thus universally distributed, for there are many,
especially among the larger forms, which have a very restricted range;
but this does not render less striking the general uniformity of the
fauna over very wide areas.
When we consider the physical environment of fresh-water animals, it
seems at first sight as if this wide distribution were the reverse of
what might have been expected, for the area occupied by them is far more
discontinuous than in the case of terrestrial or marine animals. The
inhabitants of a pond or lake are to a great extent isolated; and
although they may spread to other ponds and lakes by way of
communicating streams or rivers, where these are not too swiftly flowing
and are not interrupted by falls, yet direct passage from one river
system to another is rarely possible. Further, since practically the
whole of the fresh water on the surface of the globe is constantly
flowing, more or less rapidly, towards the sea, the smaller feebly
swimming forms tend to be swept down with the current, and ultimately
carried to perish in the sea. It follows that only those forms which
possess special adaptations for dispersal are able to flourish in fresh
water. In many cases, as will be described below, the eggs of the
smaller Crustacea can survive being dried up, and in this state they may
be blown about by wind or carried to great distances in mud, adhering to
the feet of migratory wading birds. Darwin says: "The wide-ranging power
of fresh-water productions can, I think, in most cases be explained by
their having become fitted, in a manner highly useful to them, for short
and frequent migrations from pond to pond, or from stream to stream,
within their own countries; and liability to wide dispersal would follow
from this capacity as an almost necessary consequence" ("Origin of
Species," sixth edition, chapter xiii.). In accordance with this, we
find that it is just those groups of Crustacea which show these
adaptations for dispersal that are most universally distributed in fresh
water. On the other hand, the larger Crustacea, like the Crayfishes and
River Crabs, which cannot so easily be transported from one locality to
another, have as a rule a more restricted range. These larger forms,
from their size and powers of swimming or creeping, can make their way
upstream and spread throughout a river system, and in some cases they
can leave the water and journey for short distances overland. On the
other hand, since free-swimming larvae would be liable to be swept out to
sea, most of them have a direct development, the young only leaving the
protection of the mother when they have attained the form and habits of
the adult. When all these factors have been taken into account, however,
there still remain many cases where the distribution of individual
species or of groups is hard to explain, and shows indications of dating
from a time when the outlines of continents and the connections of river
systems were different from what they are now.
Before proceeding to mention some of the more characteristic forms of
fresh-water Crustacea, it should be mentioned that in large lakes, as in
the sea, we can distinguish a littoral fauna in the shallow waters close
to the shore, a plankton fauna of the surface waters, and a deep-water
fauna. The littoral fauna does not differ in general characters from
that found in smaller ponds and gently-flowing rivers; the plankton
comprises many peculiar species showing adaptations for flotation, as in
the case of the marine plankton; and the deep-water fauna is very poor
in species and in individuals, and shows some relations with the
subterranean fauna to be mentioned later.
Of all the subclasses of Crustacea, the Branchiopoda are the most
characteristically fresh-water animals, only a few Cladocera being found
in the sea, and some Anostraca in salt lakes and brine pools.
The larger Branchiopoda (Anostraca, Notostraca, and Conchostraca) are
generally found in small, shallow ponds which are liable to be dried up
in summer. The "Fairy Shrimp" (_Chirocephalus diaphanus_; see Fig. 10,
p. 35) has been found in swarms in the water standing in deep cart-ruts
in a country lane in England, and _Apus_ sometimes appears suddenly in
rain-water puddles of a few square yards in area, which dry up after a
few weeks of hot weather. The eggs of these animals, when dried in the
mud, may remain dormant for long periods, and many species have been
hatched out from samples of dried mud brought by travellers from distant
countries. In such a sample from the Pool of Gihon at Jerusalem, it is
recorded that the eggs of _Estheria_ (see Fig. 11, p. 36) were found to
be capable of hatching after being kept dry for nine years. In some
species it is said that the eggs will not develop unless they have been
first dried, but this is not the case with _Chirocephalus_. In
favourable conditions development takes place very rapidly. Messrs.
