New species of giant Amazon fish discovered in Brazil

Scientists have discovered a new species of arapaima - a giant torpedo-shaped fish - in the central Amazon waters of Brazil.

Arapaima are huge freshwater fish native to the Amazon River in Brazil and can reach lengths of more than 2 m and weigh up to 200 kg.

"Everybody for 160 years had been saying there's only one kind of arapaima. But we know now there are various species, including some not previously recognised," said Dr Donald Stewart of the State University of New York College of Environmental Science and Forestry (ESF), who made the discovery.

Four species of arapaima were recognised in the mid-1800s, but in 1868, Albert Gunther, a scientist at the British Museum of Natural History, published an opinion that those were all one species, Arapaima gigas. Over time, Gunther's view became the prevailing wisdom.

"Until this year, no taxonomist has questioned Gunther's opinion about these iconic fishes," Stewart wrote.

He began studying the genus in Guyana and Brazil, delving into scientific literature from the 19th century and examining original specimens preserved at the National Museum of Natural History in Paris.

Stewart concluded that all four of those originally described species were, in fact, distinct. Stewart re-described one of those original species (in a paper published in the March issue of Copeia) and summarised status of the other three species.

Stewart's most recent discovery came when he examined preserved arapaima at the Instituto Nacional de Pesquisas da Amazonia in Manaus, Brazil. This new description brings the total number of species to five.

The recently identified specimen was collected in 2001 near the confluence of the Solimoes and Purus rivers in Amazonas State, Brazil.

It is distinguished from all other arapaima by several characteristics, including the shape of sensory cavities on the head, a sheath that covers part of the dorsal fin and a distinctive colour pattern. Its scientific name, A leptosoma, is in reference to its slender body.

For two centuries, arapaima have been among the most important commercial fishes in freshwaters of the Amazon.

"Failure to recognise that there are multiple species has consequences that are far reaching," Stewart said.

"For example, there is a growing aquaculture industry for arapaima, so they are being moved about and stocked in ponds for rearing. Eventually pond-reared fishes escape and, once freed, the ecological effects are irreversible.

"A species that is endangered in its native habitat may become an invasive species in another habitat. The bottom line is that we shouldn't be moving these large, predatory fishes around until the species and their natural distributions are better known," Stewart said.