The fish, a member of the carp family, has a translucent body and a head unprotected by a skeleton.
“This is one of the strangest fish that I’ve seen in my whole career,” Ralf Britz, a zoologist at the Natural History Museum in London, said. “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.”
Mature females grow to less than a third of an inch long. Males have enlarged pelvic fins and muscles that may be used in reproduction, researchers wrote in a report published by the Royal Society in London.
“You don’t wake up in the morning and think, ‘today we will find the smallest fish in the world’,” Swiss fish expert Maurice Kottelat, who helped discover the fish, said from his home in Switzerland. “What’s important is finding a complete vertebrae in a body so small.”
The previous record for small size, according to the Natural History Museum, was held by an 8mm species of Indo-Pacific goby.
The fish were found in an acidic peat swamp on the Indonesian island of Sumatra.
Indonesian peat swamps are under threat from fires lit by plantation owners and farmers, as well as unchecked development and farming. Researchers say several populations of the tiny fish, Paedocypris progenetica, have already been lost.
“The structurally complex peat swamp forests are disappearing quickly in South-East Asia,” the researchers wrote.
“Many of the peat swamps we surveyed throughout south-east Asia no longer exist and their fauna is eradicated. Populations of all the highly endemic and stenotopic miniature fishes of peat swamps have decreased or collapsed.”