In the sense of feeling they are believed to be very deficient, and it is doubtful whether they suffer any pain at all, in the true sense of the word. Probably they can be conscious of discomfort.
One of their unfailing characteristics is the possession of a stupendous appetite, and with it of a superlatively good digestion. They live to eat; and they are at it incessantly. Perhaps in their case it does not always mean greediness, but only a due satisfying of Nature’s needs.
Certainly, as a race, they are not troubled with daintiness; and if they love variety of fare, they can have the same with ease. Pretty much whatever comes in their path is gulped down without hesitation—“gobbled up in the twink of an eye.” Most of them, in cannibalistic style, feed without compunction upon other fishes.
While they may be popularly said to “breathe water,” because they take in water instead of air, they depend upon life-sustaining oxygen every whit as much as do land animals, only in a different mode.
Large quantities of oxygen are ever present, dissolved, in the sea. It is not the water flowing through the fish’s gills, but the oxygen dissolved in that water, which carries on combustion in the body, and so keeps it in life and health.
The amount of oxygen thus obtained is small; therefore the combustion is slow; therefore the blood is cold. But if, for a very short time, all oxygen could be withdrawn from the ocean, the whole multitude of fishes therein would fast die of suffocation. Sea-water alone, without oxygen, could not keep them alive.
The gills are, however, so constructed that, while they can use small supplies of oxygen dissolved in water, they cannot use large supplies of oxygen forming part of the free air. So when a fish out of water draws in atmospheric air, its gills become dry and useless, and it dies of suffocation.
A world of rapid living and dying is this which we have now in view—of killing and being killed—of an incessant struggle for existence.
Many of its inhabitants have no means of defence, save by flight; and these feed only on beings weaker than themselves; largely on the vast hordes of the young of crabs and jelly-fishes. Others are furnished with formidable weapons, wherewith they can actively attack powerful foes.
Cold-blooded though they be, and probably without the sense of physical pain, they yet have their little range of emotions.