Pirarucu, (Arapaima gigas), also called arapaima or paiche, ancient, air-breathing, giant fish of Amazonian rivers and lakes. One of the largest freshwater fishes in the world, the pirarucu attains a length of nearly 3 metres (10 feet) and a weight of 220 kg (485 pounds). The fish has a peculiar profile in that the front of the body is long and narrow, whereas the rear is flat and possesses only a rudimentary, rounded tail.

Pirarucu (Arapaima gigas). Hans Reinhard/Bruce Coleman Ltd.

The pirarucu is a fish belonging to a primitive group of carnivorous bony-tongued fishes, the Osteoglossidae, that crush their prey with a large tongue studded with teeth. Amazonian people, in fact, use dried pirarucu tongues as seed-graters. This fish is also an important protein source in the diet of people living along the river, but large pirarucu have become less common in some areas, owing to overfishing.

The pirarucu’s air bladder allows the fish to breathe air. Only very young pirarucu have functional gills. This fish seeks out oxygen-poor waters, where the other fish upon which it feeds will be torpid and slow. It builds up large fat reserves during the dry season, which it needs for the parental duties to come.

As the rainy season approaches, both sexes help excavate a hole 50 cm (20 inches) wide and 20 cm (8 inches) deep in the mud of shallow waters, digging with lower jaw, fins, and mouth. There a large female may deposit as many as 50,000 eggs, which are then fertilized by the male. He guards the nest while the female stays near to ward off predators. The newly hatched young stay close to the father’s head, which turns grayish black at this time to help camouflage the dark young. When the young grow large enough to fend for themselves, the father swims away and his head loses the dark colour.