A fish is any member of a paraphyletic group of organisms that consist of all gill-bearing aquatic craniate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups. Most fish are ectothermic (“cold-blooded”), allowing their body temperatures to vary as ambient temperatures change, though some of the large active swimmers like white shark and tuna can hold a higher core temperature.
Whale Shark (Rhincodon typus). Face portrait of an 8-meter individual filter-feeding plankton at night.
Caesio teres in Fiji
Chlamydoselachus anguineus stuffed at Aquarium tropical du Palais de la Porte Dorée (Paris, France). Citron / CC-BY-SA-3.0
Amphiprion ocellaris (Clown anemonefish) in Papua New Guinea.
Basking shark (Cetorhinus maximus).
Oscar (Astronotus ocellatus), a popular aquarium fish from South America.
A Puffer Fish, Arothron hispidus is kissing camera at Big Island of Hawaii.