CHAPTER IV

THE CHARACTERS OF THE DEEP-SEA FAUNA

The general characters presented by animals living in deep water may be considered under several headings. The most important are those that are directly or indirectly related to the fact that the animals live either in total darkness or in the faint and probably intermittent light emitted by phosphorescent animals; namely, the colour of the skin and the peculiarities of the eyes.

The colours of the skin of the deep-sea animals vary to a very remarkable extent in the different groups. It cannot be said that there is any one colour at all predominant, and it is only in certain classes that black, white, or dull-coloured animals are more numerous than others. The colours are however usually very evenly distributed, and we find but few examples of animals with spots, stripes, or other pronounced markings.

The majority of the fish are dark brown or black, but many other colours are represented. Thus _Ipnops Murrayi_, a typical deep-sea fish, is yellowish brown with colourless fins, and it exhibits a further character not uncommon in these abysmal forms, namely black buccal and branchial cavities. _Typhlonus nasus_, again, is said to be of a light brownish colour, with black fins. Many other examples could be given to show the prevalence in these regions of these black, dull, and pale uniform colours. But there are many exceptional cases. _Neoscopelus macrolepidotus_, for example—a form that according to Günther undoubtedly belongs to the bathybial region—is distinguished by its brilliant colours. It is bright red mixed with azure blue, the whole relieved by silver spots with circles of black on the abdomen.

_Prorogadus nudus_ is of a pale rose colour, with the under and lateral sides of the head bluish black.

_Rhodichthys regina_, found in 1,280 fathoms of water, is uniformly bright red in colour.

A. Agassiz says in his reports on the dredging operations on the west coast of America: ‘The coloration of the deep-sea fishes is comparatively monotonous. The tints are all a light violet base, tending more or less to brownish or brownish yellow, or even to a greenish tint, especially among the Macruridæ. Some of the Liparidæ were of a dark violet, and one species was characterised by a brilliant blue band. The Ophidiidæ, _Nemichthys_, and the like, are usually of an ashy violet tint, while in _Ipnops_ and _Bathypterois_ the tints were of a decidedly yellowish brown.’

That the deep-sea fish are usually devoid of any pronounced spots, stripes, and other markings, is now well recognised. It may not be altogether out of place, however, to refer briefly to a few exceptions.

The black circles on the abdomen of _Neoscopelus macrolepidotus_ have already been referred to.

_Halosaurus johnsonianus_, has a black spot on the tail.

_Aulostoma longipes_ has three pairs of large black spots on the ventral side, but the specimen taken in 1,163 metres of water by the ‘Talisman’ was probably a young one.

It is very probable that in all the exceptional cases, when fish taken in deep-sea water have exhibited such spots and markings, they are examples either of fish that have quite recently adopted an abysmal habitat or of young specimens exhibiting ancestral inherited characters.

In referring to a specimen of _Raja circularis_, taken by the ‘Triton’ in 516 fathoms, Günther says: ‘It is notable that the spot on each side of the back which in littoral specimens is variegated with yellow is much smaller in the deep-sea specimen and uniformly black without yellow.’

It seems to be then a very general rule among fishes that as they migrate into deeper water the spots and stripes, so conspicuous among many forms living on the surface and in shallow water, disappear, and the coloration of the body becomes more evenly distributed and uniform.

Among the Mollusca, the deep-sea Cephalopods seem to be usually violet, but an _Opisthoteuthis Agassizii_ caught by the ‘Blake’ is stated to be of a dark chocolate colour, a _Nectoteuthis Pourtalesii_ reddish-brown, and a _Mastigoteuthis_ orange brown, while of the specimens brought home by the ‘Challenger,’ _Cirroteuthis magna_ was said to be ‘rose’ when captured, and the spirit specimens of _Cirroteuthis pacifica_ and _Bathyteuthis abyssicola_ were purplish madder and purplish brown respectively.

The shells of the Gasteropods and Lamellibranchs living in the abyss are frequently so thin as to be almost transparent, and are, with very few exceptions, white or pale straw coloured. The colour of the only specimen of nudibranchiate Mollusca that has been found in the abysmal zone, namely, _Bathydoris abyssorum_, is described by Mr. Murray as follows: ‘The body of the living animal was gelatinous and transparent, the tentacles brown, the gills and protruding external generative organ orange, the foot dark purple.’

