Deep-Sea Hermit CrabPLATE XVIIA DEEP-SEA PRAWN,Nematocarcinus undulatipes. (SLIGHTLY REDUCED)(From Brit. Mus. Guide)View larger image
PLATE XVII
A DEEP-SEA PRAWN,Nematocarcinus undulatipes. (SLIGHTLY REDUCED)
(From Brit. Mus. Guide)
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Not much is known regarding the food of deep-sea animals. In the absence of plant-life they must of necessity be all carnivorous, and all ultimately dependent on the food-supply falling from above. Somespecies have been found to have the food-canal filled with Globigerina ooze, which they no doubt swallow, as earth-worms do the soil in which they burrow, for the purpose of extracting the nutriment that it contains. In one species of deep-sea Cumacea (Platycuma holti), which appears to feed in this manner, the food-canal is coiled, a condition very rare in Crustacea; in all probability this is due to the necessity for an increase of the absorptive surface, since it is common to find such an increase, either by lengthening and consequent coiling of thegut, or by infolding of its walls, in animals that have to swallow large quantities of relatively innutritious food material. Many species, however, no doubt have more selective habits of feeding. The lobster-likeThaumastocheles(Fig. 44), which was dredged by theChallengerexpedition in the West Indies at a depth of 450 fathoms, and has since been got from deep water off the Japanese coast, has one of the chelæ enormously enlarged, with long and slender fingers set with spines like the teeth of a rake. It has been suggested that this remarkable claw may be used for raking or sifting the ooze for small animals on which theThaumastochelesfeeds. A similar function may be suggested for the long and spiny first pair of walking legs in the Spider CrabPlatymaia(Fig. 45).
Thaumastocheles zaleucusFig. 44—Thaumastocheles zaleucus.Reduced.(After Spence Bate.)View larger image
Fig. 44—Thaumastocheles zaleucus.Reduced.(After Spence Bate.)
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A Deep-sea CrabFig. 45—A Deep-sea Crab(Platymaia wyville-thomsoni).Reduced.(After Miers.)View larger imageIn many deep-sea Crustacea the eggs are of very large size, indicating that the young are hatched in an advanced stage of development. For example, in the numerous species of the genusMunidopsisthe eggs are always large and correspondingly few in number, in striking contrast to the closely allied genusGalathea, from shallow water, in which the eggs are small and very numerous. Alcock mentions that a deep-sea Prawn of the genusPsathyrocaris, although only about 3½ inches long, has eggs nearly a quarter of an inch in length. It would seem that, in some way or other, the conditions are unfavourable for a free-swimming larval life; but they cannot be altogether prohibitive, for there are a goodmany characteristically deep-sea Crustacea, such as the Eryonidea, that have small eggs and presumably a larval metamorphosis.Bathynomus giganteusPLATE XVIIIBathynomus giganteus,ABOUT ONE-HALF NATURAL SIZE.(From Lankester's "Treatise on Zoology," after Milne-Edwards and Bouvier)View larger imageThe uniformity of the physical conditions over vast areas in the deep sea is no doubt the cause of the enormously wide geographical range of many species of deep-sea animals. There are many examples of this among Crustacea, and they are added to by every deep-sea dredging expedition. For example, the giant IsopodBathynomus(Plate XVIII.) was first discovered in West Indian seas, and the same species has since been dredged near Ceylon, while a second species has been found off the Japanese coast. Of the strange lobster-likeThaumastocheles(Fig. 44),mentioned above, only four specimens are known—one dredged by theChallengerin the West Indies, and three others more recently brought from Japan.Polycheles phosphorusFig. 46—Polycheles phosphorus,One of the Eryonidea, Female, from the Indian Seas.(From British Museum Guide, after Alcock.)View larger imageThe low temperatures prevailing in deep water, even in tropical seas, render it possible for many Crustacea to live there which are closely allied to, or identical with, species occurring in shallow water in the colder seas of the North and South. Many examples of this are mentioned by Dr. Alcock in his discussion of the deep-sea fauna of Indian seas; for example, the LobsterNephrops andamanicus, found at depths of 150 to 400 fathoms in the Indian seas, is very closely allied to the Norway Lobster (Nephrops norvegicus) of our own coasts. To some extent this fact affords an explanation of the phenomenon that has been called "bipolarity" in the distribution of marine animals. It has been observed that certain families, genera, and even species, are found in the Arctic and Antarctic seas, although they seem to be entirely absent from the intervening tropical zones. In some cases, however, it has been found that these forms occur in the deep sea in the warmer regions where the cold water offers them a connection between North and South without any great difference of temperature.In the early days of deep-sea exploration, when naturalists were becoming aware of the rich fauna inhabiting the abysses of the ocean, which till thenhad been supposed to be barren of all life, it was confidently expected that representatives would be discovered of some of the animals known as fossils from the earlier geological periods. It was believed that the great ocean basins had remained unchanged for vast periods of geological time, and that numerous "living fossils" would be found surviving in the depths. These hopes have not been fully realized, for the deep-sea fauna as a whole has proved to be of a comparatively modern type; nevertheless, it does include a considerable number of primitive and old-fashioned forms of life, some of which belong to groups elsewhere extinct. This is conspicuously the case among the Crustacea. The lobster-like Eryonidea, which at the present day are only found in the deep sea, were long known as fossils before they were discovered to survive as livinganimals. The existing species (Fig. 46) are all blind, with only vestiges of eye-stalks, and they may be readily distinguished by the fact that the first four, and sometimes all five, pairs of legs end in chelæ, no other Decapods having more than three pairs of chelate legs. The fossils occur in rocks of the Secondary Period, from the Trias to the early Cretaceous. Some of them, at least, had well-developed eyes, and probably lived in shallow water. This was almost certainly the habitat of those (Fig. 47) that are found preserved in a marvellously perfect state in the lithographic limestone of Solenhofen (famous for the discovery ofArchæopteryxand many other remarkable fossils), which is believed to have been deposited in a lagoon. After the early part of the Cretaceous epoch, the Eryonidea are no longer found as fossils, and it is, at all events, a probable conjecture that about that period they forsook the shallow waters for the deeper recesses of the ocean, where their descendants have held their own till the present day.Eryon propinquusFig. 47—Eryon propinquus,One of the Fossil Eryonidea, from the Jurassic Rocks of Solenhofen.