The world's tiniest fish (all 7.9mm of it) is found

Scientists have discovered the world's smallest fish in a tropical acidic swamp where the water is the colour of strong tea. Females grow no bigger than 7.9mm (0.31in) and the male measures up to 10.3mm.

The diminutive size of the new fish species means that it now has the title of the smallest known vertebrate - an animal with a backbone.

Biologists discovered the fish during an expedition to explore remote forest swamps on the Indonesian island of Sumatra, where the peat water is a hundred times more acidic than rainwater.

Scientists had thought that little if anything could survive in such peat swamps but have been astonished to discover several species of small fish including the latest specimen, named Paedocypris progenetica. A Swiss biologist, Maurice Kottelat, and Tan Heok Hui from the Raffles Museum of Biodiversity Research in Singapore, discovered the fish by sieving the water of the Sumatran swamps with a fine-mesh fishing net.

They sent specimens of the fish to the Natural History Museum in London where Ralf Britz, a zoologist and fish expert, identified it as a species new to science.

Dr Britz said that it was a member of the carp family. The males were distinguished by having a pair of large pelvic fins which were manipulated by well-developed muscles. "This is one of the strangest fish that I've seen in my whole career. It's tiny, it lives in acid and it has these bizarre grasping fins," Dr Britz said. The peat swamps on Sumatra were extensively damaged in 1997 by forest fires and are threatened by logging, the growth of towns and agriculture. Several populations of the tiny fish have already been lost, he said.

"I hope we'll have time to find out more about them before their habitat disappears completely," Dr Britz said.

The forest swamps of Indonesia were once thought to harbour very few animals but recent research shows that they have a rich and diverse range of wildlife that appears to be unique to this habitat. "These forest swamps harbour an unusually large number of species. The water is poor in minerals and this may restrain bone growth leading to the adaptation of being small," Dr Britz said. Adult Paedocypris fish are transpa rent and resemble juvenile larval fish despite being sexually mature, with males sporting the well-endowed pelvic fins.

"There are a number of miniature fish that have been described but none is smaller and none has this unique pelvic fin. It's an amazing structure," Dr Britz said.

One theory is that the fins are used by males to grasp females during mating. Another is that they are used to capture eggs as they are released by females into the murky water of the swamp. The fish probably feeds on microscopic animals such as small crustaceans called copepods and filter-feeding organisms called rotifers.

The previous record for the smallest fish was held by the Indo-Pacific goby, which is 8mm long. The smallest British fish is the marine Guillet's goby, which reaches 24mm.