A new species of fish with a penis on its head has been identified in the Mekong Delta in Vietnam, researchers said on Wednesday.

Phallostethus cuulong is the newest member of the Phallostethidae family - small fish found in South-east Asian waters that are distinguished primarily by the positioning of the male sexual organ.

Male phallostethids have a copulatory organ, termed the priapium, under the throat for holding or clasping onto females and fertilising their eggs internally, according to conservationists.

"We have scientifically identified a new penis-head fish in Vietnam," researcher Tran Dac Dinh from Can Tho University told AFP. The fish was known to Vietnamese people in the Mekong Delta but had not been described scientifically before a team identified the species last year, he said.