(From Lankester's "Treatise on Zoology," after Oppel.)View larger imageAnother group of deep-sea Crustacea which has affinities with certain fossil forms is the little family Homolodromiidæ among the Crabs. It has already been mentioned that the Dromiacea are the most primitive tribe of the Brachyura, and Professor Bouvier has shown that among these the Homolodromiidæ approach most nearly to the lobster-like forms from which the Crabs have been derived. He has further shown that the members of this familyclosely resemble in the arrangement of the grooves upon the carapace the extinct Prosoponidæ, which are known as fossils from Jurassic and Cretaceous rocks.It is in the deep sea alsothat we find the curious Hermit Crabs of the familyPylochelidæ(Fig. 37, p. 94), which are perfectly symmetrical and show no trace of having ever adopted the habit of living in Gastropod shells; so primitive, indeed, are these forms that it is not easy to find characters by which to define them from the lobster-like Thalassinidea or from the true Lobsters themselves, and, although no fossil representatives are yet known, there seems no reason to doubt that the Pylochelidæ are nearly related to the primitive stock from which the other Hermit Crabs have been evolved. Among the deep-sea Prawns there are many forms, both of Penæidea and of Caridea, which are more primitive than most of their relatives from shallow water; and although in these cases also the geological records are faulty, we may assume, if we cannot prove in detail, a general similarity to the fossil Prawns of Mesozoic rocks.When all has been said, however, perhaps the most surprising thing about the deep-sea fauna is, not that the animals are unlike those living in shallow water, but that they differ from them so little. When we consider the physical conditions of the oceanic abysses—the absolute darkness, the freezing cold, the pressure measured in tons on the square inch—it would seem inevitable that the physiological processes of deep-sea animals must differ greatly from those of animals living in shallow water; yet in very many cases these differences of function areaccompanied only by the most trivial differences in structure. To take one example, the "Pink Shrimp" (Pandalus montagui), which we may find commonly between tide-marks on our own coasts, differs only in inconspicuous details from species of the same genus living at a depth of 600 fathoms; while other genera of the family Pandalidæ range downwards to 2,000 fathoms or more, without any important divergences in structure.CHAPTER VIIFLOATING CRUSTACEA OF THE OPEN SEAIt is only rarely that the floating organisms of the surface of the sea are so large or so abundant as to catch the attention of the casual observer. Except for an occasional shoal of porpoises or of flying-fish, the waste of waters seen from the deck of a ship in mid-ocean usually seems to be barren of life. Nevertheless, there is probably no region of the ocean where the tow-net will not reveal the existence of a more or less varied fauna and flora. Sometimes, indeed, these organisms, though minute, are so numerous as to discolour the water over large areas; whalers in the Arctic seas know by the appearance of "whale-food" where whales are likely to be found, and Herring or Mackerel fishermen recognize the changes in colour of the water among the "signs" which guide them when and where to shoot their nets.The organisms which make up this "pelagic" fauna and flora may be grouped into two classes, which may be termed the "swimmers," or Necton,and the "drifters," or Plankton. The former include the larger and more active animals, such as fish, whales, and the like, whose movements are more or less independent of the movements of the water; the latter comprise the plant-life and the floating or feebly swimming animals that drift at the mercy of waves and currents. A great deal of attention has been given in recent years to the study of the plankton, and it has come to be recognized as filling a very important place in the balance of life in the sea. In the sea, as on land, all the animals are ultimately dependent on plants for their food. The larger and more conspicuous sea-weeds which grow on the sea-bottom, however, can only flourish in comparatively shallow water, and the region which they occupy forms only a narrow fringe round the land-masses of the globe. It is only necessary to look at a map of the world, showing the depth of the sea, to realize what an insignificant part of the area of the oceans contributes in this way to the food-supply of marine animals. The microscopic plant-life of the plankton, however, makes up for the individual minuteness of its constituents by their incalculable numbers. The lowly organisms known as "diatoms," familiar to the microscopist from the beauty of their flinty skeletons, are among the most numerous and important of these, and they are associated with a great variety of other single-celled algæ and allied organisms, some of them so minutethat they pass through the finest silk plankton-nets, and have to be sought for by special methods of collection recently devised for the purpose. All these organisms possess the green colouring matter (chlorophyll) that enables them to live, as the higher plants do, on the carbon dioxide and other substances dissolved in the water. The smaller animals of the plankton feed on these vegetable organisms, and in their turn serve as food for larger animals. The Herring, the Mackerel, the gigantic Basking Shark, and the still more gigantic Greenland Whale, all feed directly on the animal plankton, and we have already seen that the animals of the deep sea depend entirely on the same source of food-supply. Further, very many of the bottom-living animals of shallow water swim at the surface in the early stages of their life, and feed on the other plankton animals and plants. Indeed, it is no exaggeration to say that "all fish is diatom" in the same physiological sense as "all flesh is grass," and the study of the plankton is thus of practical importance as well as of scientific interest.Of all the minute animals that form the intermediate links in the chain between diatom and fish or whale, the Crustacea are the most important and the most numerous both in species and in individuals. The Copepoda are more richly represented than any of the other groups, and it would be difficult to find a sample of marine plankton from which they werealtogether absent. Associated with them we find one or two species of Cladocera, a larger number of Ostracoda (chiefly of the family Halocypridæ), a few Mysidacea, the Amphipoda of the suborder Hyperiidea, the Euphausiacea, and some of the shrimp-like Decapods; while the larval stages of these and other groups also form an important part of the plankton.It is necessary to make a distinction between the "neritic" plankton of shallow water near the coast and the "oceanic" plankton of the open sea. In the inshore waters the plankton consists not only of organisms that pass the whole of their life at or near the surface, but also, and very largely, of the free-swimming larvæ of bottom-living species, and of others that make occasional and temporary excursions to the surface. For example, if the tow-net be used a short distance from land—say in some sheltered bay on our own coasts—the catch will often be found to consist largely of larval Crustacea. The zoëa and megalopa stages of Crabs, the zoëa and schizopod stages of Prawns and Shrimps, are often conspicuous by their numbers, or we may find swarms of the nauplius and cypris larvæ of Barnacles. Sometimes, and especially at night, numbers of Cumacea may be found in the tow-net; and it is noteworthy that these are usually males, which leave the females burrowing in the mud at the bottom, and swarm to the surface for a brief period ofactivity. Besides all these more or less temporary visitors, however, there are numerous species, even in the inshore waters, which are adapted to a floating life, and pass their whole existence as members of the plankton. Copepoda of many kinds, some Mysidæ, Amphipods