The world's smallest fish is Paedocypris progenetica, and was only recently discovered in the last decade. Scientists discovered it living in the forest swamps on the Indonesian island of Sumatra. Mature femalesmeasure just 7.9 millimeters — less than one third of an inch! Paedocypris was the world?s smallest known vertebrate animal, which has now been superseded by a minute frog (Paedophryne amauensis) from the New Guinean rain forests.

This tiny fish, a member of the carp family, has a partially see-through body and a reduced head skeleton, which leaves the brain completely unprotected by bone.