No other group of vertebrates offers such a wide range of structural, biochemical and behavioral differences as do the fishes. They are adapted to live in desert pools and under the polar ice; in salt or fresh water or both; at the surface of weed choked ponds, in city sewers, between the rocks of torrential streams or at abyssal depths in total darkness. There are fishes that produce electricity, sense electricity and modulate frequency. Their skin may contain scales, mucous glands, taste buds, alarm cells, toxins, photophores and melanophores. They may live for only a year, 5 years, 20 years, 50 years or 150 or more. Some undergo sex reversals from functional male (protandrous) or functional female (protogynous) to the functional opposites. There are hermaphroditic, parthenogenetic, gynogenetic, oviparous and viviparous species.

The features shared by all include fins, a skeleton of cartilage or bone, metameric muscles, a gill respiratory system, a single-circuit four-chambered heart, and a brain that reflects their sensory modalities.