REMARKS ON THE DISTRIBUTION OF MARINE AND FRESH-WATER CRUSTACEA
=A. Marine.=
The great majority of the Crustacea are inhabitants of the sea. From a
Zoogeographical point of view we divide the sea into three chief
regions, each of which is characterised by a special kind of fauna—the
littoral, the pelagic, and the abyssal regions.
The =littoral= region, which comprises all the shallow coastal waters
down to about 100 fathoms, varies very greatly in its physical character
according to the nature of the coast, its geological constitution,
latitude, etc., but, on the whole, it is characterised by variability of
temperature and salinity, by the presence of sunlight, and by the
continuous motion of its waves. On the shores of the large oceans this
region is also greatly affected by the tides. It is inhabited by a vast
assemblage of Crustacea, all of which are dependent upon a solid
substratum, either of rock or sand, or of vegetable or animal growth,
upon which they may wander in search of food, or in which they may hide
themselves. In consequence, the character of the Crustacea on any shore
is largely determined by its geological nature.
Although a certain number of Entomostraca such as Copepoda
(Harpacticidae and Cyclopidae), Ostracoda (Cypridae and Cytheridae), and
a few Operculata are littoral in habit, it is the Malacostraca, from
their larger size and variety of form, which give the character to
coastal waters.
On rocky coasts, especially those affected by tides, a great many kinds
of Shore-crab are found, which hide at low tide in the rock-pools and
under stones. _Carcinus maenas_ is characteristic of the rocky coasts of
the North Sea, while it is replaced in warmer seas and all round the
tropics by Crabs of the family Grapsidae, which are typical rock-livers,
and exceedingly agile in clambering over tide-washed rocks.
Porcellanidae are also very common under stones at low tide on rocky
beaches. Such typical Shore-crabs as these are remarkably resistant to
desiccation, and can live out of water for an astonishing time; nor do
they require a change of water provided they have access to the air. The
edible crab (_Cancer pagurus_) and the lobsters (_Homarus_ and
_Palinurus_) are dependent on rocks, but they rarely come close
in-shore, preferring depths of a few fathoms.
Sandy coasts are preferred by Shrimps and Prawns, which haunt the
shallow coastal waters in shoals; and in the sand are found all the
Crabs whose respiratory mechanism is specially adapted for life in these
regions, _e.g._ Hippidea or Mole-crabs, _Corystes_, _Matuta_, _Calappa_,
etc.
Characteristic of sandy bottoms are also the Thalassinidea, such as
_Callianassa_, which excavate galleries in the sand. On tropical sandy
shores various species of _Ocypoda_ and _Gelasimus_ are conspicuous,
which have deserted the sea, and live in burrows which they excavate on
the shore. _Gelasimus_ is especially abundant in the muddy sand of
tropical mangrove swamps.
Besides the rocky and sandy coasts we must distinguish the muddy shores
and bottoms which support a large amount of vegetable and animal growth.
These, besides harbouring the greater number of Amphipods and Isopods,
are also the natural home of the Dromiacea and Oxyrhyncha, or
Spider-crabs, among which the habit is common of decking themselves out
with pieces of weed or animal growth in order to harmonise better with
their surroundings. Pagurids are also especially abundant in the deeper
waters of these coasts.
Coral-reefs support a characteristic Crustacean fauna. In the growing
coral at the reef-edge a number of small Cyclometopa are found, e.g.
_Chlorodius_, _Actaea_, _Xantho_, which are finely sculptured and often
coloured so as to harmonise with the coral. Alpheidae also, Shrimp-like
Macrura with highly asymmetrical claws, which can emit a sharp cracking
sound with the larger claw, are commonly found in pools on the reef. In
the coralshingle formed by abrasion from the reef-edge at a few fathoms
depth, Leucosiidae are found, in which, again, respiratory mechanisms
for filtering sand from the gills are present.
Besides the geological nature of the coast, latitude has a very
important bearing upon the distribution of littoral Crustacea. Indeed,
the present distribution of littoral Crustacea appears to be far more
determined by the temperature of the coastal waters than by the presence
of any land-barriers, however formidable. We may distinguish an Arctic,
Antarctic, and Circumtropical zone.
The =Arctic= zone includes the true Arctic seas, and stretches right
down through boreal regions towards the sub-tropical seas. Almost all
the truly Arctic forms penetrate fairly far south, the Arctic seas being
characterised more by the absence of temperate forms than by the
presence of forms peculiar to itself. At the same time it must be noted
that the individuals from the coldest regions often grow to an enormous
size, a characteristic which is physiologically unexplained.
A great many of the Crustacea characteristic of this region are
circumpolar, _i.e._ they are not restricted in range to either the
Atlantic or Pacific. This is especially true of the extremely northern
types, _e.g._ Crangonidae and Hippolytidae, but it is also true of a
number of Crustacea which do not now occur as far north as Greenland or
Bering Strait, so that there is no longer any free communication for
them between Pacific and Atlantic. This gives rise to a discontinuous
distribution in the two oceans, exemplified in the common Shrimp,
_Crangon vulgaris_, which is found on the temperate European coasts and
on the Pacific coasts of Japan and Eastern America. The same is true of
_Eupagurus pubescens_ and _E. bernhardus_.