likeHyperia—which is commonly found sheltering under large jellyfish—some species of actively swimming Isopods, and many other forms, are only to be captured by the tow-net; and now and then, in certain localities, winds and currents may drive into coastal waters shoals of species whose proper home is the open ocean.In a similar way the strictly neritic forms may sometimes be carried far out to sea, so that it is nowhere possible to draw a hard-and-fast line between the regions occupied by the neritic and the oceanic plankton. With increasing distance from land, however, the larval stages of bottom-living species become fewer, and finally disappear altogether, and there is left an assemblage of animals whose whole existence is passed floating at the surface or at the intermediate depths. How far down from the surface this floating fauna actually descends is a question which has been much debated. It appears now to be certain that there is no stratum of water between the surface and the bottom of the ocean which is devoid of life, although the upper layers (not at, but some distance below, the surface)are probably much more densely populated than those of the abyss. Many of the species appear to undertake more or less extensive migrations in a vertical direction, coming nearer the surface at certain stages of their life-history, and sinking into deeper water at others. Further, some species at least seem to rise to the surface at night, and to sink again during the day. Apart from these vertical movements, which are as yet only imperfectly understood, it is desirable to distinguish between the "epiplankton," comprising the organisms which inhabit the superficial strata of the ocean down to about 100 fathoms, and the "mesoplankton," found at greater depths. The plant-life which is dependent on sunlight belongs to the epiplankton, while the animals of the mesoplankton are dependent, like the bottom animals of the deep sea, on the supply of dead food material falling from above. A third division, the "hypoplankton," has been established for those animals which live immediately above the bottom, but its distinctness from the mesoplankton has not yet been satisfactorily established. Indeed, many of the swimming forms which have already been mentioned in dealing with the Crustacea of the deep sea are probably rather to be considered as belonging to the deep mesoplankton—at least, where their size and swimming powers do not entitle them to be ranked with the "necton."Conchoecia curtaFig. 48—Conchœcia curta,an Ostracod of the Plankton.× 40. (Partly after G. W. Müller.)View larger imageMany of the modifications in structure characteristicof pelagic animals may be traced to the necessity for keeping continuously afloat with a minimum of exertion. The Crustacea of the plankton never carry the heavy armour found in bottom-living species. Thus, the thick-shelled Ostracoda of the bottom are represented in the plankton chiefly by the family Halocypridæ (Fig. 48), in which the shell is thin, uncalcified, and almost membranous. Many species, particularly of the Copepoda, are seen, under the microscope, to have large globules of oil distributed through the tissues of the body, and these no doubt serve as floats, increasing the buoyancy of the animal. The same purpose is probably served, in many cases, by having large spaces, filled with fluid, within the body. This is characteristic of pelagic animals, and is well seen in many of the Crustacea in which the viscera and muscles occupy a relatively small part of the interior of the animals, the intervening spaces being filled with colourless transparent fluid. Many of theHyperid Amphipoda show this peculiarity—for example, the relatively giganticCystisoma, which is mesoplanktonic in deep water; and it reaches its extreme inMimonectes(Fig. 49), in which the anterior part of the body is, as it were, blown out into a balloon, giving the animal the aspect of a small jellyfish rather than an Amphipod.Mimonectes loveniFig. 49—Mimonectes loveni.A Female Specimen seen from the Side and from Below, showing the Distended-balloon-like Form of the Anterior Part of the Body.× 3. (After Bovallius.)View larger imageIf, as seems probable, the body-fluid of these animals is of a lower specific gravity than the sea-water, it will act like the oil-globules of the Copepoda in keeping the animals afloat. Even if the specific gravity be the same, however, the distensionof the body with fluid acts in another way, by increasing the surface exposed to friction with the surrounding water, and so retarding sinking. The principle involved is illustrated by the fact that a soap-bubble sinks much more slowly through the air than the drop of water into which it collapses. The same result is produced if the surface is increasedby outstanding spines or hairs, just as, for instance, a downy feather sinks slowly through the air, but drops rapidly if it is rolled into a ball between the fingers. This is, no doubt, one function of the spines with which plankton Crustacea, and particularly larvæ, are frequently provided, though theymay also serve in some cases as a protection against enemies. The spines have been already alluded to in describing the various larvæ, but it may be noted here that they are most strongly developed in larvæ which live in the open ocean; for example, the most elaborately armed of all Decapod larvæ are the zoëa stages ofSergestes(Fig. 50), which, like the adults, belong to the oceanic plankton. The nauplius larvæ of Cirripedes are all more or less spiny, and the spines reach an exaggerated development in the larvæ of the genusLepas(Fig. 51), of which the adults are attached to floating drift-wood or the like, and belong to the oceanic fauna, although hardly to be classed with the plankton.The Zoëa Larva of a Species of SergestesFig. 50—The Zoëa Larva of a Species ofSergestes,taken by the "Challenger" Expedition.× 25. (After Spence Bate.)View larger imageThe Nauplius Larva of a Species of BarnacleFig. 51—The Nauplius Larva of a Species of Barnacle of the Family Lepadidæ, showing greatly-developed Spines. From a Specimen taken in the Atlantic Ocean, near Madeira.× 11. (After Chun.)View larger imageThe large feathered bristles that decorate the limbs or tail of many plankton Copepoda have no doubt the same function in assisting flotation. In the genusCalocalanus(Fig. 52), for example, the tail setæ are large and brilliantly coloured feathery plumes, and in one species,C. plumulosus, one of these setæ is of relatively enormous size, five or six times as long as the body of the animal itself.Calocalanus pavoFig. 52—Calocalanus pavo,One of the Free-swimming Copepoda of the Plankton. Enlarged.(From Lankester's "Treatise on Zoology," after Giesbrecht.)View larger imageAmong the most singular of plankton Crustacea are thePhyllosomalarvæ (seeFig. 