CHAPTER VII

THE ARTHROPODA OF THE DEEP SEA

The deep-sea fauna seems to be particularly rich in marine Arthropoda, many curious and interesting forms being brought up with almost every haul of the dredge. The Arthropoda, too, being very highly organised animals, afford interesting and instructive examples of the effect of abysmal life in the modification of the sense organs and the production of varieties specially modified for the conditions of the struggle for existence in their strange habitat.

Concerning the groups of Ostracoda and Copepoda it may be said that the evidence is not yet conclusive that they include any truly deep-sea species. The largest known Ostracod, measuring somewhat more than an inch in length and probably allied to the genus _Crossophorus_, has quite recently been captured by Professor Agassiz at depths of less than 200 fathoms, but he could obtain no evidence that it descended into much deeper water than this.

Mr. Brady, in writing the report of the ‘Challenger’ Ostracoda, came to the conclusion that they do exist in very limited numbers in the most profound depths of the sea; but it is nevertheless quite possible that all the Ostracods brought on deck by the trawl or dredge were really captured either on the way down or on the way up, and are, strictly speaking, pelagic in habit.

Similar caution must be taken in dealing with the Copepoda, an order of Crustacea that is essentially pelagic in habit. The only species that has been regarded as undoubtedly abysmal is _Pontostratiotes abyssicola_, a form whose carapace and antennæ are armed with exceedingly long and strongly toothed spines, and was found in the mud brought up by the trawl from a depth of 2,200 fathoms.

_Calamus princeps_, the largest species of its genus of a deep reddish brown colour, may also belong to the fauna of the deep sea, but we have less evidence concerning the habitat of _Hemicalamus aculeatus_, _Phyllopus bidentatus_, and some of the Euchætæ.

The Amphipoda seem to be but poorly represented in the fauna of the abyss; in fact it may be considered to be still an open question whether any Amphipods habitually live in very deep water.

In the reports on the ‘Challenger’ Amphipoda, the Rev. T. R. Stebbing states that thirty-one specimens are known to come from great depths, but it would be more correct to say that these specimens were found in the dredges and trawls that had been lowered into the great depths. It should be noticed, however, that some of these specimens do show characters that suggest, at any rate, that they come from deep water. Thus the genus _Lanceola_, for example, is characterised by the smallness of the eyes and a soft membranous integument, while _Cystisoma spinosum_, found in a dredge that had been at work at a depth of over a thousand fathoms, has very large eyes.

In his report on the Crustacea of the ‘Norske Nord-havns’ expedition, Professor Sars gives a full description of many species of Amphipoda brought by the dredge from depths of over 1,000 fathoms, and nearly all of these were found to be quite blind.

The form that seems to be most peculiar to the great depths of the Northern Ocean is _Harpinia abyssi_. It was found at no less than fifteen different stations at depths ranging from 350 to 2,215 fathoms, and is characterised by its large size and the total absence of eyes.

Another point that should be considered in coming to any conclusion on the supposed habitat of such forms, is the similarity or dissimilarity of widely distributed species.

I have had occasion to point out in a previous chapter the general similarity of the abysmal fauna all over the world, a very striking phenomenon, commented on by almost every naturalist who has had a wide experience of this kind of investigation.

Among the Amphipoda we have a very striking example of this. The species _Orchomene musculosus_ was taken by the ‘Challenger’ off the southern part of Japan at a depth of 2,425 fathoms, the bottom being red clay and the temperature 35·5° Fahr. The species _Orchomene abyssorum_ was taken off the east coast of Buenos Ayres at a depth of 1,900 fathoms, the bottom being blue mud and the temperature 33·1° Fahr. To the description of this last-named species Mr. Stebbing adds, ‘had this species been taken within reasonable distance of _O. musculosus_, the resemblance is so great that one might have been tempted to disregard the points of difference as due to some other cause than difference of species.’

Such a striking similarity between two species living so far apart from one another may, when we take into consideration the depth, the character of the bottom, and the temperature from which they are supposed to have been dredged, be taken to support very strongly the view that these species are really abysmal in habit.