At the same time the boreal Atlantic and Pacific have their peculiar
forms. Thus the European and American Lobsters are confined to the
Atlantic, while the North Pacific possesses a very rich array of
Lithodinae, which cannot be paralleled in the Atlantic.
We may explain the community of many littoral forms to both the North
Atlantic and Pacific coasts by the continuous coast-line uniting them,
which in former times possibly did not lie so far north, or else was not
subjected to so rigorous a climate as now.
In the =Antarctic= zone we are presented with very different relations,
since the great continents are drawn out to points towards the south,
and are isolated by vast tracts of intervening deep sea. Nevertheless,
certain littoral forms are circumpolar, _e.g._ the Palinurid _Iasus_ and
the Crabs _Cyclograpsus_ and _Hymenosoma_. The genus _Dromidia_ is
common to Australia and South Africa, though it is apparently absent
from South America.
The Isopod genus _Serolis_ is confined to Antarctic seas. The majority
are littoral species, and they are distributed round the coasts of
Patagonia, Australia, and Kerguelen in a manner that certainly suggests
a closer connection between these shores in the past. These facts are,
on the whole, evidence in favour of the former existence of an Antarctic
continent stretching farther north and connecting Australia, Africa, and
S. America—a supposition that has been put forward to account for the
distribution of the Penguins, Struthious birds, Oligochaets, Crayfishes,
etc., in these regions (see pp. 215–217).
In considering the Arctic and Antarctic faunas the supposed phenomenon
of bipolarity must be mentioned, _i.e._ the occurrence of particular
species in Arctic and Antarctic seas, but not in the intermediate
regions. This discontinuous type of distribution was upheld for a
variety of marine animals by Pfeffer, Murray, and others, but it has
been very adversely criticised by Ortmann.[159] As far as the Arctic and
Antarctic Decapod fauna in general are concerned, the north polar forms
are quite distinct from the south polar. Typical of the former are
_Hippolyte_, _Sclerocrangon_, _Hyas_, _Homarus_, etc.; of the latter,
_Hymenosoma_, _Dromidia_, _Iasus_. It appears, however, that in certain
special cases, bipolarity of distribution may be produced owing to the
operation of peculiar causes. Two such cases seem to be fairly well
established. _Crangon antarcticus_ occurs at the two poles, and
apparently not in the intermediate regions; but, as Ortmann points out,
it is represented right down the West American coast by a very closely
related form, _C. franciscorum_. The waters on the tropical western
coasts both of Africa and America are exceedingly cool, and it appears
that in this way the _Crangon_ may have migrated across the tropical
belt, leaving a slightly modified race to represent it in this
intermediate region. The other case of bipolarity is afforded by the
“Schizopod,” _Boreomysis scyphops_, which occurs at both poles, but is
not known from the tropics. This is a pelagic species, and we know that
the Mysidae often descend to considerable depths. We also know that the
Mysidae are dependent on cold water, only occurring in boreal or
temperate waters. We may safely suppose, therefore, that the migration
of this species has taken place by their forsaking the surface-waters as
the tropics were approached, and passing down into the depths where the
temperature is constantly low even in the tropics.
The dependence of Crustacea upon the temperature of the water is also
illustrated by the distribution of the Lithodinae. The headquarters of
this family are in the boreal Pacific, with a few scattered
representatives in the boreal Atlantic. The cool currents on the western
coasts of America, however, have permitted certain forms to migrate as
far south as Patagonia, where they still have a littoral habit. In the
tropical Indo-Pacific, where a few species occur, they are only found in
deep waters. Thus at these various latitudes, by following cool currents
or migrating into deep water, they are always subjected to similar
conditions of temperature. The same kind of thing is observed in Arctic
seas, where deep-sea forms are apt to take on secondarily a littoral
habit owing to the temperature of the depths and of the shore being the
same.
Despite the impassable barriers of land which now sever the tropical
oceans, we can yet speak of a =circumtropical= zone possessing many
species common to its most widely separated parts. Such circumtropical
species, occurring on both the Atlantic and Pacific coasts of tropical
America, on the West African coast, and in the Indo-Pacific, are various
Grapsidae, _Calappa granulata_ and its allies, and certain _Albunea_.
The most striking instance of all is that of the Land-crabs. Of
_Ocypoda_, the greater number of species occur in the Indo-Pacific, but
representatives are also found on the tropical Eastern and Western
American coasts and on the West African coast, and the same is true of
_Gelasimus_. The genus _Cardisoma_, belonging to a different group of
Land-crabs, is also typically circumtropical.
For this community of the circumtropical species we may certainly
advance in explanation the comparatively recent formation of the Isthmus
of Panama. Besides the resemblance of the Crustacea on the east and west
coasts of the isthmus, we have an actual identity of species in several
cases, e.g. _Pachycheles panamensis_ and _Hippa emerita_, and the same
thing has been observed for the marine fish.
Another connexion, at any rate during early tertiary times, which
probably existed between now isolated tropical coasts, was across the
Atlantic from the West Indies to the Mediterranean and West African
coasts. Numerous facts speak for this connexion. Species of _Palinurus_
and _Dromia_ occur in the West Indies and the Mediterranean, which only
differ from one another in detail, and a connexion between these two
regions has been urged from the minute resemblances of the late
Cretaceous Corals of the West Indies with those of the Gosau beds of S.
Europe, and also of the Miocene land-molluscs of S. Europe with those at
the present time found in the West Indies.