Spencer and Hall, in describing the Branchiopoda of Central Australia,
say: "Certainly not more than two weeks after a fall of rain, and
probably only a few days, numberless specimens of _Apus_, measuring in
all about 2-1/2 to 3 inches in length, were swimming about; and, as not
a single one was to be found in the water-pools prior to the rain,
these must have been developed from the egg."
From what has been said, it is apparent that the larger Branchiopoda are
particularly well fitted to be distributed by the agency of birds, and
this is no doubt the explanation of the way in which many of the species
suddenly appear in localities where they were previously unknown, and,
after swarming for a longer or shorter time, sometimes for several
successive seasons, as suddenly vanish. A striking example of this is
afforded by _Apus cancriformis_ (see Plate II.), which formerly occurred
in several localities in the South of England, and appears more or less
irregularly in many parts of the Continent of Europe. No British
specimens had been recorded for over forty years, and the species was
believed to be extinct in this country, when it was found in 1907 by Mr.
F. Balfour Browne in a brackish marsh near Southwick, in
Kirkcudbrightshire. It can hardly be supposed that so large an animal as
_Apus_, and one so easily recognized, would have escaped notice
altogether had it occurred regularly in any part of the British Islands.
It is much more probable that the Scottish specimens found in 1907 had
developed from eggs accidentally transported by some bird from the
Continent. In 1908 a careful search in the same locality failed to
reveal a solitary specimen.
The Anostraca and Notostraca usually swim with the back downwards.
Particles of mud and of animal and vegetable matter are drawn by the
currents produced in swimming, into the ventral groove between the pairs
of feet, and are passed forwards to the mouth to serve as food. Some
species of Conchostraca are said to swim in the same inverted position;
but Messrs. Spencer and Hall, in the memoir already quoted, state that
the Australian Conchostraca swim back uppermost. They attribute the
difference in habit between the Conchostraca and Notostraca to the fact
that in the former group the valves of the shell can be rapidly closed
to protect the soft and vulnerable appendages, while no such protection
is possible in the Notostraca. They found on one occasion a specimen of
_Apus_ (Notostraca) attacked by three Water-beetles, which were tearing
its soft appendages, and they suppose that _Apus_ generally escapes such
attacks by swimming upside down.
The breeding habits of the Branchiopoda are also of interest, from the
prevalence in many species of reproduction by unfertilized eggs, or
"parthenogenesis." This may go on for many generations, and in _Apus_,
for instance, it is possible to examine thousands of specimens before
finding a single male, although, for some unexplained reason, males are
sometimes comparatively common. It is probable that males must appear
sooner or later, otherwise the series of parthenogenetic generations
will come to an end; but it is not certain that this is the case, and
there are some species of Conchostraca of which the males have never
been seen.
[Illustration: FIG. 55--THE BRINE SHRIMP (_Artemia salina_). (After
Sars.)
A, Female, under-side, x 6; B, head of male, upper side, further
enlarged, showing the large clasping antennae. The larval stages of this
species are shown in Fig. 33, p. 81]
The genus _Artemia_ (Fig. 55), among the Anostraca, is peculiar in its
habitat; for, while most of the Branchiopoda inhabit fresh or brackish
water, it flourishes in concentrated brine. In the South of Europe it is
found, as it was formerly in England, in the shallow ponds in which
sea-water is exposed to evaporation for the manufacture of salt, and in
these it occurs in such numbers as to give the water a reddish colour.
It is also found in salt lakes, like the Great Salt Lake of Utah, in the
United States, and in many other parts of the world. The specimens from
different localities often differ considerably, especially in the form
of the tail-lobes; but it has been shown that these differences are more
or less directly correlated with the degree of salinity of the water in
which the animals live, and it is probable that the forms which have
been described are all variations of a single cosmopolitan species
ranging from Greenland to Australia, and from the West Indies to Central
Asia. _Artemia_ is the only one of the Anostraca that is known to be
parthenogenetic, some colonies consisting entirely of females, while in
others males are abundant. The reddish colour above alluded to is found
also in _Branchipus_, _Apus_, and other Branchiopoda, and is due, as Sir
Ray Lankester first showed, to the presence in the body-fluids of
haemoglobin, the red colouring matter of the blood of Vertebrates, which
is important in the process of respiration.
[Illustration: FIG. 56--_Chydorus sphaericus_, A COMMON SPECIES OF
WATER-FLEA. x 50. (After Lilljeborg.)]
