The world's smallest known fish has been discovered living in a peat swamp on the Indonesian island of Sumatra.
At just 7.9mm long, the tiny fish is thought to be a distant relative of the much larger carp, say scientists.
Called Paedocypris, they manage to live in the acidic swamps by eating larvae living in the bottom of the water.
But experts are worried the fish could soon die out, because Indonesian peat swamps are disappearing due to forest fires, logging and farming.
The Royal Society report found that to keep their size down, the fish have very little protection around their brain, and females can only carry a few eggs at a time.
The adults are transparent and look a bit like a fish larva.
Dr Ralf Britz, a zoologist at the Natural History Museum, said it was the strangest fish he's ever seen: "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 new discovery is 1mm smaller than the current record holder, the Indo-Pacific goby fish.