Among the Isopoda we have several very characteristic forms—no fewer than nine distinct genera peculiar to the abysmal zone have been described by Beddard—and of these two, _Bathynomus_ and _Anuropus_, are to be regarded as types of sub-families. They seem to be very unevenly distributed over the floor of the ocean, some regions, such as the whole of the Central and Southern Atlantic and the Central and Western Pacific, produce none; whilst the waters of the east coast of New Zealand, the Crozets, and others, produce a great many varieties. Many of the deep-sea Isopoda exhibit characters that are usually associated with the bathybial life. Thus, according to Beddard, thirty-four of the deep-sea species are totally blind, and eighteen have well-developed eyes. In four species there are eyes which are evidently degenerating. If we compare, for instance, the structure of the eye of _Serolis schythei_, a species found in shallow water ranging from 4 to 70 fathoms, with the eyes of _Serolis bromleyana_, a species living in deep water ranging from 400 to 1,975 fathoms, we cannot fail to see that the latter are undergoing a process of degeneration; the retinulæ and pigment being absent, and nothing left of the complicated structure of the Isopod eye but the remnants of the crystalline cones and corneal facets (see figs. 4 and 5, p. 74).

Taking the genus _Serolis_ alone, it has been said ‘that in all the shallow-water forms the eye is relatively small but very conspicuous from the abundant deposition of pigment; in all the deep-sea forms, with the exception of _S. gracilis_, where the eye seems to be disappearing, it is relatively larger but not so conspicuous, owing to the fact that little or no pigment is present.’

In many groups of animals it has been shown that some of the deep-sea species are relatively much larger in size than the shallow-water species, and that others, more rarely, are much smaller, the abysmal fauna reminding us in this respect of the characters of the alpine flora.

The Isopoda show many examples of this largeness in size, thus _Bathynomus giganteus_, dredged by Professor Agassiz off the Tortugas at a depth of over 900 fathoms, reaches the enormous size, for an Isopod, of 9 inches (fig. 15). _Stenetrium haswelli_, again, is larger than any of the shallow-water species of the genus, and the same remark applies to the deep-sea species of the genus _Ichnosoma_, while _Iolanthe acanthonotus_, from a depth of nearly 2,000 fathoms, is considerably larger than most of the shallow-water Asellidæ.

There is another very common character of deep-sea Crustacea that is also well exemplified in the group of the Isopods, and that is the extraordinary length and number of the spines covering the body.

I have already referred to this character in the supposed deep-sea Copepod _Pontostratiotes abyssicola_, and I shall have again to refer to it in treating of the Decapoda and other groups of the Crustacea.

Besides its enormous size _Bathynomus_ possesses some other characters that may be correlated with its deep-sea environment. The respiratory organs are quite different from those of other Isopods; instead of being borne by the abdominal appendages, they are in the form of branched outgrowths from the body-wall containing numerous blood-lacunæ, and the appendages simply act as opercula to cover and protect them. The eyes of the Bathynomus too are remarkably well developed, each one bearing 4,000 facets, and they are directed not dorsally as in the Cymothoadæ, but ventrally. The cause of these curious modifications of structure in Bathynomus is by no means clear, but it is quite probable that they are connected with the conditions of pressure and light in the deep sea. It is a remarkable fact that the other deep-sea Isopods do not exhibit precisely these modifications, and it might be supposed that the same causes would produce the same or similar effects on the structure of animals belonging to the same order. That is perfectly true, but we cannot yet determine how long ago any one species has taken to a deep-sea life, or what length of time, in other words, these conditions have been at work in modifying the structure of the organism. A recent immigrant into the abyss will naturally exhibit closer affinities with its shallow-water allies than those that have dwelt in the region since secondary or tertiary times. If we take this into consideration we should expect to find considerable differences occurring between deep-sea species of the same order, which is precisely what we do find.

Concerning the Cirripedia, that curious group of profoundly modified Crustacea that includes the barnacles and acorn shells, Dr. Hoek writes in the ‘Challenger’ monograph:—

‘Though unquestionably by far the greater part of the known Cirripedia are shallow-water species, and though some of the species are capable of living at a considerable variety of depths, as, for instance, _Scalpellum stroemii_, yet it must be granted that the number of true deep-sea species of Cirripedia is very considerable.’ Only two genera, however, occur in depths of over 1,000 fathoms, and these—_Scalpellum_ and _Verruca_—occur also as fossils in secondary and tertiary deposits. The oldest of all fossil cirripedes, however, namely, _Pollicipes_, never occurs, at the present day, in deep water, but is purely littoral or neritic in habit. But what is perhaps more interesting still is the fact, that, when we come to compare the living and the fossil species, we find that in the one genus (_Scalpellum_) the deep-sea forms have preserved the more archaic characters, and in the other (_Pollicipes_) the shallow-water forms.

