How's this for a head turner? A tiny new species of fish from Vietnam sports its genitalia on its noggin.

Phallostethus cuulong is only the 22nd known species of its family, Phallostethidae, all of which bear their copulatory organs just behind their mouths.

As with all Phallostethus—"penis chest" in Greek—species, the male uses its bony "priapium" to clasp a female while he inserts sperm into her urogenital opening, also located on the head, said Lynne Parenti, curator of fishes at the National Museum of Natural History in Washington, D.C.

Parenti remembers seeing another species of priapiumfish mate at a lab in Singapore. Attached at the head and together forming a v, the fish "looked like a little pair of scissors, darting around the tank together," she said.

For many fish, such as guppies, mating is almost instantaneous, but priapiumfish "actually couple, staying together for a remarkable period of time," she noted.

Fish-Head Genitalia Still a Puzzle

P. cuulong, like most members of its family, is less than an inch (2.5 centimeters) long and mostly transparent, according to the study, published July 3 in the journal Zootaxa.

The new species was identified via measurements of nine specimens found during a field survey in shallow river waters in Vietnam's Mekong Basin—just the sort of brackish coastal habitat priapiumfish typically call home.

Such habitats have undergone heavy development in Vietnam in recent decades, but the fish have proved highly resilient and seem to have adapted to modern life. Scientists have even collected priapiumfish "in a ditch on the side of the road," Parenti said.

Partly due to these overlooked and possibly unappealing study sites, the fish "tend to be ignored by a lot of biologists," she said.

Clue to Front-Loaded Genitalia

That may explain why the fish's front-loaded genitalia remains an evolutionary mystery, she added.

There are some clues, though: For one thing, the Phallostethidae family is part of a larger group that includes many species that fertilize their eggs internally. (The vast majority of fish species fertilize their eggs outside the body.)

Many of the males in the family have physical modifications that allow them to internally fertilize females, so it makes sense that priapiumfish would also have evolved an adaptation.

For another thing, head-to-head mating is apparently "a very efficient way to do it," Parenti added. While examining preserved female priapiumfish, she has found oviducts filled with sperm, meaning almost all the eggs had been fertilized.

"There's still a lot more to discover" about Phallostethus, she added—P.cuulong is only third species found in its genus.