what is fish

A fish is an aquatic, craniate, gill-bearing animal that lacks limbs with digits. This definition includes living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups. Fish are vertebrates, meaning they have a backbone, and they breathe using gills. They can be found in both fresh and salt waters, and there are approximately 34,000 species of fish in the world. Fish are valuable for various reasons, including their role in the food supply, their importance in medical and biological research, and their significance in the study of animal behavior. In the most basic biological terms, a fish can be defined as a marine or aquatic animal that has a backbone, gills, and fins. This definition excludes invertebrates like shellfish, cuttlefish, starfish, crayfish, and jellyfish, which are not considered fish in the biological sense. Overall, fish play a crucial role in the ecosystem and have significant scientific, ecological, and economic importance.