The smaller Branchiopoda known as "Water-fleas," forming the order
Cladocera, are abundant everywhere in fresh water. _Daphnia pulex_ and
other species of the genus, and the little Lynceidae, of which _Chydorus
sphaericus_ (Fig. 56) is the commonest species, are to be found in ponds
and ditches, and often swarm in farmyard ponds where the water is foul
with decaying matter. In most gatherings from such localities only
female specimens will be found, and nearly all of these will be seen to
carry a cluster of eggs or of developing embryos in the "brood-chamber"
between the back part of the body and the shell. In _Daphnia pulex_ (see
Fig. 12, p. 37) a single brood may consist of thirty young, and
occasionally of more than twice that number. As the broods may succeed
each other at intervals of two or three days, it will be seen that the
multiplication of the species in favourable circumstances may be
exceedingly rapid. It has been calculated that in sixty days the progeny
of a single female might amount to about 13,000,000,000. In addition to
these parthenogenetic eggs, which hatch at once while still within the
brood-chamber, the Cladocera produce, at certain seasons, another kind
of egg which requires to be fertilized by the male before it will
develop. These eggs are dark in colour and are enclosed in a thick
shell, and they do not hatch at once, but are cast off when the shell of
the female is moulted. Very commonly these "resting eggs," as they are
called, are produced in the autumn and lie dormant until the following
spring, and they can survive drying or freezing without injury, while
the thin-shelled parthenogenetic eggs within the brood-chamber of the
mother are easily killed. In addition to having thick shells, the
resting eggs are further protected in most, but not in all, cases by the
moulted carapace of the parent, which is specially thickened for the
purpose. This modification of the carapace is most highly developed in
the family Daphniidae (Fig. 57), where a saddle-shaped area on the dorsal
side, known as the "ephippium," becomes thickened, and on moulting
separates from the rest of the carapace to form a compact case enclosing
the two resting eggs. The outer wall of the ephippium is divided up into
small hexagonal cells, which become filled with air, causing the
ephippium to float at the surface of the water. In this position the
ephippia readily become entangled in the feathers of birds, and in some
cases the shell is provided with spines or hooks, which facilitate
transport to other localities by such means.
[Illustration: FIG. 57--A WATER-FLEA, (_Daphnia pulex_), FEMALE, WITH
EPHIPPIUM CONTAINING TWO "RESTING EGGS." x 20. (Partly after Lilljeborg.)
The Antenna is cut short. Compare Fig. 12, p. 37.]
The appearance of males and the production of ephippial eggs--in other
words, the "sexual period"--is generally more or less restricted to one
season of the year. In most species, particularly in those which live
in lakes, the sexual period occurs in the late autumn, and the ephippial
eggs lie dormant during the winter, and hatch in the spring. In species
living in small ponds exposed to the risk of overheating or of drying up
during summer, there is often a distinct sexual period in the spring,
when ephippial eggs are produced to tide over the unfavourable
conditions of the warmer months of the year. Although no species is
known to be exclusively parthenogenetic, yet it appears that purely
parthenogenetic colonies of certain species may be found in favourable
localities, where they may reproduce from year to year without males
ever being found.
Certain species of Cladocera belong to the plankton of lakes and large
ponds, and show modifications which adapt them for a floating life. Some
of these belong to the genus _Daphnia_, and differ from the species
found in other situations by their glassy transparency. As in the case
of many marine plankton Crustacea, this transparency is probably due to
the thinness of the shell and to the general watery condition of the
body, giving the necessary buoyancy to enable the animal to remain
constantly afloat. The same effect is no doubt produced by the long
terminal spine of the carapace and by the great helmet-shaped crest into
which the upper part of the head is often produced. A form very
characteristic of the plankton of large lakes is _Bythotrephes_ (Fig.