Among the Crustacea various shades of red are the prevailing colours. ‘The deep-sea types, like _Gnathophausia_, _Notostomus_, and _Glyphocrangon_,’ says Agassiz, ‘are of a brilliant scarlet; in some types, as in the Munidæ and Willemoesiæ, the coloration tends to pinkish or yellowish pink, while in _Nephrops_ and _Heterocarpus_ the scarlet passes more into greenish tints and patches.’[1] But perhaps the most remarkable point in the colour of the crustacea is that which immediately follows the paragraph I have just quoted. ‘The large eggs of some of the deep-sea genera are of a brilliant light blue, and in one genus of Macrura we found a dark metallic blue patch on the dorsal part of the carapace in marked contrast to the brilliant crimson of the rest of the body.’

Footnote 1:

In the recent researches of the ‘Investigator’ a few crustacea of rather exceptional colour were found. Whilst the great majority of them are described to be pink or red in colour when alive, _Gnathophausia bengalensis_ is deep purple lake, _Haliporus neptunus_ lurid orange, and _Aristaeus coruscans_ bright orange.

The occurrence of this blue colour in Crustaceans of the deep sea is very remarkable, for blue is a colour, as Moseley pointed out many years ago, that is rarely met with in the fauna of the abyss, and it is certainly very exceptional in the crustacea of that zone.

Among the deep-sea Echinoderma we find a wonderful variety of coloration. Moseley says that many deep-sea Holothurians, for example, are deep purple, and Agassiz reports that in one species the colour was of a delicate green tinge. ‘We obtained,’ he adds, ‘a white _Cucumaria_ and some species of _Benthodytes_ of the same colour,’ while others vary from transparent milky white to yellow and light yellowish brown and even pinkish colours. The Crinoids are described by the authorities to be white, purple, yellow and brownish-chestnut, and of the other groups of the echinoderms we read that the star-fishes are, as a rule, of duller colours than the crustacea, but all more or less pink or red. ‘The Hymenasteridæ, on the contrary, vary from light bluish violet to deep reddish chestnut colours.’ The brittle stars are red and orange or dullish grey, while the urchins may be deep violet, claret coloured, brownish, or of a delicate pink.

It is impossible to account for this extraordinary variety of colour in the deep-sea echinoderms. It is hardly probable that it can be protective or warning in function, and it is difficult to suppose that it is due to any peculiar excretory process. Whether it is due in any way to the influence of the environment, or, like the colour of autumn leaves, to the chemical degeneration of colours that in the shallow-water ancestry were functional, are problems that must be left for the future to decide.

The colour of the deep-sea Cœlenterates has unfortunately not been recorded in all cases, but still the few observations that we have show that in this group, as in the last, almost every tint and shade are represented.

The colouring of the deep-sea jelly-fishes is said to be usually deep violet or yellowish red. However ‘a species of _Stomobrachium_,’ says Agassiz, ‘is remarkable for its light carmine colour, a tint hitherto not observed among Acalephs.’

Moseley records most minutely the colour of some of the deep-sea anemones and corals, and calls attention to the very general presence of madder brown in the soft parts. Agassiz says: ‘Among deep-sea Actiniæ, a species of a new _Cereanthus_ was of a dark brick-red, while other actinians allied to _Bunodes_ were of a deep violet. Actinauge-like forms with tentacles of a pinkish-violet tinge frequently have the column of a yellow shade. The Zoanthidæ were greyish-green.’ And again, in his narrative of the voyage of the ‘Blake,’ he records that ‘some of the deep-sea corals are scarlet, deep flesh-coloured, pinkish orange, and of other colours,’ and in referring to the Gorgonian _Iridogorgia_ he says: ‘The species are remarkable for their elegance of form and for the brilliant lustre and iridescent colours of the axis, in some of a bright emerald green, in others like burnished gold or mother-of-pearl.’

The fauna of the deep sea then, taken as a whole, is not characterised by the predominance of any one colour. The shades of red occur rather more frequently than they do in the fauna of any other zone or region, but whether this is in any way connected with the fact that red is the complementary colour to that of the phosphorescent light, in which many of these animals live, it is at present difficult to say; it is possible that, when we have further information concerning the colours of the animals living in the deeper parts of the Neritic zone, another explanation may be forthcoming.

