Members of the family  Phallostethidae are small freshwater fishes native to Southeast Asia and included in the order Atheriniformes so related to rainbowfishes and ricefishes, among others.

They’re not popular aquarium fishes, but have an interesting reproductive strategy whereby males are equipped with a unique apparatus known as the priapium which is located on the underside of the head and used for clasping a female while simultaneously entering the body cavity and fertilising eggs internally. The female subsequently lays fully-fertilised eggs rather than giving birth to live young.

Freshly collected male (above) and female specimens of P. cuulong. © L. X. Tran

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