arapaima (ărˌəpĪˈmə) [key], tropical fish, Arapaima gigas, of the Amazon basin. It is perhaps the largest of the strictly freshwater fishes, reportedly reaching a length of 15 ft (4.5 m), but averaging 7 to 8 ft (2–2.4 m) in length and 200 lb (90 kg) in weight. The dorsal and anal fins of the arapaima are placed so far back that they appear to be part of the tail fin, giving a massive appearance to the posterior region. The scales are olive-green, turning increasingly reddish in the tail region and becoming crimson near the tail fin. The swim bladder, as in all members of the order Clupeiformes, is open to the pharynx; in the arapaima it is rich in blood vessels and serves as a lung. The arapaima uses its fins to hollow out a nest in clear, shallow, sandy-bottomed areas. It is a graceful swimmer despite its bulk, and it is valued as an aquarium fish as well as for food. It is classified in the phylum Chordata, subphylum Vertebrata, class Osteichthyes, order Clupeiformes, family Osteoglossidae.