The temptation to call this a dickheaded fish is overwhelming, but the females also have genitals in the same place. And it’s an evolutionary mystery.
I saw this on National Geographic Daily News (h/t: M. Cobb), which gave a picture of the new species, Phallostethus cuulong, and noted that it’s the 22nd member of its family (Phallostethidae ["penis chest"]) to be described. All of these species have genitals on the ventral (lower) side of their heads. They’re also called “priapiumfish,” another name based on their genitals (biologists are just as salacious as everyone else).
Here are the male (top) and female (bottom) from the Nat Geo article; you can see the male’s genitals on the head:
The species, captured in the Mekong in Vietnam is described in a new paper by Kochi Shibukawa et al. (reference and link below) in Zootaxa.
The fish have internal fertilization, so the male has an intromittent organ on the head which he inserts into the female’s urogenital opening. Here are better pictures of the male and female genitalia, also from the paper:
The males have a clasping organ, the ctenactinium, which is used to hold onto the female when mating; here’s a cleared and stained speciment of a male clearly showing the genital apparatus:
Sadly, the paper doesn’t describe the behavior (it’s a straight morphological and taxonomic description), but the Nat Geo article helps answer at least one of the two obvious questions.
1. How do they mate? This gives new meaning to the phrase “dancing cheek to cheek,” since they obviously have to put their genitals next to each other; one of the fish must therefore be upside down. Nat Geo describes the act, obviously hard to see since it requires keeping them alive in captivity and being there at the right time:
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.
2. How did it evolve? To me, this is the really interesting question. But the Nat Geo article doesn’t offer much help:
Partly due to these overlooked and possibly unappealing study sites, the fish “tend to be ignored by a lot of biologists,” she said.
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.
Well, presumably the fish’s relatives, whose genitals are in the normal place further back on the body, can also mate efficiently, so that’s not much of a hypothesis. And other fish also have similar types of genital modifications. Perhaps something about the fish’s environment—shallow rivers, perhaps in areas with low visibility—explains it, but who knows?
And I’m curious about how genitals on the head evolved from genitals on the rear: what were the intermediate stages of evolution, and how were they adaptive? During that evolutionary process, the male and female genitals would have to be placed in a way that made copulation easy, so perhaps the change in position evolved concurrently in both sexes.
I have no answers here, but maybe some readers with an ichthyological bent can offer solutions.
Shibukawa, K., D. D. Tran, and L X. Tran. 2012. Phallostethus cuulong, a new species of priapiumfish (Actinopterygii: Atheriniformed: Phallotstehidae) from the Vietnamese Mekong. Zootaxa 3363:45-51.