58), which is found in the lakes of Scotland, Ireland, Wales, and the
Lake District of England. In _Bythotrephes_ the carapace does not
enclose the body, but is reduced to a small brood-sac; the abdomen,
however, is drawn out into a long spine, which may be two or three times
as long as the body. A further point of interest is the division of the
eye into a dorsal and a ventral portion, differing in structure in much
the same way as do the two divisions of the eyes in certain marine
plankton Crustacea (see p. 152). Another very remarkable lacustrine form
is _Leptodora_, the largest of all the Cladocera, being sometimes more
than half an inch in length. In this case also the carapace is very
small, and does not enclose the body. The swimming antennae are very
large, and the abdomen is long and divided into several segments.
[Illustration: FIG. 58--_Bythotrephes longimanus_, FEMALE, WITH EMBRYOS
IN THE BROOD-SAC. x 12. (After Lilljeborg.)]
_Leptodora_ is further remarkable on account of its mode of development.
The parthenogenetic eggs, as in other Cladocera, develop directly, but
the resting eggs give rise to larvae of the nauplius type.
_Holopedium_, which is found in similar situations, surrounds itself
with a mass of a jelly-like substance which it secretes. A similar
envelope of jelly is found in some marine plankton animals, though not,
so far as is known, in any Crustacea, and it no doubt serves to give
buoyancy to the animal.
The Copepoda of fresh water are as abundant and universally distributed
as the Cladocera. Species of the genus _Cyclops_ (see Fig. 14, p. 39),
easily recognized by the pear-shaped body and the two egg-packets
carried by the female, are to be found in almost every pond and ditch.
The genus _Canthocamptus_ comprises species of smaller size, with
slender, flexible body, and carrying only a single egg-packet. The
plankton of lakes and ponds includes species of _Diaptomus_ (Fig. 59),
which have a narrow body and very long antennules. The latter are held
out stiffly while the animal swims by rapid movements of the antennae and
mouth parts, making occasional sudden leaps by means of its oar-like
feet. In this genus also the egg-packet is single. The development can
easily be studied by keeping egg-carrying females of _Cyclops_ in a jar
of water, when the nauplius larvae will soon hatch out.
[Illustration: FIG. 59--_Diaptomus coeruleus_, FEMALE. x 25. (After
Schmeil.)]
Although the Copepoda, unlike the Cladocera, are not parthenogenetic, it
has been found that certain species of _Diaptomus_ produce resting eggs
capable of surviving freezing or drying. In the early part of the
breeding season the eggs have thin shells, and they hatch after a short
time. In the autumn, however, thick-shelled eggs are produced, which lie
dormant in the mud until the following spring. It has recently been
discovered that species of _Cyclops_ and _Canthocamptus_ pass through a
resting stage, in which the animal surrounds itself with a cocoon-like
capsule of mud held together by a glutinous secretion produced by glands
on the surface of the body and limbs. The encapsuled animals, in the
cases observed, lie dormant in the mud during the summer, to resume
active life in the colder months of the year. It is very probable that
they can also be dried without injury, and that the "cocoons" serve the
same purpose as the resting eggs of other species.
Numerous species of Ostracods, belonging to the genus _Cypris_ (see Fig.
13, B, p. 38), and other closely related genera, occur in fresh water.
Like the Cladocera, they reproduce largely by parthenogenesis, and the
males of many species are rarely found, while in some species they have
not yet been discovered. In Professor Weismann's laboratory at Freiburg
a colony of _Cypris_ was kept in an aquarium for eight years, and during
the whole of that time no males made their appearance, the colony
reproducing exclusively by parthenogenesis. Probably in all species the
eggs survive drying.
The common "Freshwater Shrimp" (_Gammarus pulex_), which has already
been described, may be taken as a type of a large number of Amphipoda,
for the most part closely allied, which are widely distributed in most
regions of the world, with the exception of the tropics. _G. pulex_
itself ranges from the British Islands to Mongolia. As the eggs are
carried, till they hatch, in the brood-pouch of the parent, and are not
known to survive drying, it is difficult to understand in what way
_Gammarus_ and its allies contrive to spread from one locality to
another.
The little fresh-water Isopod _Asellus aquaticus_ (Fig. 60) is common in
ponds and canals in this country. It may be recognized by its general
resemblance to a Woodlouse, with very long antennae, and with a pair of
long, slender, forked uropods projecting behind. The species is widely
distributed in Europe, and other species of the same and closely related
genera are found in North America.
[Illustration: FIG. 60--_Asellus aquaticus_, FEMALE. x 4. (After Sars.)]
