What organ do arapaima use as a lung to breathe air at the water's surface?

Arapaima use a modified swim bladder as a lung to breathe air at the water's surface.

Arapaima are giant freshwater fish native to the Amazon and Essequibo basins. Their swim bladder is richly supplied with blood vessels and functions as an air-breathing organ. The fish must rise to the surface regularly, producing a distinctive gulping sound as they inhale.

This adaptation helps arapaima survive in warm, slow-moving waters that may contain little dissolved oxygen. They still retain gills and can exchange gases in water, but adults depend heavily on atmospheric oxygen. Their air-breathing habit also makes them vulnerable to surface fishers.

Arapaima are among the largest freshwater scaled fish. They have long bodies, broad heads and large overlapping scales. Several species are now recognized within the genus, although older sources often treated arapaima as a single species, Arapaima gigas.