To account, then, for the present distribution of littoral Crustacea we
must imagine that great changes have taken place during comparatively
recent times in the coast-lines of the ocean, but the guiding principle
in both the past and present has been temperature, and this factor
enables us, despite the immense changes in the configuration of the
globe that must have taken place, to divide the coasts latitudinally
into Arctic, Antarctic, and Circumtropical zones.
=Pelagic Crustacea= belong chiefly to the Copepoda (Calanidae,
Centropagidae, Candacidae, Pontellidae, Corycaeidae), a few Ostracoda
(Halocypridae and Cypridinae), and among Malacostraca a few Amphipoda
(Hyperina), some “Schizopoda,” and among Decapoda only the Sergestidae,
if we except the few special forms which live on the floating weeds of
the Sargasso Sea, _e.g._ the Prawns _Virbius acuminatus_ and _Latreutes
ensiferus_, and the Brachyura _Neptunus sayi_ and _Planes minutus_.
Besides these Crustacea which are pelagic as adults, there is an
enormous host of larval forms, both among Entomostraca and Malacostraca,
which are taken in the surface-plankton.
In dealing with the Copepoda we have already mentioned the vast pelagic
shoals of these organisms which occur at particular times of the year,
and have an important influence on fishing industries. _Anomalocera
pattersoni_ (Fig. 27, p. 60) is a good instance of this. It is a large
Heterarthrandrian, about 3 mm. long, with the body of a fine bluish
green colour; it has a remarkable power of springing out of the water,
so that a shoal has the appearance of fine rain upon the surface of the
sea. It occurs in the open Atlantic and Mediterranean, but comes into
the coasts during violent storms; the Norwegian fishermen hail its
presence in the fjords as the sign of the approach of the summer
herring.
It was Haeckel[160] who first clearly distinguished between “neritic”
plankton, the species of which have their centres of distribution in
shallow coastal waters and die out gradually as the open ocean is
approached, and “oceanic” plankton which is habitually found in the open
sea, and though it may invade the coasts is not dependent on the
sea-bottom in any way. It appears that although these two kinds of
plankton may get mixed up by currents and storms, they are always
recruited by new generations from the neritic or oceanic stations proper
to each kind.
Common oceanic species, found chiefly in the open Atlantic and in the
North Sea, are _Anomalocera pattersoni_, _Calanus finmarchicus_,
_Centropages typicus_, _Metridia lucens_, _Oithona plumifera_, etc.
Common neritic species in the Channel and other coastal waters are
_Centropages hamatus_, _Euterpe acutifrons_, _Oithona nana_, _Temora
longicornis_, etc. It was found by Gough[161] that although the true
oceanic species invade the Channel from the open Atlantic to the west,
they become rarer and rarer as they advance up the Channel. Thus the
plankton midway between the Lizard and Ushant at all times of year is
about 70 per cent. oceanic, while at the line drawn from Portland to the
Cap de la Hague it is about 35 per cent. Seasonal changes in the
salinity of the Channel water, chiefly due to the influx of oceanic
water from the Atlantic, as observed by Matthews,[162] do not clearly
influence the distribution of oceanic and neritic forms. The influx of
highly saline water from the Atlantic was most marked during the winter
months up to February. From February to May the highly saline water
receded, and during the summer months at the line drawn between Portland
and the Cap de la Hague the salinity was rather low. This was increased
in November by a patch of oceanic water being cut off from the main mass
and passing up Channel, and it is noteworthy that during this month the
highest percentage of oceanic forms was taken in the plankton of this
region.
_Calanus finmarchicus_ affords a clear instance of the way in which the
plankton may be carried about for great distances by means of currents.
This species has its home in the subarctic seas, but is carried down in
the spring by the East Icelandic Polar stream to its spawning-place
south of Iceland; the enormous shoals produced here are carried back,
continually multiplying, along the coasts of Norway during the summer
and autumn.
Besides these great migrations, the plankton organisms perform daily
movements, the majority of the Crustacea avoiding the surface during the
day, and often going down to as much as seventy fathoms or more, and
only coming up to the surface at night. Others, however, e.g. _Calanus
finmarchicus_, behave in the converse manner, preferring the sunlit
surface to swim in.
Owing to their dispersal by means of oceanic currents the pelagic
Crustacea do not offer any very striking features in regard to their
distribution, and the possibility of always finding congenial
temperatures by passing into the upper or under strata of water enables
them to live in almost all seas. The tropical species of Sergestidae are
mostly circumtropical, _i.e._ unhindered by the present barriers of
land.
The =Abyssal= regions of the sea contain many of the most interesting
Crustacea. Families entirely confined to the abyss are the Eryonidae,
Pylochelidae, and certain Caridean Prawns (Psalidopodidae, etc.), but
there are a great number of normally littoral genera which have
representatives in deep water. If we draw the limit between the littoral
and abyssal regions at about 200 metres, we can characterise the latter
as absolutely dark except for the presence of phosphorescent organisms,
with the temperature at a little above zero, and with a comparative lack
of dissolved oxygen in the water. These conditions bring about
remarkable modifications in the structure and life-histories of the
inhabitants of the deep sea; we have already touched on the
modifications of the visual organs and on the presence of
phosphorescence in many of the animals; other points to be noticed are
the usually uniform yellowish or bright red coloration, the frequent
delicacy of the tissues without much calcification, variations in the
structure of the breathing organs, _e.g._ in _Bathynomus giganteus_ and
_Encephaloides armstrongi_, and the loss of the larval development.
