A detailed guide of everything you want and need to know about fish. A fish is a water-dwelling vertebrate with gills that doesn''t change form, as amphibians do, during its life. Most are cold-blooded, though some (such as some species of tuna and shark) are warm-blooded. There are over 29,000 species of fish, making them the most diverse group of vertebrates. Fishing is the activity of hunting for fish. Fishing is a very ancient practice that dates back at least to the Mesolithic period which began about 10,000 years ago. Fishing is the activity of hunting for fish. By extension, the term fishing is also applied to hunting for other aquatic animals such as various types of shellfish as well as squid, octopus, turtles, frogs and some edible marine invertebrates. Fish as a food describes the edible parts of water-dwelling, cold-blooded vertebrates with gills, as well as certain other water-dwelling animals such as mollusks, crustaceans, and shellfish. An aquarium (plural aquariums or aquaria) is a vivarium, usually contained in a clear-sided container (typically constructed of glass or high-strength plastic) in which water-dwelling plants and animals (usually fish, and sometimes invertebrates, as well as amphibians, marine mammals, and reptiles) are kept in captivity, often for public display; or it is an establishment featuring such displays. A detailed guide of everything you want and need to know about fish.