28, p. 72) of the Spiny Lobsters and their allies (Scyllaridea), which have been already described. These larvæ are sometimes found far out at sea, and it seems likely that their larval life is unusually prolonged, and that they may be drifted to great distances by ocean currents. At all events, they are well adapted for pelagic life, since the broad flat body, hardly thicker than a sheet of paper, can be sustained in the water like a "hydroplane" by comparatively slight efforts of the swimming legs.The watery character of the body, together with the thinness of the exoskeleton, helps to explain the glassy transparency which is a feature of most plankton Crustacea. This transparency has been regarded as a protective adaptation rendering the animals inconspicuous in the water, and it has indeed that effect to human eyes, but it is very doubtful whether the animals derive much benefit from this. Many of the animals—such as Herringand other pelagic fishes—that prey upon plankton Crustacea appear to swallow them in bulk, without much selection; and the Greenland Whale, as it swims open-mouthed through the sea, is not likely to be guided by the greater or less visibility of the Copepods that it sifts out on its baleen plates. Further, this glass-like transparency is by no means universal, for many plankton Copepoda are brightly coloured. In some, as in the beautiful blueAnomalocera, common in British waters, the colour is due to pigment in the fluids and tissues of the body; in others the feathery hairs on the body and limbs show brilliant metallic colours, produced, like the colours of a peacock's feather, not by pigments, but by the diffraction of light in the texture of the organ. The most beautiful of all Copepoda isSapphirina, in which the surface of the body absolutely sparkles with iridescent colours.The striking phenomenon known as the "phosphorescence of the sea" is familiar to every ocean voyager, and is seen from time to time on our own coast. On a dark night the crest of every wave often seems to break in a pale glow, the wake of the vessel is a trail of light, and an oar dipped in the water seems on fire. This luminosity is due to the animals of the plankton, largely to the lowly Protozoa and the jellyfishes, but in part also to certain Crustacea. A number of pelagic Copepoda have been shown by Giesbrecht to secrete, from special glands on the surfaceof the body, a substance which becomes luminous on coming in contact with the water. Even specimens which had been dried were found to give out light on being wetted. Some pelagic Ostracods of the family Halocypridæ have been observed to emit clouds of a luminous secretion from a gland in the neighbourhood of the mouth. A similar habit has been seen, as already mentioned, in certain deep-sea Prawns and Mysidacea, which may perhaps belong to the deeper part of the mesoplankton rather than to the bottom fauna. The complex light-producing organs of the Euphausiacea have already been described in dealing with deep-sea Crustacea. A great many species of this group, however, are members of the epiplankton, and in these the phosphorescent apparatus is quite as fully developed as in species coming from greater depths.Meganyctiphanes norvegica(Fig. 24, p. 56), which is one of the largest of the Euphausiacea, is common at no great depths in many places in British seas. If a jar of sea-water in which specimens of this species are swimming be brought into a dark room, a tap on the glass will cause the photophores to flash out like a row of tiny lamps along the side of the body. After shining for a few seconds the light dies out, to appear again if the tapping be repeated.There are certain peculiarities in the structure of the eyes in some plankton Crustacea which suggest that the sense of sight is of special importance totheir possessors, although we can hardly do more than guess at their special significance. Most Copepoda have only a single eye in the middle of the head, corresponding to the single eye of the nauplius larva, and of far simpler structure than the paired compound eyes of most other Crustacea. In many plankton species, however, this simple eye becomes much enlarged and complicated in various ways. The three parts of which it is normally made up may become separated from each other, and are sometimes increased in number to five, while lenses serving to concentrate the light are often developed by thickening of the overlying cuticle. The most elaborately constructed eyes are found in the family Corycæidæ. InCopilia(Fig. 53) a pair of eyes of relatively enormous size are present. Each has in front a large biconvex lens set at the end of a conical tube which extends backwards to a smaller lens (like a telescope with object-glass and eyepiece), behind which, again, are the sensory cells, corresponding to the retina, enclosed in a tube of dark pigment, the whole apparatus being more than half the length of the body. These eyes, although paired, do not correspond to the paired compound eyes of other Crustacea, but have arisen by the separation and enlargement of two of the three divisions of the typical median Copepod eye.Copilia quadrataFig. 53—Copilia quadrata(Female),a Copepod of the Family Corycæidæ, showing the Pair of Large "Telescopic" Eyes.x 20. (After Giesbrecht.)View larger imageA peculiarity of the paired compound eyes found in plankton Crustacea of several different ordersconsists in the division of each eye into two parts, which differ in structure. In many Euphausiacea and Mysidacea, especially in those haunting the deeper strata (mesoplankton), this division of the eyes is well marked, a frontal or dorsal part having the separate elements of the eye (ommatidia) greatlylengthened and with reduced pigment, while the lateral part is of more normal structure. It seems probable, from the researches of Professor Chun, that the fronto-dorsal division is adapted for the perception of very faint light, while the lateral division will give a more accurate image of brightly illuminated objects.Phronima collettiFig. 54—Phronima colletti,Male.From a Specimen taken in Deep Water near the Canary Islands.× 12. (After Chun.)View larger imageIn the pelagic Amphipoda, forming the suborder Hyperiidea, the eyes are of very large size, generally occupying almost the whole surface of the head, and giving the animals a very characteristic appearance, in contrast to the small-eyed, bottom-living Gammaridea. In the family Phronimidæ (Fig. 54) the eyes are each divided into two parts, differing in structure in the way just described.There are a few Crustacea living habitually on the high seas which cannot be reckoned as belonging either to the true plankton or to the necton, since they depend on outside help for keeping themselves afloat. Among these are the Barnacles which cluster on logs of drift-wood, and are among the most important causes of the "fouling" of ships' hulls on long voyages. The stalked Barnacles of the genusLepasare especially common in such situations, and the characters of their larvæ have been already alluded to. Certain species of sessile Barnacles are constantly found attached to large marine animals. For example,Chelonobiaadheres to the shell of Turtles, whileCoronulaand some allied genera are found on Whales.PLATE XIXLatreillia elegansLatreillia elegans,ONE OF THE DROMIACEA WHICH RESEMBLES A SPIDER-CRAB. FROM THE MEDITERRANEAN. (NATURAL SIZE)View larger imagePlanes minutusTHE GULF-WEED CRAB,Planes minutus. (SLIGHTLY ENLARGED)View larger image
Fig. 45—A Deep-sea Crab(Platymaia wyville-thomsoni).Reduced.
(After Miers.)
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In many deep-sea Crustacea the eggs are of very large size, indicating that the young are hatched in an advanced stage of development. For example, in the numerous species of the genusMunidopsisthe eggs are always large and correspondingly few in number, in striking contrast to the closely allied genusGalathea, from shallow water, in which the eggs are small and very numerous. Alcock mentions that a deep-sea Prawn of the genusPsathyrocaris, although only about 3½ inches long, has eggs nearly a quarter of an inch in length. It would seem that, in some way or other, the conditions are unfavourable for a free-swimming larval life; but they cannot be altogether prohibitive, for there are a goodmany characteristically deep-sea Crustacea, such as the Eryonidea, that have small eggs and presumably a larval metamorphosis.
