Definition: The term fish is used to refer to any aquatic vertebrate that has a 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.
Most fish are cold-blooded animals that have a streamlined body that is adapted for efficient movement in water. There are exceptions to both of these rules though. Tuna, swordfish and a few shark species are warm-blooded, not cold-blooded. Rays are flat-bodied fish that not streamlined. They move through the water at a slower pace by undulating their broad pectoral fins.
The first fish were the ostracoderms, a now-extinct group of jawless fishes that appeared in the Cambrian Period, about 510 million years ago. Ostracoderms were followed by the conodonts and the agnanthans. Fish are the most diverse group of vertebrates alive today. The ray-finned fishes alone number in the region of 24,000 species.
