What is a fish – characteristics

Ask anyone what creatures live in water, and nine times out of ten the first answer will be “fish.” A vast and colorful horde of other living things make their home in the oceans, lakes, rivers, streams, and swamps of the earth, but to man the fish has always been the most important of water dwellers.

And for good reason. Since before the dawn of history, man has relied upon the fish as food, wresting a good part of his livelihood from the water. He has found a thousand cunning ways to satisfy his needs with this creature, from making tools of its skeleton, a fertilizer of its flesh, a bone-building medicine of the rich oil in its liver, right up to using the fish itself as a decoration in his home.

Fishes dominate the waters of our planet. There are more fishes – and more different kinds of them – than of any other good-sized animals living wholly or partly in water. Today we are able to recognize more than twenty-five thousands of fishes – as many species as there are of mammals, birds, reptiles, and amphibians put together.

What is a fish? Don’t suppose this question is naive. To cover so diverse a group of animals with a single definition – one that will include all the kinds of fishes and at the same time exclude all living things that are not fish – is difficult. Fishes are so varied in the nature of their bodies and the way they use their organs, that exceptions turn up for almost every characteristic of fishes in general. For example: a number of fishes have no scales; a few have no fins; a few lack jaws; some do not breathe primarily by means of gills; and others spend practically as much time out of the water as in it.

After gathering together a number of the features unique among fishes, and taking account of the exceptional fish that lacks one or another of them, we arrive at the following definition: A fish is a cold-blooded, aquatic vertebrate with gills and a two-chambered heart.

Like most definitions, this one needs some explaining to make it fully understandable. A vertebrate is, briefly speaking, an animal with a backbone, and this characteristic alone is sufficient to separate fishes from all the invertebrates (animals without backbones) – which include such creatures as insects, spiders, shellfish of all sorts, starfish, and their relatives, all kinds of worms, sponges, and jellyfish – and also from the near-vertebrates (protochordates), the acorn-worms, sea-squirts, and lancelets.

The fact that fishes are cold-blooded, that is, have a body temperature which more or less closely follows that of the water or air in which they live, clearly distinguishes them from birds and mammals. (We must say “more or less” because a very few fishes, such as the tunas, maintain a body temperature which is somewhat higher than that of the water surrounding them.)

The fact that fishes have a two-chambered heart also separates them from the amphibians and reptiles and from the birds and mammals – all of which have three- or four-chambered hearts. The gills, too, are an important feature of fishes. Although some fishes do not depend on their gills for breathing, these organs are always present, even though they may consist of only a few undeveloped filaments.

Fishes are aquatic, with no exceptions whatsoever. Those that spend hours or perhaps days at a time out of water, still must periodically return to that element to keep from becoming dried out. Some fishes can live for months without water, and a very few even for years, but they can do this only when inactive, in a state of suspended animation. All active fishes require at least enough water to bathe their bodies and keep their respiratory, or breathing, organs moist.

Actually, it is much easier to distinguish fishes from other generally similar aquatic animals than you might gather from what we have said. You can tell fishes from whales and porpoises by their tails; in fish the tail is vertical, like a rudder, while in whales and porpoises it is horizontal. A snake and an eel can be distinguished at a glance: the snake has no gill-openings, but the eel has. Tadpoles have no paired fins, and this marks them off from all but a very few fishes – and these few exceptional fishes are so untadpole-like that they could never be confused. Recognizing fishes, you can see, is more difficult in theory than in practice.