The conditions of life in the Ocean depths are very peculiar. The light of the sun cannot penetrate beyond about two hundred fathoms; deeper than this complete darkness prevails. Hence in many species the eyes have more or less completely disappeared.
Sir Wyville Thomson mentions a kind of Crab (Ethusa granulata), which when living near the surface has well developed eyes; in deeper water, 100 to 400 fathoms, eyestalks are present, but the animal is apparently blind, the eyes themselves being absent; while in specimens from a depth of 500-700 fathoms the eyestalks themselves have lost their special character, and have become fixed, their terminations being combined into a strong, pointed beak.
In other deep sea creatures, on the contrary, the eyes gradually become more and more developed, so that while in some species the eyes gradually dwindle, in others they become unusually large.
Many of the latter species may be said to be a light to themselves, being provided with a larger or smaller number of curious luminous organs. The deep sea fish are either silvery, pink, or in many cases black, sometimes relieved with scarlet, and when the luminous organs flash out must present a very remarkable appearance.
We have still much to learn as to the structure and functions of these organs, but there are cases in which their use can be surmised with some probability. The light is evidently under the will of the fish.[61] It is easy to imagine a Photichthys (Light Fish) swimming in the black depths of the Ocean, suddenly flashing out light from its luminous organs, and thus bringing into view any prey which may be near; while, if danger is disclosed, the light is again at once extinguished. It may be observed that the largest of these organs is in this species situated just under the eye, so that the fish is actually provided with a bull's eye lantern. In other cases the light may rather serve as a defence, some having, as, for instance, in the genus Scopelus, a pair of large ones in the tail, so that "a strong ray of light shot forth from the stern-chaser may dazzle and frighten an enemy."
In other cases they appear to serve as lures. The "Sea-devil" or "Angler" of our coasts has on its head three long, very flexible, reddish filaments, while all round its head are fringed appendages, closely resembling fronds of sea-weed. The fish conceals itself at the bottom, in the sand or among sea-weed, and dangles the long filaments in front of its mouth. Little fishes, taking these filaments for worms, unsuspectingly approach, and thus fall victims.
Several species of the same family live at great depths, and have very similar habits. A mere red filament would be invisible in the dark and therefore useless. They have, however, developed a luminous organ, a living "glow-lamp," at the end of the filament, which doubtless proves a very effective lure.
In the great depths, however, fish are comparatively rare. Nor are Molluscs much more abundant. Sea-urchins, Sea Slugs, and Starfish are more numerous, and on one occasion 20,000 specimens of an Echinus were brought up at a single haul. True corals are rare, nor are Hydrozoa frequent, though a giant species, allied to the little Hydra of our ponds but upwards of 6 feet in height, has more than once been met with. Sponges are numerous, and often very beautiful. The now well known Euplectella, "Venus's Flower-basket," resembles an exquisitely delicate fabric woven in spun silk; it is in the form of a gracefully curved tube, expanding slightly upwards and ending in an elegant frill. The wall is formed of parallel bands of glassy siliceous fibres, crossed by others at right angles, so as to form a square meshed net. These sponges are anchored on the fine ooze by wisps of glassy filaments, which often attain a considerable length. Many of these beautiful organisms, moreover, glow when alive with a soft diffused light, flickering and sparkling at every touch. What would one not give to be able to wander a while in these wonderful regions!
It is curious that no plants, so far as we know, grow in the depths of the Ocean, or, indeed, as far as our present information goes, at a greater depth than about 100 fathoms.
As regards the nature of the bottom itself, it is in the neighbourhood of land mainly composed of materials, brought down by rivers or washed from the shore, coarser near the coast, and tending to become finer and finer as the distance increases and the water deepens. The bed of the Atlantic from 400 to 2000 fathoms is covered with an ooze, or very fine chalky deposit, consisting to a great extent of minute and more or less broken shells, especially those of Globigerina. At still greater depths the carbonate of lime gradually disappears, and the bottom consists of fine red clay, with numerous minute particles, some of volcanic, some of meteoric, origin, fragments of shooting stars, over 100,000,000 of which are said to strike the surface of our earth every year. How slow the process of deposition must be, may be inferred from the fact that the trawl sometimes brings up many teeth of Sharks and ear-bones of Whales (in one case no less than 600 teeth and 100 ear-bones), often semi-fossil, and which from their great density had remained intact for ages, long after all the softer parts had perished and disappeared.