World's smallest fish shrinks

There's a new contender for the title of world's smallest fish, as scientists have described a new species 0.1 mm smaller than the current record holder.

Maurice Kottelat and Tan Heok Hui, who work at the Raffles Museum of Biodiversity Research in Singapore, described the new fish as Paedocypris progenetica in a paper in the Royal Society's Proceedings B journal.

Unlike the previous record holder, Trimmatom nanus, which measured 8 mm when adult, and the one before that, the Stout infantfish, Schindleria brevipinguis, the new species is unusual in that it's a freshwater fish, not a marine species.

Fully grown at just 7.9 mm, a wopping 0.1 mm smaller than T. nanus, the new species is a member of the barb order Cypriniformes and lives in the highly acidic blackwater peat swamps of the island of Sumatra in Indonesia, and in the Malaysian part of Borneo.

Like the waters of Brazil's Rio Negro, the pH of these blackwater swamps can be extremely low - just pH 3.0 - which is over 1000 times more acidic than the average UK tapwater.

The new cyprinid fish is said to have very large and muscular pelvic fins which are believed to be used to grasp the female during reproduction.

Amazingly, the authors also described a second species, P. micromegethes from Sarawak, which is the second smallest fish ever described.

Not only is P. progenetica the world's smallest known fish, it's also the world's smallest known vertebrate organism.

Unfortunately, things are not looking good for the longterm survival of the species: "Many of the peat swamps we surveyed throughout south-east Asia no longer exist and their fauna is eradicated," the authors wrote in the paper.

The swamps of the region were previously believed to harbour few species, but further research has shown that they harbour endemic creatures that have evolved to live in the extreme conditions present there.

For more information see the paper: Maurice Kottelat, Ralf Britz, Tan Heok Hui & Kai-Erik Witte, 2006. Paedocypris, a new genus of Southeast Asian cyprinid fish with a remarkable sexual dimorphism, comprises the world's smallest vertebrate. Proc. R. Soc. B. doi:10.1098/rspb.2005.3419.