Owing to the similarity of conditions in the deep sea all over the globe
most of its inhabitants are universally distributed. It is also a
striking fact that species are found in the deep sea of the tropics
whose nearest allies occur, not in the littoral seas of the tropics, but
in those of the temperate region. This fact has already been alluded to
in dealing with the distribution of the Lithodinae. Alcock[163] remarks
that between 50–500 fathoms in the Indian Ocean are found Crabs such as
_Maia_, _Latreillia_, and _Homola_, regarded as characteristic of the
north temperate seas; the lobster _Nephrops andamanica_, taken at
150–400 fathoms, is closely allied to the Norwegian _N. norwegica_; and
nine species of “Schizopoda,” which are certainly temperate forms, occur
in the Indian Ocean at depths of 500–1750 fathoms.
=B. Fresh-Water.=[164]
If we except the Crayfishes and River-crabs, the Crustacean fauna of
running water is exceedingly poor, but in all standing fresh-water, from
the smallest pond to the large lakes and inland seas, Crustacea,
especially Entomostraca, are abundant and characteristic, and form an
important item in the food of fresh-water fishes. In small ponds a vast
assemblage of Cladocera is met with; these animals multiply with great
rapidity by parthenogenesis, especially during spring and summer, but on
the advent of untoward conditions sexual individuals are produced, which
lay fertilised winter-eggs which lie dormant until favourable conditions
again arise. As Weismann first pointed out, the frequency with which
sexual individuals are produced in the various species is closely
correlated with the liability of the water in which they live to dry up;
so that the Cladocera which inhabit small ponds usually have at least
two “epidemics” of sexual individuals, one during early summer and the
other before the onset of winter.
Besides Cladocera, the Phyllopoda (e.g. _Apus_, _Artemia_, etc.) inhabit
small pools; and also a great number of Cyclopidae. Of the other
fresh-water families of Copepoda, viz. Centropagidae and Harpacticidae,
inhabitants of small pieces of water are _Diaptomus castor_, as opposed
to the other species of _Diaptomus_ which are pelagic, and a number of
Harpacticidae (_Canthocamptus_), the members of this family living in
the weed or mud of either small ponds or else on the shores of the
larger lakes. The greater number of Ostracoda are found in similar
situations.
A district like the Broads of Norfolk, which consists partly of
slowly-moving streams and partly of extensive stretches of shallow
water, supports a Crustacean fauna intermediate in character between
that found in small ponds and the truly pelagic fauna characteristic of
deep lakes. A very complete list of the Crustacea of the Norfolk Broads,
with an interesting commentary on their distribution, is given by Mr.
Robert Gurney.[165] We miss here the pelagic Cladocera, such as
_Leptodora_, _Bythotrephes_, _Holopedium_, etc., which form so
characteristic a feature of large lakes; at the same time, besides a
rich development of the Cladocera, Cyclopidae, and Harpacticidae, which
haunt the weeds and mud of shallow waters, we find such species as
_Polyphemus pediculus_ and _Bosmina longirostris_ among Cladocera, which
are otherwise confined to large bodies of water, and a few pelagic
_Diaptomus_, e.g. _D. gracilis_. The fauna is also complicated in this
district by the proximity to the sea and the frequently high salinity of
the water, which allows a number of typically marine Copepods to pass up
the estuaries and intermingle with typically fresh-water species; such
are _Eurytemora affinis_ among the Centropagidae, and several species of
Harpacticidae (see p. 62).
The large lakes of the world, such as the continental lakes of Europe
and America, or of our own Lake District, reproduce on a small scale the
varied conditions which appertain to the ocean—as in the ocean, we can
recognise in these lakes a littoral, a pelagic, and an abyssal region.
Our knowledge of the physiography of lakes is largely due to the
classical work of Forel,[166] and the following account of the physical
conditions in the various regions is condensed from his book.
The =littoral= region is sharply marked off from the others by the
relative instability of its physical conditions, owing to the agitation
of its waters, the affluence of streams and drainage, and the constant
changes of temperature. The water in this region generally contains a
good deal of solid matter in suspension, while the shelving banks of the
lake support a wealth of vegetable growth, both of Algae and of
Phanerogams, down to about 20–25 metres. At this depth the daylight does
not penetrate sufficiently to admit of the growth of green plants, so
that this region marks the limit, both physical and biological, between
the littoral and the abyssal zones. In this littoral region there
flourish a great quantity of Entomostraca, most of which are also found
in small ponds where similar conditions of life prevail—the pelagic
species only penetrating rarely, and by accident, into its waters. At
the beginning of July Mr. H. O. S. Gibson and myself found that the
weedy littoral region of Grasmere contained almost entirely large
quantities of the Cladoceran _Eurycercus lamellatus_, and a number of
_Cyclops fuscus_ and _C. strenuus_. In the littoral zone of large lakes,
Amphipods, Isopods, and fresh-water shrimps may also be met with, but
this applies more to the lakes of the Tropics and of the Southern
Hemisphere.
