Five Arapaima species, not one

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Scientists studying the world's largest freshwater fish, Arapaima gigas, have suggested that it is actually five species not one, writes Matt Clarke.

New research has suggested that there are actually four valid Arapaima species and at least one undescribed one, which is currently known only from the Mamiraua Reserve in Brazil.

Until recently, scientists believed that there was only a single species — Arapaima gigas — which has long been considered the world’s largest freshwater fish.

Arapaima gigas was described by Achille Valenciennes in 1847, along with three other species: A. mapae, A. agassizii and A. arapaima. In 1868 Albert Gunther studied Arapaima but concluded that Valenciennes had got it wrong and there was only one species — and that was A. gigas.

However, new research by Donald Stewart and Leandro Castello which examined these ancient museum specimens has found that Valenciennes was right and there are indeed four valid species. They also found an entirely new one in the protected Mamiraua Reserve.

The findings therefore cast some doubt on the identity of the Arapaima in public aquariums and private collections. However, one thing is almost certain; they are highly unlikely to be A. gigas, mapae or agassizii, as these three are known only from the preserved holotype specimens in museum collections. Captive fish are most probably A. arapaima.

Arapaima arapaima, the most widely seen, is easy to tell apart from the others as it has a deeper caudal peduncle and has 31 rays in the caudal fin. The other three have less than 18 and have a narrow peduncle. There’s no data on the undescribed one, but as it’s from Mamiraua, it’s unlikely to be traded. The distribution of the species is not yet known.

The study raises questions over the conservation of Arapaima which are protected by CITES under the name A. gigas, since this species doesn’t appear to exist in the wild any more, or hasn’t yet been rediscovered if it does.

According to Stewart and Castello there’s no evidence that any of the specimens in zoos and aquariums are A. gigas and the other species could well be unprotected. They have suggested that the entire Arapaima genus is listed on CITES Appendix II to give all species urgently needed protection until more is known about the fish.

Arapaima have been recorded at lengths of up to 2.85m and will reach around 1m and 10kg in weight in their first year, and can reach 160cm and 45kg after three or four years and 45kg after three or four years.

This item was first published in the September 2009 issue of Practical Fishkeeping magazine. It may not be reproduced without written permission.