In Australia and New Zealand the Isopoda are represented in fresh waters
by a very peculiar group of species, forming the suborder Phreatoicidea,
which have more the aspect of Amphipods than of Isopods, since the body
is more or less flattened from side to side, instead of from above
downwards.
With regard to the mode of distribution of the fresh-water Isopoda,
there is the same difficulty as in the case of the Amphipoda, for the
eggs are carried in a brood-pouch, and do not seem to be in any way
protected against drought. It is no doubt in consequence of this that
the fresh-water species and genera of both Amphipoda and Isopoda, though
widely distributed, do not have the world-wide range of many of the more
minute Crustacea described above.
The common Crayfish, _Astacus_ (or _Potamobius_) _pallipes_, is the only
truly fresh-water Decapod found in England, although a small Prawn,
_Palaemonetes varians_, which usually inhabits brackish water, may
occasionally be found in places where the water is practically fresh.
The structure of the Crayfish is very similar to that of the Lobster,
but, as already mentioned, it differs in its mode of development, having
no free-swimming larval stage. From its size, and from the fact that the
eggs are carried by the female, the Crayfish cannot be transported from
one locality to another by the agencies which distribute the smaller
fresh-water Crustacea. On the other hand, the adult animals can live out
of the water for days, or even weeks, if they are kept moist, and the
English species is stated to leave the water occasionally, and to make
short excursions on land. Many species found in foreign countries are
still more truly amphibious in their habits. It is clear, however, that
the means of dispersal of the Crayfishes are very limited, and on this
account the problems connected with their geographical distribution
are of great interest. An admirable discussion of the subject will be
found in Professor Huxley's book on the Crayfish, and the conclusions
reached by him have hardly been modified by thirty years of subsequent
research. Only a very brief outline can be attempted here.
[Illustration: FIG. 61--MAP SHOWING THE DISTRIBUTION OF CRAYFISHES.
(Partly after Ortmann.)
The dotted areas are those occupied by the Northern Crayfishes (family
Astacidae). The black patches mark the areas inhabited by the Southern
Crayfishes (family Parastacidae)]
Crayfishes are found in the fresh waters of the Northern and Southern
Hemispheres (Fig. 61), but in each case they are practically confined to
the temperate regions, and are absent from a broad intervening tropical
zone. The Northern Crayfishes, forming the family Astacidae (or
Potamobiidae) are distinguished, among other characters, by having a pair
of appendages on the first abdominal somite, at least in the male sex;
the Southern Crayfishes have no appendages on that somite, and for this
and other reasons are regarded as constituting a distinct
family--Parastacidae. There is thus a general correspondence between the
geographical distribution of the Crayfishes and the more important
structural differences expressed in their classification. There can be
no doubt that the two families have been derived from a common stock of
marine lobster-like animals, and it is reasonable to suppose that two
branches of this stock became independently adapted to a fresh-water
habitat in the North and in the South, giving rise to the Astacidae and
the Parastacidae respectively.
The distribution of the individual genera is, however, not so easy to
understand. The species found in Europe all belong to the genus
_Astacus_, which also penetrates into Asia as far as Turkestan and the
basin of the River Obi.
Throughout the greater part of Asia no Crayfishes are found until we
come to the Far East, where we find an isolated colony in the
river-system of the Amur, in Korea, and in the north of Japan. These far
eastern Crayfishes, however, differ so much from the typical species of
_Astacus_ that they are now placed in a subgenus (sometimes regarded as
a distinct genus), _Cambaroides_. Curiously enough, the typical genus
_Astacus_ reappears again on the other side of the Pacific, where
several species occur in that part of North America which lies west of
the Rocky Mountains. East of the Rockies, again, numerous species are
found belonging to a distinct genus, _Cambarus_, which ranges from
Canada to Central America and Cuba, and this genus is allied in certain
respects to the _Cambaroides_ of Eastern Asia. If the systematic
relations of these genera have been properly interpreted, it is by no
means easy to understand in what way their present distribution has been
brought about.
[Illustration: _PLATE XX_
THE MURRAY RIVER "LOBSTER," _Astacopsis spinifer_. NEW SOUTH WALES.
