Fish represent a diverse group of fully aquatic vertebrate animals that are gill-breathing, cold-blooded, scaled, and have fins. They include around 25,000 species worldwide divided into a few jawless fish (like lampreys), cartilaginous fish (like sharks, skates, and rays), and many bony fish (like ray-finned fish). Here we divide fish according to their major habitats, which includes freshwater fish, saltwater fish, and searun fish.

Reference

Hickman CP, Roberts LS, Larson A: Animal Diversity. Boston: McGraw-Hill; 2000.