Here then we are presented with a veritable puzzle for which we can at present frame no manner of answer. Pollicipes on the one hand—like Lingula among the brachiopods—has been able to maintain itself almost unchanged amid the tremendous struggle for life of the shallow water of the tropics ever since the Lower Oolite epoch; while Scalpellum, on the other hand, has either become profoundly modified, or been driven into the abysmal depths of the ocean.

The group of the Thoracostraca, or stalk-eyed Crustacea, including lobsters, crabs, hermit crabs, prawns, and shrimps, is well represented in the deep sea. Most of them are characterised by being quite blind (in many cases even the eye-stalks are obliterated), by being protected with a dense covering of spines, by the thinness of their shells, and by their bright red or carmine colour.

The order Stomatopoda is almost entirely confined to the shallow waters of the tropical or temperate shores. Not a single species is known to inhabit the deep sea, and only a very few specimens have been captured in more than a few fathoms of water.

The Schizopoda, however, present us with many curious abysmal forms. Most of the genera of this order belong to the pelagic plankton, and many of them are known to possess the power of emitting a very strong phosphorescent light. Several genera, however, such as _Gnathophausia_, _Chlaraspis_, _Eucopia_, _Bentheuphausia_, &c., never seem to leave the great depths of the ocean, and nearly all of these genera are distinguished by being quite blind or possessing very much reduced or rudimentary eyes.

If we compare, for example, the pelagic _Euphausia latifrons_ (fig. 16) with the nearly allied but abysmal _Bentheuphausia amblyops_ (fig. 17), the difference in this respect between a Schizopod living in the sunlight and one living in the darkness of the deep-sea is very apparent.

The pelagic Schizopoda are usually quite pale and transparent; the deep-sea forms on the other hand are frequently if not invariably of a bright red colour, as is the case with many other deep-sea Crustacea to which reference will be made later on.

Passing on to the group of the Decapoda, we find that the most interesting of all the abysmal cray-fish is the family of the Eryonidæ; indeed, in some respects the discovery of these curious forms may be reckoned among the most valuable results of the ‘Challenger’ Expedition. They are characterised by the dorsal depression of the anterior part of the cephalothorax, the absence of a rostrum, and the absence or very rudimentary condition of the eyes (fig. 18).

Their nearest relations seem to be certain genera of Crustacea that are found in jurassic strata, in the lias, and more particularly in the lithographic slates of Solenhofen.

They have a very wide bathymetrical range extending from a depth of 250 fathoms (_Polycheles crucifera_) to a depth of 2,000 fathoms (_Willemoesia_).

But there are many other curious forms of the macrurous crustacea that deserve a passing mention. The graceful _Nematocarcinus gracilipes_, distinguished by the extraordinary length of the antennæ and last four pairs of legs, these appendages being three or four times the length of the body, is by no means rarely met with in depths of over 400 fathoms.

The genus _Glyphus_ captured by the ‘Talisman’ is remarkable for the development of a peculiar pouchlike arrangement on the abdomen for the protection of the larvæ during the younger stages of their existence.

The proof of the existence of a peculiar cray-fish, _Thaumastocheles zaleuca_, at a depth of 450 fathoms, was one of the most important contributions to carcinology made by the ‘Challenger’ Expedition. The chelæ of this remarkable form are of great but unequal length and armed with long tooth-like spines giving it an appearance not unlike that of the jaws of some carnivorous fish. The shell is soft and the abdomen broad and flattened. There are no eyes nor even eye-stalks, but ‘in front of the carapace,’ as Sir Wyville Thomson remarks, ‘between the anterior and upper edge and the insertions of the antennæ, in the position of the eyes in such forms as _Astacus fluviatilis_, there are two round vacant spaces, which look as if the eye-stalks and eyes had been carefully extirpated and the space they occupied closed with a chitinous membrane.’ The deep-sea prawn, _Psalidopus_, recently taken in 500 fathoms of water by the ‘Investigator,’ affords us an example of a common bathybial character, the whole body being covered with an extraordinary array of sharp needle-like spines.

