arapaima [key], tropical fish, genus Arapaima, of the Amazon basin. Arapaimas are 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 arapaimas 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 is open to the pharynx; it is rich in blood vessels and serves as a lung. Arapaimas use the fins to hollow out a nest in clear, shallow, sandy-bottomed areas. They are graceful swimmers despite their bulk, and they are valued as an aquarium fish as well as for food. Arapaimas are classified in the phylum Chordata, subphylum Vertebrata, class Actinopterygii, order Osteoglossiformes, family Arapaimidae.
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