Bathynomus giganteusPLATE XVIIIBathynomus giganteus,ABOUT ONE-HALF NATURAL SIZE.(From Lankester's "Treatise on Zoology," after Milne-Edwards and Bouvier)View larger image
PLATE XVIII
Bathynomus giganteus,ABOUT ONE-HALF NATURAL SIZE.
(From Lankester's "Treatise on Zoology," after Milne-Edwards and Bouvier)
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The uniformity of the physical conditions over vast areas in the deep sea is no doubt the cause of the enormously wide geographical range of many species of deep-sea animals. There are many examples of this among Crustacea, and they are added to by every deep-sea dredging expedition. For example, the giant IsopodBathynomus(Plate XVIII.) was first discovered in West Indian seas, and the same species has since been dredged near Ceylon, while a second species has been found off the Japanese coast. Of the strange lobster-likeThaumastocheles(Fig. 44),mentioned above, only four specimens are known—one dredged by theChallengerin the West Indies, and three others more recently brought from Japan.
Polycheles phosphorusFig. 46—Polycheles phosphorus,One of the Eryonidea, Female, from the Indian Seas.(From British Museum Guide, after Alcock.)View larger image
Fig. 46—Polycheles phosphorus,One of the Eryonidea, Female, from the Indian Seas.(From British Museum Guide, after Alcock.)
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The low temperatures prevailing in deep water, even in tropical seas, render it possible for many Crustacea to live there which are closely allied to, or identical with, species occurring in shallow water in the colder seas of the North and South. Many examples of this are mentioned by Dr. Alcock in his discussion of the deep-sea fauna of Indian seas; for example, the LobsterNephrops andamanicus, found at depths of 150 to 400 fathoms in the Indian seas, is very closely allied to the Norway Lobster (Nephrops norvegicus) of our own coasts. To some extent this fact affords an explanation of the phenomenon that has been called "bipolarity" in the distribution of marine animals. It has been observed that certain families, genera, and even species, are found in the Arctic and Antarctic seas, although they seem to be entirely absent from the intervening tropical zones. In some cases, however, it has been found that these forms occur in the deep sea in the warmer regions where the cold water offers them a connection between North and South without any great difference of temperature.
In the early days of deep-sea exploration, when naturalists were becoming aware of the rich fauna inhabiting the abysses of the ocean, which till thenhad been supposed to be barren of all life, it was confidently expected that representatives would be discovered of some of the animals known as fossils from the earlier geological periods. It was believed that the great ocean basins had remained unchanged for vast periods of geological time, and that numerous "living fossils" would be found surviving in the depths. These hopes have not been fully realized, for the deep-sea fauna as a whole has proved to be of a comparatively modern type; nevertheless, it does include a considerable number of primitive and old-fashioned forms of life, some of which belong to groups elsewhere extinct. This is conspicuously the case among the Crustacea. The lobster-like Eryonidea, which at the present day are only found in the deep sea, were long known as fossils before they were discovered to survive as livinganimals. The existing species (Fig. 46) are all blind, with only vestiges of eye-stalks, and they may be readily distinguished by the fact that the first four, and sometimes all five, pairs of legs end in chelæ, no other Decapods having more than three pairs of chelate legs. The fossils occur in rocks of the Secondary Period, from the Trias to the early Cretaceous. Some of them, at least, had well-developed eyes, and probably lived in shallow water. This was almost certainly the habitat of those (Fig. 47) that are found preserved in a marvellously perfect state in the lithographic limestone of Solenhofen (famous for the discovery ofArchæopteryxand many other remarkable fossils), which is believed to have been deposited in a lagoon. After the early part of the Cretaceous epoch, the Eryonidea are no longer found as fossils, and it is, at all events, a probable conjecture that about that period they forsook the shallow waters for the deeper recesses of the ocean, where their descendants have held their own till the present day.
Eryon propinquusFig. 47—Eryon propinquus,One of the Fossil Eryonidea, from the Jurassic Rocks of Solenhofen.(From Lankester's "Treatise on Zoology," after Oppel.)View larger image
Fig. 47—Eryon propinquus,One of the Fossil Eryonidea, from the Jurassic Rocks of Solenhofen.(From Lankester's "Treatise on Zoology," after Oppel.)
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Another group of deep-sea Crustacea which has affinities with certain fossil forms is the little family Homolodromiidæ among the Crabs. It has already been mentioned that the Dromiacea are the most primitive tribe of the Brachyura, and Professor Bouvier has shown that among these the Homolodromiidæ approach most nearly to the lobster-like forms from which the Crabs have been derived. He has further shown that the members of this familyclosely resemble in the arrangement of the grooves upon the carapace the extinct Prosoponidæ, which are known as fossils from Jurassic and Cretaceous rocks.
It is in the deep sea alsothat we find the curious Hermit Crabs of the familyPylochelidæ(Fig. 37, p. 94), which are perfectly symmetrical and show no trace of having ever adopted the habit of living in Gastropod shells; so primitive, indeed, are these forms that it is not easy to find characters by which to define them from the lobster-like Thalassinidea or from the true Lobsters themselves, and, although no fossil representatives are yet known, there seems no reason to doubt that the Pylochelidæ are nearly related to the primitive stock from which the other Hermit Crabs have been evolved. Among the deep-sea Prawns there are many forms, both of Penæidea and of Caridea, which are more primitive than most of their relatives from shallow water; and although in these cases also the geological records are faulty, we may assume, if we cannot prove in detail, a general similarity to the fossil Prawns of Mesozoic rocks.
When all has been said, however, perhaps the most surprising thing about the deep-sea fauna is, not that the animals are unlike those living in shallow water, but that they differ from them so little. When we consider the physical conditions of the oceanic abysses—the absolute darkness, the freezing cold, the pressure measured in tons on the square inch—it would seem inevitable that the physiological processes of deep-sea animals must differ greatly from those of animals living in shallow water; yet in very many cases these differences of function areaccompanied only by the most trivial differences in structure. To take one example, the "Pink Shrimp" (Pandalus montagui), which we may find commonly between tide-marks on our own coasts, differs only in inconspicuous details from species of the same genus living at a depth of 600 fathoms; while other genera of the family Pandalidæ range downwards to 2,000 fathoms or more, without any important divergences in structure.
It is only rarely that the floating organisms of the surface of the sea are so large or so abundant as to catch the attention of the casual observer. Except for an occasional shoal of porpoises or of flying-fish, the waste of waters seen from the deck of a ship in mid-ocean usually seems to be barren of life. Nevertheless, there is probably no region of the ocean where the tow-net will not reveal the existence of a more or less varied fauna and flora. Sometimes, indeed, these organisms, though minute, are so numerous as to discolour the water over large areas; whalers in the Arctic seas know by the appearance of "whale-food" where whales are likely to be found, and Herring or Mackerel fishermen recognize the changes in colour of the water among the "signs" which guide them when and where to shoot their nets.
