The world's tiniest fish with a transparent body and exposed brain has been found in Indonesia's peat swamps on the island of Sumatra, experts from five countries said.

The mature females are as small as 7.9 millimetres, making some fully-grown adults of the carp species about the size of mosquitoes, according to the researchers from Singapore, Indonesia, Germany, Switzerland and Britain in a report published Wednesday by the Royal Society in London.

The fish is also the tiniest creature on earth to possess a backbone.

"It came as a surprise that this was the world's smallest vertebrate," The Straits Times quoted Tan Hoek Hui, an expert with the Raffles Museum of Biodiversity Research at the National University of Singapore, as saying.