This story appears in the April 2017 issue of National Geographic magazine.
The fish’s eyes are ideally suited to helping it thrive and propagate. The fish’s loins, on the other hand, are a somewhat trickier fit (literally) for that purpose.
Anableps anableps is commonly called a four-eyed fish, but it actually has only two. They look like four because a horizontal band of tissue splits the eye into two lobes, each with its own pupil and separate vision. Swimming near the surface as it typically does, the fish can see in and out of water simultaneously.
“It’s pretty awesome how they can differentiate a predator under and above the water at the same time and process all those images to know which way to go,” says Erik Kalen, assistant curator of aquatics at the Oklahoma City Zoo. This almost certainly gives the species an evolutionary advantage, he says.
But that trait isn’t A. anableps’s only anatomical quirk. The female’s genital opening and the male’s sex organ—a pipelike modified fin called a gonopodium—are turned to the right on some fish and to the left on others. This means that a righty male is built to copulate only with a lefty female, and vice versa—which statistically halves the chances of finding a compatible mate. “I can’t tell you any advantage to that,” Kalen says.
At his zoo’s four-eyed-fish exhibit, he says, “it’s mass pandemonium when we get into breeding season.” Keepers monitor “which males are left-erect and which are right-erect and who’s breeding with who.” Kalen has sometimes seen a male “struggle extremely hard to go the other way” and mate with a same-sided female, “but I’m not sure it’s successful.” That could become clear in about 12 weeks, the species’ typical gestation period.