(MUCH REDUCED)
THE LAND CRAYFISH, _Engaeus cunicularis_. TASMANIA (NATURAL SIZE)]
The Southern Crayfishes have an even more scattered and discontinuous
range. In New Zealand the genus _Paranephrops_ occurs, in Australia and
Tasmania the genera _Astacopsis_ (Plate XX.), _Cheraps_ and _Engaeus_
(Plate XX.). A single species of _Cheraps_ has been recorded from New
Guinea, but no Crayfishes are found in any part of the Malay
Archipelago, in Southern Asia, or on the continent of Africa, although,
curiously enough, a single species of a peculiar genus (_Astacoides_) is
found in Madagascar. In South America species of _Parastacus_ are found
in Southern Brazil, Argentina, and Chili. It is evident that these
various genera of Parastacidae, which are now so widely isolated from
each other, must have reached their present habitats when the relative
distribution of land and sea in the Southern Hemisphere was very
different from what it is now. What exactly the nature of the land
connection between the various islands and continents was, whether by
way of an Antarctic continent or otherwise, is a question that can only
be suggested here. To attempt to answer it would involve the
consideration of the distribution of many other groups of animals
besides Crayfishes.
Before leaving the Crayfishes, it may be mentioned that certain species
have become adapted to almost terrestrial habits. A number of species of
_Cambarus_ in North America are often found at considerable distances
from open water, burrowing in damp earth, their burrows reaching down to
the ground-water. In many cases they throw up chimney-like piles of mud
at the mouths of their burrows, and in places their chimneys are so
numerous as to "hamper farming operations by interfering with the
harvesting machines, clogging and ruining them." The species of
_Engaeus_ (Plate XX.), found in Tasmania, are there known as "Land
Crabs," and burrow in marshy places and in the forests up to an
elevation of 4,000 feet.
[Illustration: _PLATE XXI_
_Palaemon jamaicensis_, A LARGE FRESHWATER PRAWN OF THE FAMILY
PALAEMONIDAE. WEST INDIES. (MUCH REDUCED)]
The broad equatorial belt which separates the regions inhabited by the
Northern and the Southern Crayfishes is characterized by the presence of
several other groups of fresh-water Decapoda. The large River Prawns,
which are found nearly everywhere within the tropics, belong to the
genus _Palaemon_ (Plate XXI.), which is very closely related to the
common marine Prawns (_Leander_) of our own coasts. Some of these Prawns
grow to a foot or more in length of body, and the large claws may
measure as much again. From the Crayfishes, for which they are sometimes
mistaken, they may be easily distinguished by the fact that the large
pincer-claws are not the first, but the second pair of legs. Another
widely-spread group of River Prawns, for the most part of small size, is
the family Atyidae (Plate XXII.), in which the two pairs of pincer-claws
are feeble, and have the fingers tipped with brushes of long hairs, used
in sweeping up minute particles of food from the mud. The distribution
of these Prawns presents many difficult problems, as an example of which
we may mention the presence of identical or closely related species in
the fresh waters of West Africa and of the West Indies.
[Illustration: _PLATE XXII_
_Atya scabra_, A FRESH-WATER PRAWN OF THE FAMILY ATYIDAE, WEST INDIES
(REDUCED)]
The Brachyura (or Crabs) include many species that live in fresh water.
Some of these, like the species of _Sesarma_ (see Plate XXIII.) and some
other genera of the family Grapsidae, are common throughout the tropics,
passing up the rivers from the brackish water of estuaries, and being
often found long distances inland in quite fresh water. The true River
Crabs, however, belong to the family Potamonidae, and are very common
throughout the warmer regions of the globe. One species, _Potamon edule_
(Plate XXIII.), formerly called _Telphusa fluviatilis_, is found in the
South of Europe (Italy, Greece, etc.). Very numerous species, as yet
only imperfectly known, occur throughout the whole of Africa, in
Southern Asia, and in the Malay Islands, extending to Australia in the
south and Japan on the north. In the New World the River Crabs are found
in South America, and extend north to Mexico and the West Indian
Islands. Many of the River Crabs are amphibious in habits, and may be
found burrowing in marshy ground or in damp forests. The young are
hatched from the egg with all the appendages developed, and they remain
clinging to the abdomen of the mother until after the first moult, when
they are perfectly-formed little Crabs (see Fig. 31, p. 78).