The organisms which make up this "pelagic" fauna and flora may be grouped into two classes, which may be termed the "swimmers," or Necton,and the "drifters," or Plankton. The former include the larger and more active animals, such as fish, whales, and the like, whose movements are more or less independent of the movements of the water; the latter comprise the plant-life and the floating or feebly swimming animals that drift at the mercy of waves and currents. A great deal of attention has been given in recent years to the study of the plankton, and it has come to be recognized as filling a very important place in the balance of life in the sea. In the sea, as on land, all the animals are ultimately dependent on plants for their food. The larger and more conspicuous sea-weeds which grow on the sea-bottom, however, can only flourish in comparatively shallow water, and the region which they occupy forms only a narrow fringe round the land-masses of the globe. It is only necessary to look at a map of the world, showing the depth of the sea, to realize what an insignificant part of the area of the oceans contributes in this way to the food-supply of marine animals. The microscopic plant-life of the plankton, however, makes up for the individual minuteness of its constituents by their incalculable numbers. The lowly organisms known as "diatoms," familiar to the microscopist from the beauty of their flinty skeletons, are among the most numerous and important of these, and they are associated with a great variety of other single-celled algæ and allied organisms, some of them so minutethat they pass through the finest silk plankton-nets, and have to be sought for by special methods of collection recently devised for the purpose. All these organisms possess the green colouring matter (chlorophyll) that enables them to live, as the higher plants do, on the carbon dioxide and other substances dissolved in the water. The smaller animals of the plankton feed on these vegetable organisms, and in their turn serve as food for larger animals. The Herring, the Mackerel, the gigantic Basking Shark, and the still more gigantic Greenland Whale, all feed directly on the animal plankton, and we have already seen that the animals of the deep sea depend entirely on the same source of food-supply. Further, very many of the bottom-living animals of shallow water swim at the surface in the early stages of their life, and feed on the other plankton animals and plants. Indeed, it is no exaggeration to say that "all fish is diatom" in the same physiological sense as "all flesh is grass," and the study of the plankton is thus of practical importance as well as of scientific interest.
Of all the minute animals that form the intermediate links in the chain between diatom and fish or whale, the Crustacea are the most important and the most numerous both in species and in individuals. The Copepoda are more richly represented than any of the other groups, and it would be difficult to find a sample of marine plankton from which they werealtogether absent. Associated with them we find one or two species of Cladocera, a larger number of Ostracoda (chiefly of the family Halocypridæ), a few Mysidacea, the Amphipoda of the suborder Hyperiidea, the Euphausiacea, and some of the shrimp-like Decapods; while the larval stages of these and other groups also form an important part of the plankton.
It is necessary to make a distinction between the "neritic" plankton of shallow water near the coast and the "oceanic" plankton of the open sea. In the inshore waters the plankton consists not only of organisms that pass the whole of their life at or near the surface, but also, and very largely, of the free-swimming larvæ of bottom-living species, and of others that make occasional and temporary excursions to the surface. For example, if the tow-net be used a short distance from land—say in some sheltered bay on our own coasts—the catch will often be found to consist largely of larval Crustacea. The zoëa and megalopa stages of Crabs, the zoëa and schizopod stages of Prawns and Shrimps, are often conspicuous by their numbers, or we may find swarms of the nauplius and cypris larvæ of Barnacles. Sometimes, and especially at night, numbers of Cumacea may be found in the tow-net; and it is noteworthy that these are usually males, which leave the females burrowing in the mud at the bottom, and swarm to the surface for a brief period ofactivity. Besides all these more or less temporary visitors, however, there are numerous species, even in the inshore waters, which are adapted to a floating life, and pass their whole existence as members of the plankton. Copepoda of many kinds, some Mysidæ, Amphipods likeHyperia—which is commonly found sheltering under large jellyfish—some species of actively swimming Isopods, and many other forms, are only to be captured by the tow-net; and now and then, in certain localities, winds and currents may drive into coastal waters shoals of species whose proper home is the open ocean.
In a similar way the strictly neritic forms may sometimes be carried far out to sea, so that it is nowhere possible to draw a hard-and-fast line between the regions occupied by the neritic and the oceanic plankton. With increasing distance from land, however, the larval stages of bottom-living species become fewer, and finally disappear altogether, and there is left an assemblage of animals whose whole existence is passed floating at the surface or at the intermediate depths. How far down from the surface this floating fauna actually descends is a question which has been much debated. It appears now to be certain that there is no stratum of water between the surface and the bottom of the ocean which is devoid of life, although the upper layers (not at, but some distance below, the surface)are probably much more densely populated than those of the abyss. Many of the species appear to undertake more or less extensive migrations in a vertical direction, coming nearer the surface at certain stages of their life-history, and sinking into deeper water at others. Further, some species at least seem to rise to the surface at night, and to sink again during the day. Apart from these vertical movements, which are as yet only imperfectly understood, it is desirable to distinguish between the "epiplankton," comprising the organisms which inhabit the superficial strata of the ocean down to about 100 fathoms, and the "mesoplankton," found at greater depths. The plant-life which is dependent on sunlight belongs to the epiplankton, while the animals of the mesoplankton are dependent, like the bottom animals of the deep sea, on the supply of dead food material falling from above. A third division, the "hypoplankton," has been established for those animals which live immediately above the bottom, but its distinctness from the mesoplankton has not yet been satisfactorily established. Indeed, many of the swimming forms which have already been mentioned in dealing with the Crustacea of the deep sea are probably rather to be considered as belonging to the deep mesoplankton—at least, where their size and swimming powers do not entitle them to be ranked with the "necton."
Conchoecia curtaFig. 48—Conchœcia curta,an Ostracod of the Plankton.× 40. (Partly after G. W. Müller.)View larger image
Fig. 48—Conchœcia curta,an Ostracod of the Plankton.× 40. (Partly after G. W. Müller.)
