Tiny blue-bellied fish discovered in the Rio Negro

Douglas Heaven, reporter

(Image: Natural History Museum, London)

At only 17 millimetres long, this blue-bellied night wanderer won't break any angling records. But for the researchers who fished it out of the Rio Negro, the largest tributary of the Amazon, it must have been catch of the season.

The fish - since named Cyanogaster noctivaga - is not only a new species, but an entirely new genus. And it was found in a stretch of river that has been particularly well explored, making it even more of a surprise.

A member of the Characidae family, the fish is marked out from its relatives by an unusual mouth and teeth - including a single conical tooth in the outer row - and slightly different fins. It is fully transparent except for its head and striking blue belly.

Nocturnal and only 7 millimetres longer than Paedocypris progenetica, the world's smallest fish, it wasn't easy to spot. But the team learned to look out for a sudden blue streak in their nets. To get pictures, they had to transfer specimens to a specially rigged photo tank quickly, without taking them out of the water, otherwise they died and turned white.

"It's a strange little animal," says Ralf Britz, a fish expert at the Natural History Museum in London, who took part in the Amazonian expedition in October 2011.

Journal reference: Ichthyological Exploration of Freshwaters, vol 23, p 297

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