What
animal sees above and
below water at the same time?
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Clear vision is a valuable adaptation, helping animals to find food, avoid predators and locate suitable mates. That's why eyes with lenses have evolved independently in 6 different animal phyla. Because we are land-based mammals, our eyes are well-adapted for seeing out of the water, but our underwater vision is only really clear when we use a face mask that traps air between our eyes and a glass face plate. At least two animals have evolved adaptations for seeing clearly both above and below water. And they do this at the same time! These are the "whirly-gig beetle" and Anableps anableps, or "Cuatro Ojos" the 4-eyed fish.
Anableps is a minnow about 10 cm in length that lives in quiet freshwater lagoons of Central and South America. It feeds on both aerial and aquatic prey; anything from insects flying above the water to smaller fish or crustaceans living below. During the day, it spends most of its time poised at the surface looking for possible prey, the water line bisecting its eye.
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| Anableps sees well both above and below the water because, over time, its ancestors evolved eyes with a two-fold function. Like the eyes of other fish, those of Anableps have a cornea, lens, and retina. But in Anableps, natural selection has favoured an eye with two regions of retina; the ventral region is used for vision above water and the dorsal region for vision below water (see diagram below). |
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| If you keep in mind that vision in water requires a lens that is more curved than vision in air (see our essay on this topic) then it makes sense that the lens of the Anableps eye is asymmetrical. It is thicker and more curved along the axis of light coming in from below water, and thinner and less curved along the axis where rays travel in from the air. With these adaptations the eye of Anableps can focus on objects both in the air and in the water at the same time. |
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