Although they are commonly called the 4-eyed fish, Anableps species actually have just 2 eyes each divided into 2 parts, one adapted for seeing above the water and the other for seeing below.1 These fishes swim at the surface so that the water level separates each eye horizontally (Figure 1). The 2 halves of the eye are divided by a band of pigment at the waterline. Like a pair of conjoined twins, each eye has 2 pupils that share a single lens, retina, and optic nerve (Figure 2). The top half of the lens is flattened as in a human eye, whereas the bottom half is rounded, which is typical for a fish eye. Also, the upper cornea is thicker, is flatter, and contains a much higher concentration of glycogen than the lower cornea.2 The resulting difference in refractive power provides an undistorted image of objects both above and below the water level. There is also an iris flap that shields the upper pupil from the glare of light reflected from the water surface. As if wearing bifocals, these amazing fishes can see their prey under water and predators in the air at the same time.
Rozenbaum I, Ritch R. Four-Eyed Fish. Arch Ophthalmol. 2008;126(5):733. doi:10.1001/archopht.126.5.733