The =pelagic=[167] region is distinguished from the littoral by the
greater purity and transparency of its waters, and by the relative
stability of the temperature, the annual range, even at the surface, in
Geneva being from 4°–20° C., while at 100 metres the water has a uniform
temperature of 4° or 5° C. The upper strata are, of course, brightly
illuminated, but at 20 metres there is hardly sufficient light for green
plants to grow, and at 100 metres it is completely dark. The inhabitants
of this region, known collectively as plankton, spend their whole life
swimming freely in the water, sometimes at the surface and sometimes in
the deeper strata. They consist chiefly of Diatoms, Protozoa, Rotifera,
and Crustacea. The pelagic Crustacea, especially the Cladocera, are
often the most curiously and delicately built creatures. _Leptodora
hyalina_, which is quite transparent, is the largest of them, attaining
to three-quarters of an inch in length, though _Bythotrephes longimanus_
is nearly as large if we include the immense spine which terminates the
body. _Holopedium gibberum_, which is the commonest of all in Grasmere
lake, but not so frequently met with in the other English lakes, is
peculiar in that its body is enveloped in a spherical mass of
transparent jelly, sometimes a quarter of an inch in diameter, so that
the contents of a tow-net jar full of _Holopedium_ have something of the
consistency of boiled sago. The enormous quantities in which these
animals often occur during summer is very astonishing; but to be truly
appreciated tow-nettings should be taken at the surface of the lake
either during night-time when there is not much moonlight, or else on a
dark still day when there is a quiet drizzle falling on the surface of
the water. In bright sunshine the plankton passes below the surface into
the lower strata, and can be usually taken by sinking the tow-net some
10–20 feet, or to even greater depths in the water. The exact reason of
these periodic migrations out of the light, and their dependence on
other physical conditions, such as temperature and the agitation of the
water, is not clearly understood. It appears, however, that when the
water is rough, plankton always passes into the deeper regions. Besides
the species mentioned, the minute Bosminidae, whose trunked heads are
suggestive under the microscope of elephants, and _Polyphemus pediculus_
are among the commonest pelagic Cladocera, though neither _Polyphemus_
nor _Bythotrephes_ ever form shoals. The above-mentioned genera are
characteristic of the larger lakes in the Northern Hemisphere. Our
knowledge of the Crustacean plankton of tropical lakes and of those of
the Southern Hemisphere is limited (but see p. 216).
A very important constituent of lake-plankton is furnished by the
Copepoda, especially of the genus _Diaptomus_. With the exception of
_Holopedium_, by far the commonest Crustacean in Grasmere during July
was found by Mr. Gibson and myself to be _D. caeruleus_.
At the same time a number of Cyclopidae, e.g. _Cyclops strenuus_, may
occur in the pelagic region in considerable quantities, though they were
never found by us in such numbers as _Diaptomus_.
The life-cycle of the pelagic Entomostraca has been studied in both the
Cladocera and the Copepoda. In some of the Cladocera Weismann at first
supposed that males had altogether disappeared, and that reproduction
was entirely parthenogenetic. It appears, however, that all the pelagic
species have at least one sexual period, namely, in the autumn, when
resting eggs are produced which lie dormant during the winter. The
pelagic Copepods may either produce resting eggs for the winter
(_Diaptomus_), or else the winter is passed through in the Nauplius
stage, the larvae hibernating in the mud until the spring (Cyclopidae).
We have so far only dealt with fresh-water Entomostraca. There are, in
addition, a number of Malacostraca which inhabit fresh water, and some
of these are found in the abyssal region of the great lakes, which must
now be considered.
The physical conditions of the =abyssal= region are still more stable
than those of the pelagic region, since the water is never disturbed,
the bottom is always composed of a fine mud, the temperature is constant
at 4°–5° C., and there is a total absence of light. It was hardly
expected that animals would inhabit this region, until Forel discovered
_Asellus aquaticus_ in a depth of forty metres in the Lake of Geneva,
and subsequently showed that quite a number of animals, including a
_Hydra_, several worms, Molluscs, Crustacea, and larval Insects, may be
found in these or even much greater depths.
The Crustacea of the abyssal region are two in number, and these have
been found in a number of European lakes; _Niphargus puteanus_, a blind
Amphipod closely allied to _Gammarus_; and _Asellus forelii_, allied to
_A. aquaticus_ and _A. cavaticus_, which may be either quite blind or
else retain the rudiments of eyes.
These two Crustacea, under a practically identical form, are also found
in the subterranean waters of Europe, and Forel considers that they have
arrived in the abysses of the lakes from the subterranean channels, and
are not derivatives of the littoral fauna.[168]
Having completed our short review of lacustrine Crustacea, we may deal
with the =subterranean and cave Crustacea=,[169] which, as far as light
and temperature are concerned, are subjected to very similar conditions
to those dwelling at the bottom of deep lakes. The inhabitants of the
subterranean waters have been chiefly brought to light in Artesian
wells, etc., while the cavedwellers have been investigated especially at
Carniola and in the American caves.
A number of species of Cyclopidae and Cypridae, many of which are blind
and colourless, have been brought up in well-water. The Amphipod
_Niphargus puteanus_ has long been known from a similar source in
England[170] and all over Europe, and several other blind Gammarids
inhabit the subterranean waters and caves in various parts of the world.
