The arapaima also known as the pirarucu, or paiche are any large species of bony tongue in the genus Arapaima native to the Amazon and Essequibo basins of South America. These fish are among the world’s largest freshwater ones, reaching as much as 3 m (9.8 ft). They are considered an important food fish. As such their numbers have declined in the native range due to overfishing and habitat loss. In contrast, arapaima have been introduced to several tropical regions outside the native range (within South America and elsewhere) where they are sometimes considered invasive species. Its local name, pirarucu, derives from the indigenous words for “pira” meaning “fish” and “urucum” meaning “red”.
Arapaima was traditionally regarded as a monotypic genus, but later several species were distinguished. As a consequence of this taxonomic confusion, most earlier studies were done using the name A. gigas, but this species is only known from old museum specimens and the exact native range is unclear. The regularly seen and studied species is A. arapaima, which is relatively thickset compared to the remaining species.
Arapaima can reach lengths of more than 2 m (6 ft 7 in), in some exceptional cases even more than 2.5 m (8 ft 2 in) and over 100 kg (220 lb). The maximum recorded weight for the species is 200 kg (440 lb), while the longest recorded length was 4.52 m (15 ft). As a result of overfishing, large arapaima of more than 2 m (6 ft 7 in) are seldom found in the wild.
The arapaima is torpedo-shaped with large blackish-green scales and red markings. It is streamlined and sleek, with its dorsal and anal fin set near its tail.
Arapaima scales have a mineralised, hard, outer layer with a corrugated surface under which lie several layers of collagen fibres. In a structure similar to plywood, the fibres in each successive layer are oriented at large angles to those in the previous layer, increasing toughness. The hard, corrugated surface of the outer layer, and the tough internal collagen layers work synergistically to contribute to their ability to flex and deform while providing strength and protection—a solution that allows the fish to remain mobile while heavily armored. The arapaima has a fundamental dependence on surface air to breathe. In addition to gills, it has a modified and enlarged swim bladder, composed of lung-like tissue, which enables it to extract oxygen from the air.