Moseley points out that there are no blue animals known to live in deep water, and it might be added that green is extremely rare as a colouring matter in abysmal animals, although the phosphorescent light given out by some of the echinoderms is green.

Blue, as a colouring matter of marine animals, living on the surface or in shallow water, is not uncommonly met with, distributed in the form of bands or stripes, but green is extremely common in fishes, crustacea and cœlenterates, and it is a point of very considerable importance that in this respect there is a very great difference between the deep-sea and the shallow-sea faunas.

If a considerable collection of living abysmal forms could be placed upon one table and a similar collection of shallow-water forms upon another, I believe that the first general impression upon the mind of one who saw them both for the first time would be the presence of green colours in the last-named collection, and the absence of it in the other.

The eyes of the animals that live in deep-sea water undergo curious modifications. If the fauna of the abysmal region were confined to conditions of absolute darkness, we should expect to find either a total absence of eyes or mere rudiments of them only in those forms that have recently migrated from the shallow water. This is the case with the fauna of the great caves. There is probably total darkness in these underground lakes and streams, and there is only the remotest possibility of the animals living in them ever seeing, even temporarily, a ray of sunlight or even a glimmer of phosphorescence during the whole of their life-time. We find then that the cave fauna is totally blind.

The conditions in the deep sea are not quite the same. In some regions there is probably a very considerable illumination by phosphorescent light, and it is quite possible that many of the characteristic deep-sea forms may occasionally wander into shallower regions where faint rays of sunlight penetrate, or even that the young stages of some species may be passed at or near the surface of the sea. Taking these points into consideration, then, it is not surprising to find that, in the deep seas, there are very few animals, belonging to families usually provided with eyes, that are quite blind.

In the majority of cases we find that the eyes are either very large or very small. Only in a small minority of cases do we find that the eyes are recorded to be moderate in size. The relation between the large-eyed forms and the small-eyed forms is not the same in all the regions of deep seas. In depths of 300 to 600 fathoms the majority are large-eyed forms. This is as we should expect, for it is more than probable that many of these forms occasionally wander into shallower waters where there is a certain amount of sunlight.

In depths of over 1,000 fathoms, the small-eyed and blind forms are in a majority, although many large-eyed forms are to be found.

Among fishes, for example, we find the species of _Haloporphyrus_ found in depths of 300–600 fathoms with large eyes; and so with _Dicrolene_, _Cyttus abbreviatus_, and many other forms that are known to live in water of less depth than 700 fathoms; while on the other hand in _Melanocetus Murrayi_, _Ipnops Murrayi_, many deep-sea eels and other fish that are truly abysmal and live chiefly in depths of over 1,000 fathoms, the eyes are either very small or absent.

Some interesting examples may be found in the species of widely distributed genera to illustrate these points. Thus in _Neobythites grandis_, from 1,875 fathoms, the eye is small, only one-eleventh the length of the head, but in _Neobythites macrops_, _N. ocellatus_, and _N. gillii_ from shallower water it is much larger.

N. grandis 1,785 fms. Eye 1⁄11th length of the head N. macrops 375 fms. Eye 2⁄9 length of the head N. ocellatus 350 fms. Eye ¼ length of the head N. gillii 111 fms. Eye 1⁄(3⅔) length of the head

Similarly in the species of the widely distributed deep-sea genus _Macrurus_: the species _M. parallelus_, _M. japonicus_, _M. fasciatus_, &c., usually living in water less than 1,000 fathoms deep, have large and in some cases very large (_M. fasciatus_) eyes, but _Macrurus filicauda_, _M. fernandezianus_, _M. liocephalus_, _M. Murrayi_, _M. armatus_ have small eyes.

Some deep-sea fish have their eyes reduced to a mere rudiment; such as _Ceratias uranoscopus_, _C. carunculatus_, _Melanocetus Murrayi_, _Typhlonus nasus_, and _Aphyonus gelatinosus_, but not even a rudiment of an eye is to be found in _Ipnops Murrayi_.

But the fish of the greatest depths are by no means always characterised by small eyes. _Malacosteus_, a typical deep-sea form, has very large eyes, and so have _Bathylagus_, living in the enormous depth of 3,000 fathoms, and _Bathytroctes_, in 1,090 and 2,150 fathoms.