[Illustration: _PLATE XXIII_
THE RIVER-CRAB OF SOUTHERN EUROPE, _Potamon edule_ (OR _Telphusa
fluviatilis_) (REDUCED)
_Sesarma chiragra_, A FRESHWATER CRAB OF FAMILY GRAPSIDAE. FROM
BRAZIL. (SLIGHTLY REDUCED)]
The groups which have been mentioned are all characteristic inhabitants
of the fresh waters over considerable areas of the surface of the globe.
There are, however, in addition to these, certain Crustacea which occur
in isolated localities, and have no close allies in fresh waters
elsewhere. In the streams of Southern Brazil and Chili there is found a
small Crustacean (_AEglea laevis_--Plate XXIV.), not unlike the Galatheas
of our own coasts, which is interesting as being the only species of the
Anomura found in fresh water. Still more remarkable are the Syncarida,
which are represented by two species of "Mountain Shrimps" (see Fig. 84,
p. 264) in Tasmania, and by a third species found near Melbourne. These
forms have no near allies among living Crustacea, but appear to be
related, as will be shown in a later chapter, to certain fossil
Crustacea found in Palaeozoic rocks.
[Illustration: _PLATE XXIV_
_AEglea laevis._ SOUTH AMERICA. (NATURAL SIZE)]
Belonging to a different category from any of those mentioned are
certain Crustacea closely allied to, or identical with, species living
in the sea, which inhabit inland lakes where no direct passage from the
sea is now possible. Attention was first called to these in the case of
some of the large lakes of Sweden, in which Professor Loven found some
Crustacea--_Mysis relicta_ (see Fig. 16, p. 47), _Mesidotea entomon,
Pontoporeia affinis_--almost or quite identical with species inhabiting
the Baltic, the Arctic Ocean, and the North Atlantic. There is
geological evidence to show that these lakes were once fjords, or arms
of the sea, and have become cut off from communication with the Baltic
by gradual elevation of the land. The marine animals which they
contained would thus be imprisoned, and as the water became less and
less salt, by the inflow of rivers, certain species which were able to
accommodate themselves to the altered conditions would survive. Some of
the species living in the Swedish lakes have since been found to have a
wider distribution. Thus, _Mysis relicta_, which should perhaps be
reckoned as only a variety of the _Mysis oculata_ of Arctic seas, has
been found in lakes in Russia, North Germany, and North America (Lake
Superior and others), and has lately been discovered in Lough Neagh and
some other lakes in Ireland.
The brackish waters of the Caspian Sea contain a very remarkable
assemblage of animals, including many Crustacea, which, although now
quite isolated from the oceans, are certainly of marine, and in part of
Arctic, origin. Among these are some species closely allied to or
identical with those of the Swedish lakes already mentioned, together
with a great variety of species of Mysidacea, Cumacea, and Amphipoda,
which appear to have been evolved from marine forms since the Caspian
was cut off from communication with the Arctic Ocean.
To such assemblages of animals derived from marine species and isolated
in inland lakes the name of "relict" faunas has been given. It is
necessary to use caution, however, in extending this explanation of
their origin to every case of peculiar lake faunas. For example, there
are difficulties in the way of supposing that Lake Baikal was ever in
open and direct communication with the sea, although it contains many
animals, such as seals, which are certainly of marine origin. The chief
Crustacea of the lake are numerous species of Amphipods belonging to the
genus _Gammarus_, and other genera closely related thereto, and for
these, at all events, there is no need to assume a "relict" origin.
