Fishes make up more than half of the 55,000 species of living vertebrates. Along with this remarkable taxonomic diversity comes an equally impressive habitat diversity. Today, and in the past, fi shes have occupied nearly all major aquatic habitats, from lakes and polar oceans that are ice-covered through much of the year, to tropical swamps, temporary ponds, intertidal pools, ocean depths, and all the more benign environments that lie within these various
extremes. Fishes have been ecological dominants in aquatic habitats through much of the history of complex life. To colonize and thrive in such a variety of environments, fishes have evolved obvious and striking anatomical, physiological, behavioral, and ecological adaptations.