What is a fish?
It may in fact be unrealistic to attempt to define a “fish”, given the diversity of adaptation that characterizes the thousands of species alive today, each with a unique evolutionary history going back millions of years and including many more species. Recognizing this diversity, one can define a fish as “a poikilothermic, aquatic chordate with appendages (when present) developed as fins, whose chief respiratory organs are gills and whose body is usually covered with scales” (Berra 2001, p. xx), or more simply, a fish is an aquatic vertebrate with gills and with limbs in the shape of fins (Nelson 2006). To most biologists, the term “fish” is not so much a taxonomic ranking as a convenient description for aquatic organisms as diverse ashag fishes, lampreys, sharks, rays, lungfishes, sturgeons, gars, and advanced ray-finned fishes.
Definitions are dangerous, since exceptions are often viewed as falsifications of the statement (see, again, Berra 2001). Exceptions to the definitions above do not negate them but instead give clues to adaptations arising from particularly powerful selection pressures. Hence loss of scales and fins in many eel-shaped fishes tell us something about the normal function of these structures and theirin appropriateness in benthic fishes with an elongate body. Similarly, homeothermy in tunas and lamnid sharks instructsus about the metabolic requirements of fast-moving predatorsin open sea environments, and lungs or other accessory breathing structures in lungfishes, gars, African catfishes, and gouramis indicate periodic environmental conditions where gills are inefficient for transferring water-dissolved oxygen to the blood. Deviation from “normal” in these and other exceptions are part of the lesson that fishes have to teach us about evolutionary processes.