Among the crabs many curious forms have been found in deep water extending down to depths of over 2,500 fathoms. They are nearly all characterised by blindness and a remarkable development of tooth-like spines covering the carapace and limbs.

The remarkable _Lithodes ferox_, from a depth of from 450 to 800 fathoms, is perhaps the most perfectly armed crab—in the way of spines—that exists. Every part of the body and limbs is so covered with spines that one has to be extremely careful in handling even a dead specimen.

This is only one of the many examples that might be given to illustrate this curious feature of the deep-sea Crustacea. Among the crabs alone we have such forms as _Galathodes Antonii_, _Pachygaster formosus_, _Dicranodromia mahyeuxii_ covered with a fierce armature of spines or bristles; but there are nevertheless some species in which this character is not particularly noticeable, and in these we usually find some other protection against their enemies. An interesting example of this has been described by A. Agassiz in a crab allied to the Maiadæ, ‘in which the dorsal face appears like a bit of muddy area covered by corals, with a huge white arm resembling a fragment of an Isis-like gorgonian.’ It is evident that this is a case in which the animal is protected by its resemblance to the surroundings.

The hermit crabs of the abyss, too, are not usually characterised by any very great development of spines. They find their protection in the shells they inhabit. Some of the deep-sea hermit crabs carry about with them on their shells a sea anemone, as we find to be frequently the case among the shallow-water species. _Pagurus abyssorum_, from a depth of 3,000 fathoms, is an example of this.

In cases where there is a scarcity of gasteropod shells the hermit crabs are obliged to find some other form of protection for their bodies. The ‘Blake’ found in the West Indies a hermit crab that had formed for itself a case of tightly compressed sand, and another curious form, named _Xylopagurus rectus_, makes its home in pieces of bamboo or in the holes in lumps of water-logged wood.

The last group of the Arthropoda we need refer to is that of the Pycnogonida, those curious creatures seemingly made up entirely of legs, and by some naturalists considered to be related to the Crustacea and by others to the scorpions and spiders.

Like the Brachiopoda the Pycnogonida are not usually found in greater depths than 500 fathoms. Out of the twenty-seven known genera, only five extend into the abyss, and not one of these can be called a true deep-sea genus.

There are three genera, _Nymphon_, _Collosendeis_, and _Phoxichilidium_, that show a very wide distribution over the floor of the ocean, and are capable of existing at the greatest depths, and of these the species of the genus Nymphon have a truly remarkable range extending from the shore to a depth of 2,225 fathoms.

‘As a rule,’ says Hoek, ‘the deep-sea species are slender, the legs very long and brittle, and the surface of the body smooth.’ They have further, either no eyes at all or rudimentary eyes without pigment, and in many cases—as, for example, _Collosendeis_—they are distinguished for reaching to a gigantic size compared with their shallow-water relatives.

The Tunicata is the group of animals that includes all those curious vegetable-like organisms found upon our coasts that are familiarly known as sea-squirts, or Ascidians, besides the salps, pyrosomas, and the microscopic appendicularias of the pelagic plankton.

Notwithstanding the apparent simplicity of their adult structure, naturalists are now agreed that they must be removed from the Mollusca, with which they have hitherto been most frequently associated, and placed in the group of the Vertebrata. It is the study of embryology that has led to this unexpected conclusion, for we find, when we study the larval forms, that they possess both a notochord and gill-slits, two features that are characteristic of the group of the Vertebrata.

The species of the group Perennichordata, which includes all those Tunicates that possess a notochord persistent through life, are chiefly pelagic in habit, the little creatures, rarely more than two or three millimetres in length, swimming or drifting about with the sagittas, copepods, ctenophores, and medusæ that compose the pelagic plankton. Fol has recently described a gigantic form belonging to this group, reaching a size of thirty millimetres in length, called _Megalocercus abyssorum_, which he dredged from a depth of 492 fathoms; and other species have been recorded down to a depth of 710 fathoms in the Mediterranean Sea.

Among the simple Ascidians we find no family that is peculiar to deep water; but the Cynthiidæ and Ascidiidæ both contain genera that are abysmal, and the Molgulidæ have one species, _Molgula pyriformis_, that extends into the abysmal zone to a depth of 600 fathoms.

