Zoologger is our weekly column highlighting extraordinary animals – and occasionally other organisms – from around the world

A more forward approach (Image: L.X. Tran) Cleared and stained specimen of male Phallostethus cuulong (Image: L.X. Tran)

Species: Phallostethus cuulong

Habitat: surface waters of the Mekong River in Vietnam


The male fish, a Phallostethus cuulong just 2 centimetres long, weaves between drifting vegetation in the sluggish waters of a canal. He closes in on a female, swims alongside her and tries to mate with her.

But to an outside observer, he seems to be doing it wrong. His head is right next to the female’s, but he’s at a 45-degree angle so his rear end is well below hers. Sounds misguided, but actually he’s doing it exactly right – it’s just that his gonads are on his head.

This is the challenge faced by all priapiumfish, a little-known group of Asian fish that have their reproductive organs on their chins, just behind their mouths. How does this Cronenbergian arrangement work?

Phallic fish

P. cuulong is only the 22nd known priapiumfish, which are named after the ancient Greek fertility deity, Priapus. They all belong to a family called Phallostethidae and live in south-east Asia.

The new species was discovered in July 2009 by Koichi Shibukawa of the Nagao Natural Environment Foundation in Tokyo, Japan. He saw one swimming alone in a canal near the Mekong River in Vietnam, and managed to catch it in a net. Working with colleagues at Can Tho University in Vietnam, he realised it was a new species.

Male priapiumfish don’t have a penis like humans and other mammals. Instead they have a unique organ called a priapium, which faces backwards and looks like a muscular nozzle. It’s actually a modification of the fish’s pectoral and pelvic fins.

The priapium of P. cuulong has two attachments, both of which look frankly dangerous. At the bottom near the tip, there is a forward-facing serrated saw, or ctenactinium. Further forward, right under the head, there is a forward-facing rod called the toxactinium.

Machinery of love

No one has seen P. cuulong mating, but based on observations of other species it’s likely that the saw and rod are used for grasping the female during mating. One goes on either side of her head, holding her still while the male transfers his sperm.

To help with this, the priapium tends to be shunted to one side. The six male P. cuulong that Shibukawa found all had theirs on the right: other species tend to be left-priapiumed, or a mixture of the two.

The system seems to work. The oviducts of female priapiumfish tend to be stuffed full of sperm, so pretty much every egg gets fertilised.

Why, evolution, why?

We don’t know why priapiumfish evolved their peculiar gonads, says Lynne Parenti of the Smithsonian Institution in Washington, DC.

They belong to a big group called the Atherinomorpha, which includes many other species with modified fins that transfer sperm. In most cases, like guppies and splitfins, it’s the anal fin that’s modified. The priapiumfish are a variation on this theme, Parenti says.

It’s not just the reproductive organs that are attached to the priapiumfish’s head: so too is its bottom. The fish’s anus is on its priapium, slightly forward from the genital opening. Its guts perform a U-turn to reach it.

“There’s not much going on at the back of these fish,” Parenti says.

Journal reference: Zootaxa, vol 3363, p 45