Biologists recently discovered a new species of freshwater fish called the llanos mosquitofish, or Gambusia quadruncus, in which the male members of the species have genitalia with four pronounced hooks on them, while the females feature a colorful anal spot. Now, it's quite possible that these fish have some kind of S&M thing going on, but the scientists who are studying them believe they know what's really going on — and it ain't pretty.
Female fish, like the females of most species, have to be careful about who they mate with. Unlike males, who simply work to spread their genome far and wide, females have to bear the real costs of reproduction. Consequently, they need to be really picky about who they mate with.
As a result, some female fish have evolved a defence against those guy fish who won't take no for an answer: a blocking device (essentially a big ball of tissue) that obstructs most of the genital pore, thus restricting entry of the male's penis (or more accurately, the gonopodial tip). Armed with this shield, female fish can then behaviorally allow the males who they fancy to mate with them.
But evolution doesn't care for these tricks, and where there's a will, there's often a way. To counter this defense, male llanos mosquitofish have evolved 4-pronged genitalia as a way to overcome female resistance. The hooks on the gonopodial tip allow the males to latch on to the females' gonopore and transfer their sperm.
That said, the biologists have posited an alternative theory which suggests that the hooks may stimulate the female in a way that causes them to respond better to effective sperm transfer (which is kind of kinky when you think about it).
As for the colorful anal spot on the females, the biologists suspect that it's a signaling mechanism to help the males find, uh, their way to the right spot. It's also possible that the spot is a way of helping the fish distinguish themselves from other species. Inter-species breeding is a problem for fish, often resulting in offspring with reduced fitness (hybrids are bad).
And you thought fish were boring.
You can read the entire study at Journal of Fish Biology.
Image: Brian Langerhans, NC State University.