Arapaima gigas, also known as pirarucu or simply arapaima, is a species of arapaima native to the basin of the Amazon River. Once believed to be the sole species in the genus, it is among the largest freshwater fish. The species is an obligate air-breather, so needs to come to the surface regularly to gulp air. Taxonomy Arapaima gigas was originally regarded as the only species in the genus Arapaima, but the subsequent identification of further species, together with the rarity of specimens and the loss of several type specimens, has led to some uncertainty regarding classification within the genus and the identity of described individuals. Description The species is among the largest known freshwater fish, commonly measuring and reportedly exceptionally reaching lengths of up to . Adults may weigh up to . A. gigas has a streamlined body with dorsal and anal fins set well back towards the tail. While the body is mainly gray to gray-green, its Brazilian local name pirarucu derives from an indigenous word for "red fish", thought to refer to either the red flecks on the scales towards the tail, or the reddish-orange color of its meat. The fish have "flexible, armor-like scales" made up of "a hard, mineralized outer layer" and "a tough-but-flexible inner layer" that help protect it from attacks by piranhas. Distribution A. gigas is native to freshwater in the basin of the Amazon River; it is known to occur in Bolivia, Brazil, Guyana, and Peru. In Bolivia known as paiche, it is considered an invasive species, affecting local native species and the ecosystem. It was first found in 1976, and presumably introduced from Peru being washed out of a Peruvian fish farm by a flood. The species has been introduced to parts of East Asia, both for fishing purposes and accidentally. The fish are found in flooded forest areas where they reproduce during the wet season; they relocate to lakes after water levels drop. A 13-million-old fossil of arapaima (or very similar species) has been found in Colombia, in the Villavieja Formation, which dates from the Miocene era. Physiology Morphology changes occur as A. gigas undergoes the transition from water-breather to air-breather 8–9 days after hatch. During the transition to air-breathing, the structure of the gills changes, making them better adapted for ion absorption, but less able to undergo gas diffusion. Once developmental changes in the gills take place, the lamella