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Many of the modifications in structure characteristicof pelagic animals may be traced to the necessity for keeping continuously afloat with a minimum of exertion. The Crustacea of the plankton never carry the heavy armour found in bottom-living species. Thus, the thick-shelled Ostracoda of the bottom are represented in the plankton chiefly by the family Halocypridæ (Fig. 48), in which the shell is thin, uncalcified, and almost membranous. Many species, particularly of the Copepoda, are seen, under the microscope, to have large globules of oil distributed through the tissues of the body, and these no doubt serve as floats, increasing the buoyancy of the animal. The same purpose is probably served, in many cases, by having large spaces, filled with fluid, within the body. This is characteristic of pelagic animals, and is well seen in many of the Crustacea in which the viscera and muscles occupy a relatively small part of the interior of the animals, the intervening spaces being filled with colourless transparent fluid. Many of theHyperid Amphipoda show this peculiarity—for example, the relatively giganticCystisoma, which is mesoplanktonic in deep water; and it reaches its extreme inMimonectes(Fig. 49), in which the anterior part of the body is, as it were, blown out into a balloon, giving the animal the aspect of a small jellyfish rather than an Amphipod.
Mimonectes loveniFig. 49—Mimonectes loveni.A Female Specimen seen from the Side and from Below, showing the Distended-balloon-like Form of the Anterior Part of the Body.× 3. (After Bovallius.)View larger image
Fig. 49—Mimonectes loveni.A Female Specimen seen from the Side and from Below, showing the Distended-balloon-like Form of the Anterior Part of the Body.× 3. (After Bovallius.)
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If, as seems probable, the body-fluid of these animals is of a lower specific gravity than the sea-water, it will act like the oil-globules of the Copepoda in keeping the animals afloat. Even if the specific gravity be the same, however, the distensionof the body with fluid acts in another way, by increasing the surface exposed to friction with the surrounding water, and so retarding sinking. The principle involved is illustrated by the fact that a soap-bubble sinks much more slowly through the air than the drop of water into which it collapses. The same result is produced if the surface is increasedby outstanding spines or hairs, just as, for instance, a downy feather sinks slowly through the air, but drops rapidly if it is rolled into a ball between the fingers. This is, no doubt, one function of the spines with which plankton Crustacea, and particularly larvæ, are frequently provided, though theymay also serve in some cases as a protection against enemies. The spines have been already alluded to in describing the various larvæ, but it may be noted here that they are most strongly developed in larvæ which live in the open ocean; for example, the most elaborately armed of all Decapod larvæ are the zoëa stages ofSergestes(Fig. 50), which, like the adults, belong to the oceanic plankton. The nauplius larvæ of Cirripedes are all more or less spiny, and the spines reach an exaggerated development in the larvæ of the genusLepas(Fig. 51), of which the adults are attached to floating drift-wood or the like, and belong to the oceanic fauna, although hardly to be classed with the plankton.
The Zoëa Larva of a Species of SergestesFig. 50—The Zoëa Larva of a Species ofSergestes,taken by the "Challenger" Expedition.× 25. (After Spence Bate.)View larger image
Fig. 50—The Zoëa Larva of a Species ofSergestes,taken by the "Challenger" Expedition.× 25. (After Spence Bate.)
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The Nauplius Larva of a Species of BarnacleFig. 51—The Nauplius Larva of a Species of Barnacle of the Family Lepadidæ, showing greatly-developed Spines. From a Specimen taken in the Atlantic Ocean, near Madeira.× 11. (After Chun.)View larger image
Fig. 51—The Nauplius Larva of a Species of Barnacle of the Family Lepadidæ, showing greatly-developed Spines. From a Specimen taken in the Atlantic Ocean, near Madeira.× 11. (After Chun.)
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The large feathered bristles that decorate the limbs or tail of many plankton Copepoda have no doubt the same function in assisting flotation. In the genusCalocalanus(Fig. 52), for example, the tail setæ are large and brilliantly coloured feathery plumes, and in one species,C. plumulosus, one of these setæ is of relatively enormous size, five or six times as long as the body of the animal itself.
Calocalanus pavoFig. 52—Calocalanus pavo,One of the Free-swimming Copepoda of the Plankton. Enlarged.(From Lankester's "Treatise on Zoology," after Giesbrecht.)View larger image
Fig. 52—Calocalanus pavo,One of the Free-swimming Copepoda of the Plankton. Enlarged.(From Lankester's "Treatise on Zoology," after Giesbrecht.)
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Among the most singular of plankton Crustacea are thePhyllosomalarvæ (seeFig. 28, p. 72) of the Spiny Lobsters and their allies (Scyllaridea), which have been already described. These larvæ are sometimes found far out at sea, and it seems likely that their larval life is unusually prolonged, and that they may be drifted to great distances by ocean currents. At all events, they are well adapted for pelagic life, since the broad flat body, hardly thicker than a sheet of paper, can be sustained in the water like a "hydroplane" by comparatively slight efforts of the swimming legs.
The watery character of the body, together with the thinness of the exoskeleton, helps to explain the glassy transparency which is a feature of most plankton Crustacea. This transparency has been regarded as a protective adaptation rendering the animals inconspicuous in the water, and it has indeed that effect to human eyes, but it is very doubtful whether the animals derive much benefit from this. Many of the animals—such as Herringand other pelagic fishes—that prey upon plankton Crustacea appear to swallow them in bulk, without much selection; and the Greenland Whale, as it swims open-mouthed through the sea, is not likely to be guided by the greater or less visibility of the Copepods that it sifts out on its baleen plates. Further, this glass-like transparency is by no means universal, for many plankton Copepoda are brightly coloured. In some, as in the beautiful blueAnomalocera, common in British waters, the colour is due to pigment in the fluids and tissues of the body; in others the feathery hairs on the body and limbs show brilliant metallic colours, produced, like the colours of a peacock's feather, not by pigments, but by the diffraction of light in the texture of the organ. The most beautiful of all Copepoda isSapphirina, in which the surface of the body absolutely sparkles with iridescent colours.
The striking phenomenon known as the "phosphorescence of the sea" is familiar to every ocean voyager, and is seen from time to time on our own coast. On a dark night the crest of every wave often seems to break in a pale glow, the wake of the vessel is a trail of light, and an oar dipped in the water seems on fire. This luminosity is due to the animals of the plankton, largely to the lowly Protozoa and the jellyfishes, but in part also to certain Crustacea. A number of pelagic Copepoda have been shown by Giesbrecht to secrete, from special glands on the surfaceof the body, a substance which becomes luminous on coming in contact with the water. Even specimens which had been dried were found to give out light on being wetted. Some pelagic Ostracods of the family Halocypridæ have been observed to emit clouds of a luminous secretion from a gland in the neighbourhood of the mouth. A similar habit has been seen, as already mentioned, in certain deep-sea Prawns and Mysidacea, which may perhaps belong to the deeper part of the mesoplankton rather than to the bottom fauna. The complex light-producing organs of the Euphausiacea have already been described in dealing with deep-sea Crustacea. A great many species of this group, however, are members of the epiplankton, and in these the phosphorescent apparatus is quite as fully developed as in species coming from greater depths.Meganyctiphanes norvegica(Fig. 24, p. 56), which is one of the largest of the Euphausiacea, is common at no great depths in many places in British seas. If a jar of sea-water in which specimens of this species are swimming be brought into a dark room, a tap on the glass will cause the photophores to flash out like a row of tiny lamps along the side of the body. After shining for a few seconds the light dies out, to appear again if the tapping be repeated.