The result of recent deep-sea work, then, has been to show that as we proceed from shallow shore water to depths of 500 to 900 fathoms the eyes of the fish become larger, but in greater depths than 1,000 fathoms the eyes of some fish become considerably reduced, but those of others become still more enlarged. In the greatest depths of the ocean in fact it seems very probable that nearly all the fish are characterised by either very large eyes or very small ones.

We cannot expect to learn very much at present from the study of the eyes of deep-sea mollusca. The Cephalopods form the only class of this Phylum whose genera invariably possess large and well-developed eyes, and there does not seem to be any very marked increase or decrease in the size of the eyes of the few deep-sea cuttlefish that are known to us.

The eye of _Nautilus_ is certainly remarkably interesting, but as this genus is the only representative of its order, and is known at times to float upon the surface of the ocean, it would certainly be erroneous to attribute the peculiarity of the structure of its eye to its ‘temporary’ deep-sea habits. We are still ignorant of the usual habitat of the remarkable genus _Spirula_, notwithstanding the fact that many of the tropical beaches are very largely composed of its empty shells. Whether it is a deep-sea dweller or not, we know nothing at present of the character of its eye, so that it can throw no light upon the problems we are now discussing.

Among the deep-sea gasteropods we find the same irregularity in the possession of eyes that we have just described among fishes. Thus a species of _Pleurotoma_, dredged by the ‘Porcupine,’ in 2,090 fathoms, has a pair of well-developed eyes on short footstalks, but _Pleurotoma nivalis_, obtained by the ‘Talisman,’ is blind. Again a species of _Fusus_, obtained by the ‘Porcupine,’ in 1,207 fathoms, is provided with well-developed eyes, but _Fusus abyssorum_, obtained by the ‘Talisman,’ is blind. Among the Lamellibranchs there are very few genera that possess well-marked eyes. The genus _Pecten_ is one of those that in shallow waters possess numerous highly complicated visual organs situated on the edge of the mantle. In the deep-sea species, _Pecten fragilis_, these eyes are wanting, but we have not sufficient evidence at present to enable us to assert that all the deep-sea species of this genus are blind.

Among the Crustacea there is a very general tendency to lose the eyes at a depth of a few hundred fathoms of water.

In _Ethusa granulata_, for example, the eyes disappear at 500 fathoms and the eye-stalks become firmly fixed, greater in length, and take the place of the rostrum which disappears. In some forms—such as _Thaumastocheles zaleuca_ and _Willemoesia_—the eye-stalks themselves have completely disappeared.

In the deep-sea Isopoda some forms lose their eyes entirely, but _Bathynomus giganteus_ possesses a pair of enormous eyes, each provided with 4,000 facets.

To illustrate the distribution of eyes in this group, we may take as an example the genus _Serolis_. All the species of this genus are provided with eyes except _Serolis antarctica_—a species that extends from 600 to 1,600 fathoms.

The eyes of all the deep-sea species are relatively larger than those of the shallow-water ones, except _Serolis gracilis_, whose eyes seem to be disappearing.

But these large eyes of the deep-sea species of _Serolis_ are not capable of any greater perceptive power. In fact, the evidence of degeneration they show, both in minute structure and in the diminution of pigment, proves that they can be of very little use to these animals for perception (see Figs. 4 and 5).

This increase in size, accompanied by degeneration of structure, is just what we should expect to find in the eyes of deep-sea animals, and it is difficult to explain why it is that we do not find more examples of it.

If the animals that now live in the depths of the sea are descended from the shallow-water forms of bygone epochs, they must have passed through many different habitats with diminished light until they reached their present dark abode in the abyss.

In every new region they came to, the forms with larger and better eyes would be at an advantage in the fainter light, and would be more likely to survive and transmit their favourable variation in this respect to their offspring than their less fortunate neighbours. Thus down to the depth of the limit of sunlight we should expect to find, as we do find in fishes, large-eyed species.

Beyond the limit of direct sunlight the eyes would be of very little use to them, the pigment would disappear and the tissues become degenerate. This is precisely what has occurred in the genus _Serolis_.

The disappearance of the sense of sight in the animals of the deep sea is sometimes accompanied by an enormous development of tactile organs.

