Fish are aquatic vertebrates that have skin covered with scales, two sets of paired fins, some unpaired fins, and a set of gills. Fish do not represent a single clade but are instead paraphyletic. They include hagfishes, lampreys, lobe-finned fishes, cartilaginous fishes and ray-finned fishes.

Bony fishes (Osteichthyes) are a group of aquatic vertebrates characterized by having a bony skeleton (in contrast to the cartilaginous fishes, a group whose skeleton consists of cartilage, not bone). Bony fishes include two sub groups, the ray-finned fishes and the lobe-finned fishes.