The arapaima, pirarucu, or paiche (genus Arapaima) is a genus of bonytongues native to the Amazon and Essequibo basins of South America. They are among the largest freshwater fish in the world, reaching lengths of as much as 4.5 m (15 ft). They are important food fish and 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.
Arapaima has traditionally been regarded as a monotypic genus, but following recent studies it has been established that there are several species. As a consequence of this taxonomic confusion, most earlier studies have been 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, although a very small number of A. leptosoma also have been recorded in the aquarium trade. The remaining species are virtually unknown: A. agassizii is only known from old detailed drawings (the type specimen itself was lost during World War II bombings) and A. mapae is only known from the type specimen. A. arapaima 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 6.52 m (15 ft). As one of the most sought-after food fish species in South America, it is often captured primarily by handheld nets for export, by spearfishing for local consumption, and consequently, large arapaima of more than 2 m (6 ft 7 in) are seldom found in the wild today.
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 back near its tail. Its local name, paiche, derives from the indigenous words for "red" and "fish".
Arapaima scales have a highly mineralised, very 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 right angles to those in the previous layer for maximum toughness. The hard corrugated surface of the outer layer, the soft but tough internal orthogonal collagen layers, and the hydration of the scales all contribute to their ability to flex and deform while remaining strong - 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. This is an adaptation to the often hypoxic conditions of the Amazon floodplains, but requires the arapaima to surface for air every 5 to 15 minutes. This tendency to stay at the surface makes it more vulnerable to attacks from spear fishermen.
Commercial fishing of the arapaima has been banned by the Brazilian government due to its commercial extinction. Fishing is allowed only in certain remote areas of the Amazon basin, and must be catch-and-release, or harvesting by native peoples for consumption. Because the arapaima produces boneless steaks, it is considered a delicacy; some 7000 tons per year were taken from 1918 to 1924, the height of its commercial fishing. The demand for the arapaima has led to farming of the fish by the ribeirinhos (as Brazilians call those living on the riverbanks).
The diet of the arapaima consists of fish, crustaceans, and even small land animals that walk near the shore. The fish is an air-breather, using its labyrinth organ, which is rich in blood vessels and opens into the fish's mouth, an advantage in oxygen-deprived water that is often found in the Amazon River. This fish is therefore able to survive in oxbow lakes with dissolved oxygen as low as 0.5 ppm. In the wetlands of the Araguaia, one of the most important refuges for this species, it is the top predator in such lakes during the low water season, when the lakes are isolated from the rivers and oxygen levels drop, rendering its prey lethargic and vulnerable.
Due to the geographic ranges arapaima inhabit, the animal's life cycle is greatly affected by the seasonal flooding. You may find pictures with arapaima with slightly different coloring, this is because arapaima actually change color when they reproduce. The arapaima lays its eggs during the months when the water levels are low or beginning to rise. They build a nest about 50 cm wide and 15 cm deep, usually in muddy-bottomed areas. As the water rises, the eggs hatch and the offspring have the flood season to prosper, during May to August. Therefore, the yearly spawning is regulated seasonally. The arapaima male is a mouthbrooder, like his relative, the Osteoglossum, meaning the young are protected in his mouth until they are older. The female arapaima helps to protect the male and the young by circling them and fending off potential predators.