Ichthyology is the study of fishes. Fishes are aquatic, cold-blooded animals that have backbones and have 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 fishes divided into 57 orders and 482 families. The number of species of fishes exceeds all other species of vertebrates combined. Although over 97% of the surface water on earth is saltwater, about 41% of all species of fishes 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.
To learn about basic Illinois fish identification, visit our outreach page.