By Roland Pease

BBC science correspondent



The world's smallest known fish can measure as little as 7.9mm

Individuals of the Paedocypris genus can be just 7.9mm long at maturity, scientists write in a journal published by the UK's Royal Society.

But they warn long-term prospects for the fish are poor, because of rapid destruction of Indonesian peat swamps.

The fish have to survive in pools of acid water in a tropical forest swamp.

"This is one of the strangest fish that I've seen in my whole career," said Ralf Britz, a zoologist at the Natural History Museum in London, UK.

"It's tiny, it lives in acid and it has these bizarre grasping fins. I hope we'll have time to find out more about them before their habitat disappears completely."

The new fish was discovered by Maurice Kottelat (from Switzerland) and Tan Heok Hui from the Raffles Museum of Biodiversity Research in Singapore, while working with colleagues from Indonesia and with Kai-Erik Witte from the Max Planck Institute in Germany.

Ralf Britz helped analyse the animal's skeleton and the complex structure of the pelvic fin.

Human threat

Paedocypris can sustain their small bodies grazing on plankton near the bottom of their water pools.

To keep their size down, the fish have abandoned many of the attributes of adulthood - a characteristic hinted at in their name.

Their brain, for example, lacks bony protection and the females have room to carry just a few eggs.

The males have a little clasp underneath that might help them fertilize eggs individually.

Being so small, the fish can live through even extreme drought, by seeking refuge in the last puddles of the swamp; but they are now threatened by humans.

Widespread forest destruction, drainage of the peat swamps for palm oil plantations and persistent fires are destroying their habitat.

Science may have discovered Paedocypris just in time - but many of their miniature relatives may already have been wiped out.

There have been claims for even smaller fish but some researchers dispute whether the specimens measured were truly adult forms.