In the genus _Culeolus_ and in _Fungulus cinereus_ and _Bathyoncus_, all deep-water Ascidians, there is a very curious modification of the branchial sac, the stigmata being apparently not formed, in consequence of the suppression of the fine inter-stigmatic vessels. This peculiar feature is only found in the deep-sea simple Ascidians and, as we shall see presently, in one species of the deep sea compound Ascidians, but it is not apparently an essential character of those living in the abysmal zone, notwithstanding the fact that it is found in such widely separated genera; for _Corynascidia_, _Abyssascidia_, and _Hypobythius_, living in depths lying between 2,000 and 3,000 fathoms below the surface, have branchial sacs of the ordinary type. Professor Herdman is of opinion that this simple form of branchial sac is not a primitive form, but most probably a modification of a more complicated type.

In _Culeolus Murrayi_ there is a remarkably abundant supply of blood-vessels to the tunic, and these send special branches to a number of small papilliform processes on its outer surface. This system of highly vascular processes probably constitutes, as Professor Herdman suggests, an additional or complementary respiratory apparatus. All these modifications of the branchial system are of particular interest, for we find so many instances of a similar kind among the inhabitants of very deep water. I need only refer here to the modifications of this system in the Isopod _Bathynomus_ already referred to (p. 129), and to the reduction in the number of the gills of many of the deep-sea fishes. Why there should be such modifications is a question upon which the physical and natural history investigations of the conditions of life in the great depths of the ocean at present throw no light.

In a previous chapter I have referred to the fact that many of the bathybial animals are characterised by being stalked. Among the simple Ascidia we find many examples of stalked kinds living in deep water, such as _Culeolus_ and _Fungulus_, but also several exceptions, such as _Bathyoncus_, _Styela bythii_, and _Abyssascidia_, that are sessile. It is a noteworthy fact, however, that the genus that has the most deep-sea species—namely, _Culeolus_—is a genus that is provided with a very long stalk. Furthermore, the only known stalked forms of the very large family Ascidiidæ are the abysmal genera _Corynascidia_ and _Hypobythius_.

The most remarkable character of the genus _Hypobythius_ is the simple condition of its branchial sac, reminding one of the structure of this organ in the shallow-water genus _Clavelina_. ‘There are no folds and there are no internal bars,’ to quote the description given by Professor Herdman; ‘only a single system of vessels can be recognised, branching and anastomosing so as to form a close network, the small rounded meshes of which are the stigmata. The tentacles and dorsal lamina cannot be made out.’

Among the compound Ascidians only four families extend into the abysmal zone, namely, the Botryllidæ, Polyclinidæ, Didemnidæ, and Cœlocormidæ, and of these only one species, _Pharyngodictyon mirabile_, of the family Polyclinidæ, extends into water of greater depth than 1,000 fathoms. In _Pharyngodictyon_ we find the same curious simplification of the branchial sac that we have just referred to in the genera of simple Ascidians, _Culeolus_, _Fungulus_, and _Bathyoncus_. _Cœlocormus Huxleyi_ from a depth of 600 fathoms is a very peculiar form and the type of a separate family, the Cœlocormidæ.

The free-swimming Tunicata included in the group _Ascidiæ salpiformes_, which contains the genus _Pyrosoma_, and the order Thaliacea containing the salps, are in all probability mainly confined to the surface waters. A few specimens of _Pyrosoma_ were captured by the ‘Challenger’ dredges which came up from very deep water, but it is doubtful at what point in the journey to the surface the specimens entered the net.

The most remarkable form of free-swimming Tunicate that has come to light is _Octacnemus bythius_, a form that is probably allied to _Salpa_. It was found twice, once in the dredge that came from a depth of 1,070 fathoms, and once from 2,160 fathoms. The tunic of the animal is gelatinous and hyaline, but the most important feature it possesses is an imperforate membrane separating the branchial sac from the peribranchial cavity. Octacnemus, in other words, possesses no true stigmata, these structures being represented only by little pits in the walls of the branchial sac. This curious and extremely interesting modification of the respiratory organs points very strongly to the conclusion that Octacnemus is truly a deep-sea animal.