Among Isopods, _Asellus cavaticus_ is recorded from wells and caves in
various parts of Europe, _Caecidotea stygia_ and _C. nickajackensis_
from the Mammoth and Nickajack Caves in America, and two very remarkable
blind Isopods are described by Chilton from the subterranean waters of
New Zealand, viz. _Cruregens fontanus_, whose nearest allies are the
marine Anthuridae, and the Isopods _Phreatoicus typicus_ and _P.
assimilis_, which bear an extraordinary resemblance superficially to
Amphipods. Besides these, a small number of subterranean Decapoda are
known which retain the eye-stalks but are without functional ommatidia.
These are _Troglocaris schmidtii_, in Hungary, related to the
fresh-water Atyid _Xiphocaris_ of East Indian and East Asiatic fresh
waters rather than to the South European _Atyephyra_; _Palaemonetes
antrorum_, from artesian wells in Texas; and several species of
_Cambarus_ from the Eastern United States. A blind species of
_Cambarus_, _C. stygius_, has been described from the caves of Carniola,
and if this determination is correct, is the sole _Cambarus_ occurring
outside America.
It will be seen from the above account that the subterranean Crustacea
are an exceedingly interesting and, in many respects, archaic group,
many of which have survived in these isolated and probably uncompetitive
districts, while many secular changes were going on in the quick world
overhead.
The remaining fresh water =Malacostraca= may be mentioned under the
headings of the groups to which they belong.
Only one “Schizopod,” apart from _Paranaspides_, is known from
fresh-water lakes, viz. _Mysis relicta_, which was discovered in 1861 by
Lovén in the Scandinavian lakes, and has since been found in the Finnish
lakes, the Caspian Sea, Lake Michigan, and other localities in N.
America, and Lough Erne in Ireland. This species is closely related to
_Mysis oculata_ of Greenlandic seas.
In the Southern Hemisphere we have a species of _Anaspides_, _A.
tasmaniae_, occurring in mountain streams and tarns in Tasmania, a
related form which haunts the littoral zone of the Great Lake in
Tasmania, and a small species, _Koonunga cursor_, occurs in a little
stream near Melbourne.
Of the Isopoda certain genera, viz. _Asellus_ and _Monolistra_, are
confined to fresh water, others, such as _Sphaeroma_, _Idothea_,
_Alitropus_, and _Cymothoa_, have occasional fresh-water
representatives. Packard[171] describes a remarkable blind Isopod,
_Caecidotea_, from the Mammoth Cave of Kentucky, which occupies a very
isolated position, and in the same work gives a very complete exposition
of the cave-fauna of North America and Europe.
The Phreatoicidae are a curious family of Isopods confined to the fresh
waters of Australia and New Zealand, which bear a remarkable resemblance
to Amphipods, being laterally compressed and possessing a subchelate
hand on the anterior thoracic leg. These Isopods are exceedingly common
in small mountain pools and in streams in Tasmania, and in the Great
Lake in that country I have recently found a number of species which,
together with some species of Amphipods, make up the dominant feature in
the Crustacean fauna. One of these species may grow to fully an inch in
length. The family is confined to the temperate regions, and is usually
found on mountains. A number of species are known from the mainland of
Australia, one coming from a high elevation on Mount Kosciusko, and
another (_Phreatoicopsis_) from the forests of Gippsland attaining a
great size, and living among damp leaves, etc.
The fresh-water Amphipoda all belong to the families Talitridae,
Gammaridae, and Haustoriidae (see p. 137).
Among the Talitridae, or Sand-hoppers, _Orchestia_ and _Talitrus_ have
marine as well as fresh-water and land representatives, while the
American _Hyalella_ is entirely from fresh water, most of the species
being peculiar to Lake Titicaca. Many of these animals are partly
emancipated from an aquatic life. Thus _Orchestia gammarellus_, which is
common on the sea-shore of the Mediterranean, frequently penetrates far
inland, and was found in large numbers by Kotschy near a spring 4000
feet up on Mount Olympus.
_Talitrus sylvaticus_ is very common among fallen leaves and decaying
timber in Tasmania and Southern Australia, many miles from the sea, and
often at an elevation of several thousand feet.
Among the Gammaridae, certain genera, e.g. _Macrohectopus_
(_Constantia_), from Lake Baikal, are purely fresh-water. An enormous
development of Gammaridae was discovered by Dybowsky in Lake Baikal,
comprising 116 species, and lately a number more have been found by
Korotneff.[172] The majority of these were originally placed in the
genus _Gammarus_, but Stebbing has rightly created a number of peculiar
genera for them. Certain species are, however, placed in more widely
distributed genera, e.g. _Gammarus_ and _Carinogammarus_, which is also
represented in the Caspian Sea. Korotneff found some remarkable
transparent pelagic forms (_Constantia_) swimming in the abyssal regions
at about 600 metres depth, the majority of them being blind, but some
possessing rudimentary eyes, often on one side only.
Besides various species of _Gammarus_, a number of other Gammaridae are
frequently found in brackish water. Among Haustoriidae _Pontoporeia_ has
representatives in both the oceans and inland lakes of the northern
hemispheres (see p. 137).
Of the Decapoda, seven families are typically fresh-water in habitat—the
Aegleidae, containing the single species _Aeglea laevis_, related to the
Galatheidae, which inhabits streams in temperate S. America; the
Atyidae, a family of Prawns from the tropical rivers and lakes of the
New and Old World, and in the Mediterranean region. A number of
Palaemonidae are found in fresh water, e.g. _Palaemonetes varians_ in
Europe and N. America, while several species of _Palaemon_ occur in
lakes, streams, and estuaries of the tropical Old and New World.
