Bifocal Fish Sees Differently above and below Water Line
“Hey, four-eyes!” That playground taunt is more accurate when applied to Anableps anableps—a fish related to the guppy. It lives in the brackish waters of mangrove swamps in central and South America, and hunts for food at the water's surface... its bulging eyes submerged halfway. Which poses an evolutionary problem—should those eyes be attuned to the greenish light streaming through the mangroves? Or the yellowish rays drifting up through murky water? Well, these fish eyes see both.
Anableps doesn't actually have four eyes—just the usual two. But each eye has two pupils, one above water, one below. And each pupil sends incoming visual info to a different side of the fish's retina.
Cones in each half of the retina are adapted to produce different light-filtering pigments. So cones hit by underwater rays are primed to sense longer-wavelength yellow light. Cones hit by daylight are sensitive to shorter-wavelength green light. The finding appears in the journal Biology Letters. [Gregory Owens et al., "In the Four-Eyed Fish (Anableps anableps), the Regions of the Retina Exposed to Aquatic and Aerial Light Do Not Express the Same Set of Opsin Genes"]
The entire arrangement makes it easy for this bifocal fish to spot a tasty bug flying above the water, or a bit of algae below. For when it comes to evolution, the ayes have it.
[The above text is a transcript of this podcast.]