Arapaima

The arapaima is among the largest of freshwater fish, with adults measuring 2 to 2.5m, and it has to come to the surface to breathe air.

It lives in slow-moving, oxygen-poor freshwaters of the Amazon River Basin in South America, including floodplains, oxbow lakes and swamped forests, and is known to leap out of the water when threatened.

A primitive lung, formed from a modified swim bladder, sends it up for a gulp every 5 to 15 minutes, so it stays near the surface, usually within about 2m of it, and cannot linger deep. Seasonal flooding decides how much water it has, and its core habitats are generally only 2 to 8m deep from top to bottom.

Breeding comes in the dry season, when arapaimas build a nest just 15cm deep on the sandy bottom and guard their young.

The bonytongue name comes from a toothed bone on the floor of the mouth, the "tongue", whose teeth bite against others on the roof of the mouth.