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

There are currently four genera, and the new species has been assigned to Phallostethus based on a number of shared characters, but differs from them in the number of caudal vertebrae, teeth on paradentary and serrae on the second ctenactinium, the latter being a curved, rod-like clasper used during reproduction.

It’s known only from slowly-flowing canals and rivers with soft, muddy substrates in  Soc Trang and Tra Vinh Provinces, Vietnam, within the Mekong watershed. Like other phallostethids it tends to be found close to the surface and has a small adult size with the largest known specimen measured at just 24.5 mm SL.

The specific name is the Vietnamese name for the Mekong Delta, where the type series was collected and translates literally as ‘nine dragons’ in reference to the nine major Mekong tributaries in Vietnam.

For further information please refer to the full, open access paper: Shibukawa, K, D. Dac Tran, and L. X. Tran. 2012. Phallostethus cuulong, a new species of priapiumfish (Actinopterygii: Atheriniformes: Phallostethidae) from the Vietnamese Mekon. Zootaxa 3363: 45-51