National Geographic : 2017 Apr
anatomical quirk. The female’s genital opening and the male’s sex organ— a pipelike modified fin called a gono podium—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 foureyedfish exhibit, he says, “it’s mass pandemonium when we get into breeding season.” Keepers moni tor “which males are lefterect and which are righterect and who’s breeding with who.” Kalen has sometimes seen a male “struggle extremely hard to go the other way” and mate with a samesided female, “ but I’m not sure it’s successful.” That could become clear in about 12 weeks, the species’ typical gestation period. The fish’s eyes are ideally suited to help ing 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 foureyed 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 WHEN SEX IS SO RIGHT (OR LEFT) By Patricia Edmonds | EXPLORE | BASIC INSTINCTS The male (below left) and female four-eyed fish were photographed at the Oklahoma City Zoo. FOUR-EYED FISH HABITAT/RANGE Fresh or brackish waters on the Atlantic coast of Central, South America CONSERVATION STATUS Not yet assessed for the IUCN Red List OTHER FACTS Only three species in A. anableps’s family have two-lobed eyes.