Thus, among fishes we find _Bathypterois_, a form that possesses extremely small eyes, provided with enormously long pectoral fin rays that most probably possess the functions of organs of touch.

Among the Crustacea we find the blind form, _Galathodes Antonii_, with an extraordinary development in length of the antennæ, and _Nematocarcinus_, with enormously long antennæ and legs.

The subject of the power of emitting phosphorescent light possessed by some deep-sea animals is much more difficult to deal with.

The presence of distinct organs in many of the deep-sea fish that can only be reasonably interpreted as phosphorescent organs, the presence of well-developed and evidently functional eyes in many deep-sea animals, and many other considerations render it very highly probable that some, if not many, forms emit a phosphorescent light.

The power and constancy of the light emitted, however, must for the present remain a matter of conjecture. We cannot judge at all of the amount of light given out by an animal in deep water by its appearance when thrown out of a dredge upon the deck. Whether the phosphorescent light given out by an Alcyonarian or a Crustacean is more or less at a temperature of 40° Fahr. and a pressure of one ton per square inch than it is at 60° Fahr. and the ordinary barometric pressure of the sea-level, is a question that has not yet been brought to an experimental test.

Whatever the answer to this question may be, the fact remains that a greater percentage of animals from the deep sea exhibit some sort of phosphorescent light when brought on deck than animals that live in shallow water.

The curious organs possessed by some fishes that are supposed to be organs for the emission of phosphorescent light have recently been subjected to a minute examination by von Lendenfeld.

It has been known for some years now, that the slime secreted by the skin glands of certain sharks is highly phosphorescent. It is not difficult, then, to understand how it came about that certain fish developed complicated phosphorescent organs.

From the phosphorescent slime secreted by a simple skin gland to the most complicated eye-like phosphorescent organ, we have a series of intermediate forms that are quite sufficient, even in the imperfect state of our knowledge at the present day, to enable us to understand the outlines of the evolution of these peculiar and interesting organs.

We can distinguish two kinds of phosphorescent organs in the deep-sea fish. There are the curious eye-like or ocellar organs situated usually in one or more rows down the sides of the fish’s body, forming as it were a series of miniature bull’s-eye lanterns to illuminate the surrounding sea (fig. 6); and various glandular organs that may be situated at the extremity of the barbels or in broad patches behind the eyes or in other prominent places on the head and shoulders.

Ocellar organs have been known for many years to occur on the sides of the interesting pelagic fish, _Scopelus_. Most of the species of this genus live in the open sea at moderate depths, coming to the surface only at night, but other species are found in almost every depth down to 2,000 fathoms of water.

In _Opostomias micripnus_, a dark black fish living at a depth of over 2,000 fathoms, there are two rows of ocellar organs running down the sides of the body from the head to the tail. In the living animal they are said to shine with a reddish lustre. In addition to these, the conspicuous organs, there are groups of fifty, a hundred, or even more very much smaller organs situated on the sides and back of the fish, each of which is lenticular in shape and consists of a number of short polygonal tubes containing a granular substance with rounded bases resting on the subjacent tissue. The whole organ is covered by a simple continuation of the cuticle of the body-wall. The granular substance contained in the tubes is most probably the seat of luminosity.

As a type of the glandular organs we may take one of the sub-orbital organs found on the head of _Pachystomias microdon_.

In this fish there are two very conspicuous white organs immediately below the eye. The anterior one, which lies below and in front of the eye, is oval, with its upper margin slightly concave. In section it is seen to be surrounded by a thin layer of black pigment, and to consist of a reticular glandular substance in which is embedded a hammer-shaped lens-like body. Between these two structures there is interposed a thick layer of light reflecting spicules.

The exact part that is played by the different components of these curious phosphorescent organs is not yet known, but sufficient has been said to indicate to the reader the degree of complexity that these organs may reach in the fish of the great depths of the ocean.

But the power of emitting phosphorescent light is by no means confined to the group of fishes. Some of the Macrurous Decapoda among the Crustacea are known to be phosphorescent. In the case of _Heterocarpus Alphonsi_, for example, the naturalists of the ‘Investigator’ found that ‘clouds of a pale blue highly luminous substance, which not only illuminated the observers’ hands and surrounding objects in the vessel in which the creature was confined, but also finally communicated a luminosity to the water itself, were poured out apparently from the bases of the antennæ.’