There are certain peculiarities in the structure of the eyes in some plankton Crustacea which suggest that the sense of sight is of special importance totheir possessors, although we can hardly do more than guess at their special significance. Most Copepoda have only a single eye in the middle of the head, corresponding to the single eye of the nauplius larva, and of far simpler structure than the paired compound eyes of most other Crustacea. In many plankton species, however, this simple eye becomes much enlarged and complicated in various ways. The three parts of which it is normally made up may become separated from each other, and are sometimes increased in number to five, while lenses serving to concentrate the light are often developed by thickening of the overlying cuticle. The most elaborately constructed eyes are found in the family Corycæidæ. InCopilia(Fig. 53) a pair of eyes of relatively enormous size are present. Each has in front a large biconvex lens set at the end of a conical tube which extends backwards to a smaller lens (like a telescope with object-glass and eyepiece), behind which, again, are the sensory cells, corresponding to the retina, enclosed in a tube of dark pigment, the whole apparatus being more than half the length of the body. These eyes, although paired, do not correspond to the paired compound eyes of other Crustacea, but have arisen by the separation and enlargement of two of the three divisions of the typical median Copepod eye.
Copilia quadrataFig. 53—Copilia quadrata(Female),a Copepod of the Family Corycæidæ, showing the Pair of Large "Telescopic" Eyes.x 20. (After Giesbrecht.)View larger image
Fig. 53—Copilia quadrata(Female),a Copepod of the Family Corycæidæ, showing the Pair of Large "Telescopic" Eyes.x 20. (After Giesbrecht.)
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A peculiarity of the paired compound eyes found in plankton Crustacea of several different ordersconsists in the division of each eye into two parts, which differ in structure. In many Euphausiacea and Mysidacea, especially in those haunting the deeper strata (mesoplankton), this division of the eyes is well marked, a frontal or dorsal part having the separate elements of the eye (ommatidia) greatlylengthened and with reduced pigment, while the lateral part is of more normal structure. It seems probable, from the researches of Professor Chun, that the fronto-dorsal division is adapted for the perception of very faint light, while the lateral division will give a more accurate image of brightly illuminated objects.
Phronima collettiFig. 54—Phronima colletti,Male.From a Specimen taken in Deep Water near the Canary Islands.× 12. (After Chun.)View larger image
Fig. 54—Phronima colletti,Male.From a Specimen taken in Deep Water near the Canary Islands.× 12. (After Chun.)
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In the pelagic Amphipoda, forming the suborder Hyperiidea, the eyes are of very large size, generally occupying almost the whole surface of the head, and giving the animals a very characteristic appearance, in contrast to the small-eyed, bottom-living Gammaridea. In the family Phronimidæ (Fig. 54) the eyes are each divided into two parts, differing in structure in the way just described.
There are a few Crustacea living habitually on the high seas which cannot be reckoned as belonging either to the true plankton or to the necton, since they depend on outside help for keeping themselves afloat. Among these are the Barnacles which cluster on logs of drift-wood, and are among the most important causes of the "fouling" of ships' hulls on long voyages. The stalked Barnacles of the genusLepasare especially common in such situations, and the characters of their larvæ have been already alluded to. Certain species of sessile Barnacles are constantly found attached to large marine animals. For example,Chelonobiaadheres to the shell of Turtles, whileCoronulaand some allied genera are found on Whales.
PLATE XIXLatreillia elegansLatreillia elegans,ONE OF THE DROMIACEA WHICH RESEMBLES A SPIDER-CRAB. FROM THE MEDITERRANEAN. (NATURAL SIZE)View larger imagePlanes minutusTHE GULF-WEED CRAB,Planes minutus. (SLIGHTLY ENLARGED)View larger image
PLATE XIX
Latreillia elegansLatreillia elegans,ONE OF THE DROMIACEA WHICH RESEMBLES A SPIDER-CRAB. FROM THE MEDITERRANEAN. (NATURAL SIZE)View larger image
Latreillia elegans,ONE OF THE DROMIACEA WHICH RESEMBLES A SPIDER-CRAB. FROM THE MEDITERRANEAN. (NATURAL SIZE)
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Planes minutusTHE GULF-WEED CRAB,Planes minutus. (SLIGHTLY ENLARGED)
THE GULF-WEED CRAB,Planes minutus. (SLIGHTLY ENLARGED)
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The little "Gulf-weed Crab" (Planes minutus—Plate XIX.) is found clinging to floating drift-weed nearly everywhere throughout the temperate and tropical seas of the globe, and is especially common in the area known as the Sargasso Sea, in mid-Atlantic. It is occasionally drifted to the south coasts of the British Islands. In Sloane's "Natural History of Jamaica," published in 1707-1725, it is stated of the Gulf-weed Crab that "Columbus, finding this alive on the Sargasso floating in the sea, conceived himself not far from some land, on the first voyage he made on the discovery of the West Indies."
A few other Crustacea also form part of thepeculiar fauna which is associated with the Sargasso weed, notably a swimming Crab,Neptunus sayi, and two or three species of Prawns. All of these are coloured olive-green, like the weed among which they live.
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 muchsurprise 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 waterand journey for short distances overland. On the other hand, since free-swimming larvæ 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 thesea, 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; seeFig. 10, p. 35) has been found in swarms in the water standing in deep cart-ruts in a country lane in England, andApussometimes 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 ofEstheria(seeFig. 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 withChirocephalus. 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 ofApus, measuring in all about 2½ to 3 inches in length, were swimming about; and, as not a single one was to befound 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 byApus cancriformis(seePlate 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 asApus, 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 withthe 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 ofApus(Notostraca) attacked by three Water-beetles, which were tearing its soft appendages, and they suppose thatApusgenerally 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 inApus, 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 thecase, and there are some species of Conchostraca of which the males have never been seen.