One of the most remarkable lakes in the world from a zoological point of
view is Lake Tanganyika in Africa. When it was found that this lake
contained a fauna very different from that of the other great lakes of
Africa, it was rashly assumed that it must be of relict origin, and some
remarkable speculations were indulged in as to the former connection
between the lake and the sea. Further research, while it has greatly
emphasized the peculiar nature of the fauna, has entirely disposed of
the view that it originated in this way. The Crabs and Prawns, for
example, are not nearly related to marine forms, but belong to groups
that are characteristic of fresh waters in the tropics. While Nyassa and
the Victoria Nyanza have as yet only yielded a single species of Prawn,
and that one of enormously wide distribution (from the Nile to
Queensland), Tanganyika contains no fewer than twelve species, all of
which are peculiar to the lake, while all except one belong to genera
unrepresented elsewhere. Similarly, the Crabs found in the other great
lakes of Africa belong to commonplace types of River Crabs of the genus
_Potamon_; in Tanganyika, in addition to some of these, there are three
species of a remarkable genus, _Platytelphusa_, not known from any other
locality. The Copepoda and Ostracoda of Tanganyika comprise a remarkably
large number of species, many of them peculiar to the lake. A most
unusual feature is the entire absence of Cladocera. It is not easy to
explain the occurrence of this remarkable fauna in Tanganyika, but the
evidence from other groups of animals, such as Mollusca and fishes,
tends to suggest that the lake must have been, until recently,
completely isolated from the other lakes and river-systems of Africa,
that it had no outlet, and that the water was consequently more or less
brackish. Under these conditions the fauna of the lake, originally
similar to that of the other African lakes, has evolved along lines of
its own.
[Illustration: FIG. 62--A WELL SHRIMP (_Niphargus aquilex_). x 7. (After
Wrzesniowski.)]
A very interesting division of the fresh-water fauna is constituted by
those animals which inhabit underground waters. In the South of England
there is found not unfrequently in the water of wells a small colourless
transparent Amphipod known as the "Well Shrimp" (_Niphargus
aquilex_--Fig. 62), distinguished from the common fresh-water _Gammarus_
by the slenderness of its body, by the elongation of the last pair of
tail appendages (uropods), and by the absence of eyes. The proper
habitat of _Niphargus_ is not actually in the wells, but in the
subterranean reservoirs and streams by which the wells are fed. These
subterranean channels intercommunicate over wide areas, and are now
known in many parts of the world to contain a peculiar assemblage of
animals which become accessible to the naturalist in wells and in the
streams and lakes of large caves. Further, the scanty "abyssal" fauna of
deep lakes is partly made up of species which enter the lakes by
subterranean channels, and find a suitable habitat in the deep water.
Species of _Niphargus_, for example, have been dredged in Lough Mask in
Ireland and in some of the Swiss lakes.
Several species of blind Crayfishes have been found in caves in North
America, the best known being one (_Cambarus pellucidus_--Plate XXV.)
found in the Mammoth Cave in Kentucky; and blind Prawns belonging to
various genera have been discovered in caves in America and Europe.
[Illustration: _PLATE XXV_
THE BLIND CRAYFISH OF THE MAMMOTH CAVE OF KENTUCKY, _Cambarus
pellucidus_. (NATURAL SIZE)]
A very remarkable feature of the subterranean fauna is that a number of
the animals appear to be more closely allied to marine species than to
any known from fresh waters above-ground. This is especially the case
with some of the Isopoda belonging to typically marine families like the
Cirolanidae and Anthuridae, and it has been suggested that these have been
derived from marine species which have entered the underground waters
directly from the sea by way of submarine fissures in the crust of the
earth.
The environment in which these subterranean animals live resembles that
of the deep-sea animals in the absence of light, and the consequent
absence of plant-life. They must ultimately depend for food on animal
and vegetable debris washed down from the surface, but the food-supply
must be scanty, for the water in which they live is usually very clear
and free from organic matter. It is not surprising to find that nearly
all of them are blind, and the few species provided with visual organs
which have been described, from caves, are probably only temporary or
accidental immigrants. Whether the degeneration of the eyes is the
direct effect of disuse, or is due to natural selection ceasing to keep
the eyes up to the standard of usefulness, is a question which has been
much debated, and its answer, were we sure of it, would settle some
of the most fundamental problems of the evolution theory.
At all events, we do not find in any truly subterranean species large
and peculiarly modified eyes like those of many deep-sea animals, and
this may be associated with the complete darkness of their habitat, not
lighted by phosphorescent organisms as the deep sea is. In another
respect these animals differ from those of the deep sea, for they are
all colourless or nearly so; while many of the inhabitants of the deep
sea, as we have already seen, are brilliantly coloured.