The expeditions of Moore and Cunnington to Lake Tanganyika brought back
an exceedingly rich collection of Prawns, comprising twelve species, all
of which are peculiar to the lake,[173] and this is all the more
surprising since Lakes Nyasa and Victoria Nyanza are only known to
contain one species, _Caridina nilotica_, which ranges all over Africa
and into Queensland and New Caledonia. The Tanganyika species, however,
all belong to purely fresh-water genera, and do not afford any
suggestion that they are part of a relict marine fauna. It would appear
that they have been differentiated in the lake itself during a long
period of isolation.
Two groups of Brachyura, viz. the Thelphusidae and the Sesarminae (a
sub-family of the Grapsidae), are fresh-water. _Thelphusa fluviatilis_
is an inhabitant of North Africa, and penetrates into the temperate
regions of the Mediterranean, and is said to be exceedingly common in
the Alban Lake near Rome. Both these families have representatives on
land, e.g. _Potamocarcinus_ in Central and South America, and certain
species of _Sesarma_, and the closely related Gecarcinidae of the West
Indies.
The remaining families to be dealt with are the two Crayfish families—
the Astacidae and the Parastacidae—which live in rapidly moving rivers
and streams, and occasionally in lakes. A few species of both families
have taken to a subterranean mode of life, and excavate burrows in the
earth, _e.g._ the Tasmanian Crayfish, _Engaeus fossor_. The distribution
of the Crayfishes has long engaged the attention of naturalists. It was
first seriously studied by Huxley,[174] and has subsequently been
followed up, especially in North America, by Faxon[175] and
Ortmann,[176] but our knowledge of the South American and Australian
forms is still very incomplete. The Astacidae inhabit the northern
temperate hemisphere, the Parastacidae the southern temperate
hemisphere, the tropical belt being practically destitute of Crayfishes.
Of the Astacidae the genus _Astacus_ (_Potamobius_), including the
common Crayfishes of Europe, occurs in Europe and in North America west
of the Rockies. The genus _Cambaroides_, which in certain respects
approaches _Cambarus_, is found in Japan and Eastern Asia. The very
large genus _Cambarus_, on the other hand, only occurs in North America
east of the Rockies, so that _Cambaroides_ occupies a very isolated
position. The occurrence of a _Cambarus_, _C. stygius_, in the caves of
Carniola, has been recorded by Joseph, so that it would appear that this
genus had a much wider range formerly than now.
Of the southern temperate Parastacidae, Australia and Tasmania have the
genera _Astacopsis_ and _Engaeus_; New Zealand has _Paranephrops_, South
America _Parastacus_, and Madagascar _Astacoides_. The last named genus
is rather isolated in its characters, possessing a truncated rostrum and
a highly modified branchial system, but it agrees with all the other
Parastacine genera, and differs from the Astacidae in the absence of
sexual appendages on the first abdominal segment, and in the absence of
a distinct lamina on the podobranchiae. The largest crayfish in the
world is _Astacopsis franklinii_, found in quite small streams on the
north and west coast of Tasmania. Specimens have been caught weighing
eight or nine pounds, and rivalling the European Lobster in size.
Crayfishes appear to be entirely absent from Africa.
It seems reasonable to suppose that the two families of Crayfishes
characteristic respectively of the northern and southern hemispheres
have been independently derived from marine ancestors, which have
subsequently become extinct. Their complete absence in the tropics is
striking, and Huxley drew attention to the fact that it is exactly in
those regions where the Crayfishes are absent that the other large
fresh-water Malacostraca are particularly well developed, and _vice
versa_. Thus the large fresh-water Prawns are typically circumtropical
in distribution, while the South African rivers abound with River-crabs,
which, in general, are found wherever Crayfishes do not occur.
A few of the more interesting features in connection with the
distribution of fresh-water Crustacea have now been touched upon. With
regard to the origin of this fauna, we can see that a number of the
species are comparatively recent immigrants from the sea, working their
way up the estuaries of rivers, a proceeding which can be observed to be
taking place to-day in a district like the Broads of Norfolk. Others,
again, but these are few, appear to be true relict marine animals left
stranded in arms of the sea that have been cut off from the main ocean,
and have been gradually converted into fresh-water lakes and seas. Such
are, perhaps, _Mysis relicta_ and the rich Gammarid fauna of Lake
Baikal, a lake that, in the presence of Seals, Sponges, and other marine
forms, has clearly retained some of the characters of the ocean from
which it was derived. The majority of the fresh-water species, however,
have probably been evolved _in situ_, and their origin from marine
ancestors is lost in an obscure past. The Crustacean fauna of the
Caspian Sea[177] shows us in an interesting manner the effects of
isolation and changes in salinity, etc., on the inhabitants of a basin
which once formed part of the ocean. The waters of the Caspian Sea are
not fresh, but they are on the average about one-third as salt as that
of the open ocean. The Crustacea, described by Sars, belong to
undoubtedly marine groups, _e.g._ the Mysidae, Cumacea, and Amphipoda
Crevettina, but the remarkable feature of these Caspian Crustacea is the
great variety of peculiar species representing marine genera which are
very poorly represented in the sea, thus indicating that the variety of
the fauna is not due to a great variety of species having been shut up
in the Caspian Sea to begin with, but rather that, after the separation
from the sea, the isolated species began to vary and branch out in the
most luxuriant way—whether from lack of competition or owing to the
changing conditions of salinity it is difficult to say. As an example,
the Cumacea of the Caspian Sea are ten in number, all belonging to
peculiar genera related to _Pseudocuma_, except one species which is
included in that genus. These Caspian forms make up the Family
Pseudocumidae, which contains in addition only two marine forms of the
genus _Pseudocuma_ (see p. 121). A very similar condition is found in
the numerous Amphipods of the Caspian Sea. Considering the enormous
changes that must have taken place in the distribution of land and water
even during Tertiary times, it is astonishing that the fresh waters of
the world do not contain more species in common with the ocean, but it
must be considered that the limited area and comparatively uniform
conditions of fresh-water lakes and streams would only permit a limited
number of these forms to survive which could most easily adapt
themselves to the changed conditions. And these would in all probability
be the littoral species that were in the habit of passing up into the
brackish waters of estuaries and lagoons, so that the uniform and
limited nature of the fresh-water fauna can be accounted for to a
certain extent by this hypothesis.