‘The _Willemoesia_, too, was luminous at two circumscribed points somewhere near the orifices of the genital glands.’

Again, all the Alcyonarians dredged by the ‘Challenger’ in deep water were found to be brilliantly phosphorescent when brought to the surface, the light consisting, according to Moseley, of red, yellow, and green rays only.

Among the Echinoderms we have not many recorded instances of a phosphorescent light being emitted, but it is quite possible that many, if not all of them, may possess this power. The curious deep-sea form _Brisinga_, that was first discovered by Ch. Asbjörnsen, is known to be so brilliantly phosphorescent that it has been called a veritable _gloria maris_, and writing of the curious brittle-star _Ophiacantha spinulosa_ (dredged by the ‘Porcupine’ in 584 fathoms of water), Professor Wyville Thomson remarks that the light was of a ‘brilliant green, coruscating from the centre of the disc, now along one arm, now along another, and sometimes vividly illuminating the whole outline of the star-fish.’

According to Filhol many of the abysmal Annelid worms are in the habit of emitting a vivid phosphorescent light, and capable thereby of illuminating the medium in which they live.

We have now considered all those characters exhibited by deep-sea animals that may be associated with the absence of direct sunlight. To run through them again briefly we may say: that the deep-sea species, belonging to classes of animals that usually possess eyes, show some modification in the size of their eyes, in that they are either very large, very small, or altogether wanting. That deep-sea animals are nearly always uniformly coloured. Very frequently they are black or grey or white, less frequently bright red, purple, or blue. But whatever the colour may be, spots, stripes, bands, and other markings of the body are very rarely seen. That deep-sea animals are brilliantly phosphorescent, the light being emitted either by special organs locally situated on the head, body, or appendages, or by the general surface of the body.

But there are some other characters that cannot be thus associated with the absence of sunlight.

In the first place bathybial fish, mollusca, crustacea, and other animals usually possess a remarkably small amount of lime in their bones and shells.

In fishes we are told that the bones have a fibrous, fissured, and cavernous texture, are light, with scarcely any calcareous matter, so that the point of a fine needle will readily penetrate them without breaking. In some the primordial cartilage is persistent in a degree rarely met with in surface fishes, and the membrane bones remain more or less membranous or are reduced in extent, like the operculum, which is frequently too small to cover the gills.

This cannot be due in all cases to a deficiency of carbonate of lime in the sea water, for we find these characters well marked in some of the fish, such as _Melanocetus Murrayi_, _Chiasmodus niger_, and _Osmodus Lowii_, that are found on the Globigerina mud.

Then again, the shells of the deep-sea Lamellibranchs, Gasteropods, Brachiopods, and Crustacea are very frequently remarkably thin and transparent, a character that is probably more generally due to a weakness in absorptive or secretive activity than to a deficiency in the supply of lime.

There are one or two characters of the deep-sea fish that it is not easy to account for, and it is necessary only to mention their occurrence without attempting to offer any explanation of them.

One of the most common of these is the very dark pigment occurring in certain parts of the epithelium of the mouth and respiratory passages and the endothelium of the peritoneum. For example, in _Bathysaurus mollis_, living at a depth of 2,000 fathoms, the mouth and buccal cavities are black. The same thing occurs in _Ipnops Murrayi_, and indeed in all the strictly deep-sea forms.

Another important character of very frequent occurrence is the reduction in size, length, and number of the gill laminæ.

Among invertebrates we may mention as a fact of some interest, dependent perhaps on the soft character of the bottom, the preponderance of stalked forms over those of more sessile habits.

Thus among the Alcyonaria the characteristic forms of the deep water are the Pennatulids, and more particularly the genus _Umbellula_ with its long graceful stem and terminal tuft of polyps. Among the Echinoderma we find many forms of stalked Crinoids. Among the Tunicates several curious genera characterised by their long peduncles.

Taking the fauna as a whole, Moseley regarded it as similar in some respects to the flora of the high mountains. Some forms are dwarfed in size, such as the species of Radiolaria, Cerianthus, some of the Cephalopods, &c., while others are very much larger than their shallow-water allies, such as the Pycnogonids, nearly all the Crustacea, Alcyonarians (as regards the size of the polypes), Siphonophora, and many others.