More than seven-tenths of the earth’s surface is covered with water, and practically all of this vast area is inhabited by fishes. The seven seas, the many lakes, streams, ponds, and swamps all accommodate typical groups of fishes of one kind or another.
Ocean waters contain by far the greatest proportion of fishes – both in number of species and individuals. In the sea, fish are found from shallow, temporary tide pools down to the utmost depths of the ocean; in fact almost everywhere except uninhabitable regions such as the lower levels of the Black Sea, which are completely devoid of oxygen. Some fishes are what might be called “blue water” dwellers, living out in the middle of the ocean and never seeing or coming close to land during their whole lives. Others remain near shore, around coral reefs, or at the mouths of rivers. Still others live on the bottom, be it rocky or muddy, in calm depths, or near surf-washed beaches.
Fresh-water fishes are subject to even greater extremes in the waters they inhabit. The streams of the world – from source to mouth – provide living places for various types of fish. Fishes are found in mountain torrents and sluggish rivers, ice-cold lakes and hot volcanic springs, foul swamps and clear, crystalline pools, soda-charged waterholes and acid lakes, sun-heated ponds and murky caves. Some fishes even make temporary pools their homes, and when these dry up, the fishes either travel overland to find water elsewhere, or hole up in mud until the rains come; they may also die, but leave their drought-resistant eggs behind to carry on the race.
Few of these waters are entirely isolated from all others, and there are fishes adaptable enough to live in more than one kind. For instance, a number can thrive in either fresh or salt water, or in places either warm or cold.
So it is not surprising that fishes, being subjected to such diverse conditions of existence, should show widely different structures and functions, and should vary so greatly in their life histories and patterns of behavior.
But this variety of dwelling places alone does not account for the diversity of fish life, because in a single, relatively uniform locality, as that around a coral reef, as many as two hundred species may be found – eloquent evidence that there are additional factors involved in the evolution of fish.
There is hardly anything that lives in water, either plant or animal, that is not eaten by some fish. Fishes prey extensively upon one another, too. Most bizarre perhaps are those relationships in which fishes make their homes inside living sponges, snails, bivalves, starfish, and sea-cucumbers, or in which they share the burrows of shrimps and worms, or live among the stinging tentacles of jellyfishes and sea-anemones.