Who says genitals have to be between your legs? A new species of fish has literally turned the genital game on its head and is quietly running with it in the murky Meking River.

Discovered in 2009 by zoologist Koichi Shibukawa from the Nagao Natural Environment Foundation in Tokyo, Japan, and described in a recent issue of Zootaxa, Phallostethus cuulong is a small, near-translucent-skinned species from Vietnam. It is one of 21 known southeast Asian species in the Phallostethidae (literally "penis standing up" in Greek) family, and they are commonly known as priapiumfish, after the elaborate, muscular sex organ, called a priapium, that sits just under the male's throat.

Shibukawa and colleagues collected five male P. cuulong fish and three females from the shallow, brackish river waters in the Soc Trang and Tra Vinh Provinces of Vietnam. Both sexes range from 20mm to 24mm long, lack a first dorsal fin, and their bodies taper from the wide, upward-turned head area towards a slender tail. Where the sexes differ in this species is that while the females have very rudimentary pelvic fins, which branch out from the throat area, the males have none, this area occupied instead by the priapium. This prompted Lynne R. Parenti, curator of the Division of Fishes at the Smithsonian NMNH, to suggest in 1986 that the major bones of the priapium were derived from parts of the pelvic and pectoral fin, which sits just past the gills.

The backwards-facing priapium has both reproductive and digestive functions, with the opening of the anus set in one end and a genital pore set in the other. It has two conspicuous externalised bones - the curved toxactinium, which juts out from under the mouth, runs down the throat and connects under the skin to the rest of the priapium structure, and the short, serrated ctenactinium, which looks like a tiny, toothy lower jaw bone. Scientists have suggested that the reason this bone is lined with seven hook-like serrae is for the purpose of grasping onto a female's head during copulation. Because her oviduct is convenienrly located at her throat.

Besides the clasping component of this organ, there is also what's known as the papillary component, which is responsible for transferring sperm bundles into the female. Once the female's oviduct is filled with sperm, she is able to lay eggs that were internally fertilised. This method of internal fertilisation is extremely rare in fish, as the eggs of over 90% of known bony fish species are fertilised outside the female's body. According to Parenti, who was interviewed Michael Marshall over at New Scientist, no one knows exactly why P. cuulong and priapiumfish evolved to have their genitals on their heads, but one thing’s for sure, she said, "There's not much going on at the back of these fish”.

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"Guys. Hey guys. I think I'm going to win the Tour de France this year."

"For God's sake, just tell us why you have your genitals on your head."

"I seriously can't have a conversation with anyone in this river."

Sometimes it's hard to convince other animals that you're going to do something extraordinary when you're just a little fish. But when you're just a little fish, you don't really have much to lose, so P. cuulong will enter the Tour de France.

"I just feel like I'd be really good at sitting for a very long time," he'll tell a giant catfish.

"Well, you're going to need a pretty impressive helmet to protect... whatever it is you've got going on up there," the giant catfish will reply.

"It's called a priap--"

"I'm never going to remember that."

The giant catfish will have a point though, so P. cuulong will put together some kind of Vader helmet, pack his tiny dry-mud suitcase, and take himself to France.

And he'll kill it.

"Who ... is ... that guy?" Some cyclists will wheeze as P. cuulong whips past them somewhere in the Pyrenees.

And "What... is with... that giant... helmet...??" others will cough, shifting uncomfortably on their bike seats.

"What the christ is going on? He hasn't even got any legs!" Someone from the crowd will point out incredulously.

But somehow, against all odds, a skinny little priapiumfish from Vietnam will win the Tour de France, and when everyone asks him how the hell he did it, how the hell he managed to out-sit a bunch of very experienced cyclists for 32,000 km and three weeks, P. cuulong will remove his giant helmet and be like, "Easy. My nuts are up here."

Read about P. cuulong at New Scientist.