Scientists have discovered a fish living in forest swamps on the Indonesian island of Sumatra that is only 7.9mm long.
The species of fish belongs to the carp family and is called Paedocypris progenetica . It is the world's smallest vertebrate or backboned animal.
The tiny, see-through Paedocypris fish have the appearance of larvae and have a reduced head skeleton, which leaves the brain unprotected by bone.
They live in dark tea-coloured waters with an acidity of pH3, which is at least 100 times more acidic than rainwater.
'This is one of the strangest fish that I've seen in my whole career', said Ralf Britz, zoologist at the Natural History Museum.
'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 swamps were once thought to harbour very few animals, but recent research has revealed that they are highly diverse and home to many species that occur nowhere else.
In 1997 the peat swamps were damaged by large forest fires and they are still threatened by logging, urbanisation and agriculture. Several populations of Paedocypris have already been lost.
The new fish was discovered by fish experts Maurice Kottelat (from Switzerland) and Tan Heok Hui from the Raffles Museum of Biodiversity Research in Singapore. They were working with their colleagues from Indonesia and with Kai-Erik Witte from the Max Planck Institute in Germany. Ralf Britz, at the Natural History Museum , helped analyse its skeleton and the complex structure of the pelvic fin.
The previous record for smallest vertebrate was held by an 8mm species of Indo Pacific goby. The UK's smallest fish is the marine Guillet's goby, Lebetus guilleti, reaching 24mm in length.
The paper is being published online in the journal Proceedings of the Royal Society: Biological Sciences