The Stomiatidæ are almost entirely confined to water from 450 to nearly 2,000 fathoms in depth. They may be distinguished from the Scopelidæ by the long hyoid barbel close to the symphysis of the lower jaw, but like many of the genera of that family they have wide gaping mouths armed with a profusion of vicious looking teeth and a series of luminous spots on the sides of the head and body. (See [Frontispiece].)
In Eustomias obscurus, found in depths of over 1,000 fathoms in the Atlantic by the ‘Talisman,’ the barbel is provided with a terminal swelling, shaped like a dumb-bell, which may be capable of emitting a phosphorescent light and serve the animal as a lure for the attraction of its prey. The genus Malacosteus, too, presents us with some of the most remarkable forms that are found in the abysmal zone. The mouth is of enormous size and the integuments of the abdomen present very definite longitudinal folds, leaving no doubt that this fish is able, like several others living in deep water, to swallow prey of an enormous size.
But a perfectly unique structure in this fish ‘is a thin cylindrical muscular band which connects the back part of the mandibular symphysis with the hyoid bone. It is probably the homologue of a muscular band which, in other Stomiatids, stretches on each side from the mandible to the side of the hyoid, the two bands coalescing into an unpaired one in Malacosteus. It is, in the present state of preservation, much elongated, like a barbel, but during life it is notably contractile, and serves to give to the extremity of the mandible the requisite power of resistance when the fish has seized its prey, as without such a contrivance so long and slender a bone would yield to the force of its struggling victim.’
Fig. 22.—Saccopharynx ampullaceus; a deep-sea eel, with the head of a large fish, which it has swallowed, showing through the thin integuments of the body. (From Günther.)
Belonging to the family of the Salmons we find one genus Bathylagus that is undoubtedly an abysmal form. Although there may be some doubt as to the exact depth at which the specimens were captured, the thinness of the bones, the enormous size of the eyes, and other bathybial characters prove that they must live in very deep water. Closely allied to the salmon and the herrings is the family of the Alepocephalidæ, a family that contributes several forms to the fauna of the deep sea, but they do not possess any characters that call for special comment. Their vertical distribution varies between 345 and 2,150 fathoms.
The family Halosauridæ contains five species all confined to the abysmal zone. They have long bodies tapering to a finely pointed tail, and the head is provided with a snout that projects considerably in front of the mouth.
Of the family of the Eels there are several representatives in the deep sea. They are characterised by a combination of true eel characters with special modifications due to a bathybial existence. ‘To enable them to seize upon prey more powerful than themselves certain organs have undergone a degree of specialisation, as is observed in bathybial members of other families with a similar mode of life; the jaws are exceedingly elongate and the whole gape, the pharynx and stomach capable of enormous distension.’ The head is very large, the eye very small and the tail long and tapering (fig. [22]).
The lessons we learn from the study of the fishes of the deep sea are particularly instructive. It would take far more space than can be afforded here to fully illustrate all of the points that seem clear to us, but I hope I have said sufficient to show that the fish fauna is made up of genera and species belonging to several widely separate families of the Teleostei; that some of them show, in a very marked way, what may be looked upon as peculiarly bathybial characters, whilst others are but slightly modified from their shallow-water representatives. These facts by themselves lend support to the view that the fauna of the deep sea has been derived from the fauna of shallower water by successive migrations at different periods of the world’s history. Those that exhibit in a most marked degree the special bathybial characters are probably those whose immigration took place long ago, whilst those more closely related to shallow-water forms are, comparatively speaking, recent importations. The occurrence of Scopelidæ and Sternoptychidæ in deep water suggests, as Moseley pointed out many years ago, that the fauna is partly derived from the pelagic plankton. But while these points may seem clear to us, there are others that still require much more investigation and consideration. The whole question of the function and use of the phosphorescent organs, the mucous glands, the barbels and elongated fin rays, the mode of life, the deposition of ova and their development, afford problems which in the present state of our knowledge must remain unsolved. Let us hope that in the future there may be a new stimulus given to deep-sea research, and these problems may be again seriously studied and eventually solved.