The fauna of the deep sea

by Sydney J. Hickson

It is probable that the sea water by virtue of the free carbonic acid it contains in solution is able to exert a solvent action upon the calcium carbonate shells of animals as they sink to the bottom, and during the long and very slow journey from the surface to the bottom of the deepest seas these shells are completely dissolved.

The first to be dissolved would be the thin delicate shells of the Pteropods and Heteropods, for besides the fact that they present a wider surface to the solvent action of the water they are probably influenced more by tide and currents, sink more slowly and erratically, and thus have a longer journey to perform.

Then the smaller but more solid and compact shells of the Foraminifera are dissolved, and lastly, in the deepest water only the siliceous skeletons of the radiolaria and diatoms are able to reach their last resting place at the bottom of the ocean.

These four oozes then are characteristic of the floor of the deep oceans. In the proximity of land and in inland seas where deep water occurs, other muds are found differing from one another in accordance with the character of the coasts in their vicinity. It is not necessary to give a detailed account of them, but a few remarks on some of the more pronounced forms may not be without interest.

The blue mud contains eighty per cent. of a mixture of quartz, mica, felspar, hornblende, and other minerals, mixed with a considerable quantity of decomposing animal and vegetable substance, the calcareous remains of foraminifera, mollusca, worms, echinoderms, alcyonaria and corals, and the siliceous skeletons of radiolaria and diatoms.

The green mud is characterised by a large percentage of glauconite.

The red muds characteristic of the Brazilian coast contain a large amount of ochreous matter brought into the sea by the great rivers.

In the neighbourhood of active volcanoes there is a characteristic volcanic mud, and in the coral seas the deep-sea deposits contain a large percentage of the calcareous remains of dead corals.

One more character of the deep-sea region must be referred to before we pass on, and that is the absence of vegetable life. It has not been determined yet with any degree of accuracy where we are to place the limit of vegetable life, but it seems probable that below a hundred fathoms no organisms, excepting a few parasitic fungi, are to be found that can be included in the vegetable kingdom. While then the researches of recent times have proved beyond a doubt that there is no depth of the ocean that can be called azoic, they have but confirmed the perfectly just beliefs of the older naturalists that there is a limit where vegetable life becomes extinct. It is not difficult to see the reason for this. All plants, except a few parasites and saprophytes, are dependent upon the influence of direct sunlight, and as it has been shown above that the sunlight cannot penetrate more than a few hundred fathoms of sea water, it is impossible for plants to live below that depth.

The absence of vegetable life is an important point in the consideration of the abysmal fauna, for it is in consequence necessary to bear in mind that the food of deep-sea animals must be derived from the surface. It is possible that deep-sea fish, in some cases, feed upon one another and upon deep-sea crustacea, that deep-sea crustacea feed upon deep-sea worms, that deep-sea echinoderms feed upon deep-sea foraminifera, and so on through all the different combinations; but the fauna would soon become exhausted if it had no other source of food supply. This other source of food supply is derived from the bodies of pelagic organisms that fall from the upper waters of the ocean, and is composed of protozoa, floating tunicates, crustacea, fish, and other animals, together with diatoms and fragments of sea-weed.