Ichthyology is the study of fish. Fish are aquatic, ectothermic (i.e., relying on external sources of heat to regulate body temperature) animals that have backbones, fins, and gills throughout life. This definition covers a wide variety of species and, consequently, a great diversity of forms and habits.
Currently recognized as valid are about 25,000 extant species of fish, divided into 57 orders and 482 families. The number of species of fish exceeds all other species of vertebrates combined. Although over 97% of the surface water on Earth is saltwater, about 41% of all species of fish live in freshwater. The large number of freshwater fishes is a product of the propensity for populations in freshwater bodies to become isolated, and then to differentiate and, eventually, speciate.