These so-called "blackwater" swamps are a unique landscape of flooded trees growing in waterlogged, soft, peaty soil that is often meters thick.
The water is stained reddish-black, like very dark tea, appearing black at the surface. It is extraordinarily acidic, having a pH3 value of only three, the same as a sour apple.
The scientists needed a special stereoscopic microscope to accurately measure the fish.
The smallest adult specimen they netted was a mature
Paedocypris progenetica female, found in Sumatra, that came to just 7.9 mm (0.31 of an inch) from nose to tail, making it not only the world's smallest fish but the smallest vertebrate too.
She nudged out the previous record holder, a marine fish of the Western Pacific called the dwarf goby (
Trimmatom nanus), which comes in at 8 mm (0.32 of an inch) at sexual maturity.
The team also found a related Paedocypris species,
P. micromegethes, in Sarawak in Malaysian Borneo.
At 8.8 mm (0.35 of an inch),
P. micromegethes is the second-smallest freshwater vertebrate ever found.
The fish was discovered by Maurice Kottelat and Tan Heok Hui, researchers at the Raffles Museum of Biodiversity Research at the National University of Singapore.
They were assisted by Ralf Britz of Britain's National Museum of Natural History and Kai-Erik Witte at the Max Planck Institute for Developmental Biology in Tuebingen, Germany.
Kottelat said
P. progenetica has "a very rudimentary skull" which leaves the brain exposed.
Evolutionary pressures have caused the fish to develop highly modified fins to survive in its special environment. Males also have a tough pad on the front of the pelvic girdle that may be used to help them clutch onto females during mating.
"The discovery of such a tiny and bizarre fish highlights how little we know about the diversity of Southeast Asia," said Kottelat. "This is all the more serious because the habitat of this fish is disappearing very fast, and the fate of the species is now in doubt."