We have seen in dealing with the marine Crustacea of the littoral zone
that the chief condition determining their distribution is temperature,
and that the world may be divided into three chief areas of distribution
for these animals, viz. the north temperate hemisphere, the tropics, and
the south temperate hemisphere. It seems that the same division holds
good for fresh-water Crustacea. We have already seen that the Crayfishes
follow this rule, being practically absent from the tropics, and
represented in the two temperate hemispheres by two distinct families,
the Astacidae in the north and the Parastacidae in the south.
Characteristic of the tropical belt are the absence of Crayfishes and
the great development of Prawns and River-crabs. In the case of
Entomostraca the great majority of the genera are cosmopolitan,
especially those which live in small bodies of water liable to dry up,
because these forms always have special means of dissemination in the
shape of resting eggs which can be transported in a dry state by
water-birds and other agencies to great distances; but those genera
which inhabit large lakes are more confined in their distribution. The
Copepod genus _Diaptomus_, characteristic of lake-plankton, ranges all
over the northern hemisphere and into the tropics, but it is almost
entirely replaced in the southern hemisphere by the related but distinct
genus _Boeckella_,[178] which occurs in temperate South America, New
Zealand, and southern Australia, and was found by the author to be the
chief inhabitant in the highland lakes and tarns of Tasmania,
_Diaptomus_ being entirely absent. Of the Cladocera there are a number
of pelagic genera (e.g. _Leptodora_, _Holopedium_, _Bythotrephes_)
entirely confined to the lakes of the northern hemisphere. The
distribution of _Bosmina_ is interesting. This genus is distributed all
over the north temperate hemisphere in lakes and ponds of considerable
size, not liable to desiccation; in the New World it passes right
through the tropics into Patagonia,[179] the chain of the Andes
doubtless permitting its migration. In the tropics of the Old World it
is unknown, but it turns up again, as the author recently found, as a
common constituent in the plankton of the Tasmanian lakes. There is
another instance of a group of Crustacea, characteristic of the north
temperate hemisphere, being entirely absent from the tropics, at any
rate of the Old World, but reappearing in the temperate regions of
Australasia. The commonest fresh-water Amphipods in this region belong
to the genus _Neoniphargus_, intermediate in its characters between the
northern _Niphargus_ and _Gammarus_, but grading almost completely into
the latter. Both _Niphargus_ and _Gammarus_ are absolutely unknown from
the tropics, but whether, like _Bosmina_, they occur in the Andes and
temperate South America is not known; it seems, however, probable that
they have reached Southern Australia by way of South America rather than
through the tropics of Asia and Australia, where there is no range of
mountains to permit the migration of a group of animals apparently
dependent on a temperate climate. The other common fresh-water Amphipod
in temperate Australia and New Zealand is _Chiltonia_, whose nearest
ally is _Hyalella_ from Lake Titicaca on the Andes, and temperate South
America.
The Anaspidacea and Phreatoicidae, which are so characteristic of
temperate Australia, and are generally of an Alpine habit, have never
been found in South America, but the Anaspidacea are represented by
numerous marine forms in the Permian and Carboniferous strata of the
northern hemisphere, so that it is probable that this group reached the
southern hemisphere from the north through America.
The distribution of the fresh-water Crustacea, therefore, in the
temperate southern hemisphere affords strong evidence in favour of the
view that the chief land-masses of this hemisphere, which are at present
separated by such vast stretches of deep ocean, were at no very remote
epoch connected in such a way as to permit of an intermixture of the
temperate fauna of New Zealand, Australia, and South America. While this
connexion existed, a certain number of forms characteristic of the
northern hemisphere, which had worked through the tropics by means of
the Andes, were enabled to reach temperate Australia and New Zealand.
The existence of a coast-line connecting the various isolated parts of
the southern hemisphere would, of course, also account for the community
which exists between their littoral marine fauna. It is impossible to
enter here into the nature of this land-connexion which is becoming more
and more a necessary hypothesis for the student of geographical
distribution, whatever group of animals he may choose, but it may be
remarked that the connexion was probably by means of rays of land
passing up from an Antarctic continent to join the southernmost
projections of Tierra del Fuego, Tasmania, and New Zealand.
TRILOBITA
BY
HENRY WOODS, M.A.
St. John’s College, Cambridge, University Lecturer in Palaeozoology