How many eyes does a four-eyed fish have?
The four-eyed fishes are a genus, Anableps, of fishes in the family Anablepidae. They have eyes raised above the top of the head and divided in two different parts, so that they can see below and above the water surface at the same time.
Like their relatives, the onesided livebearers, four-eyed fishes mate only on one side, right-"handed" males with left-"handed" females and vice versa.
These fish inhabit fresh and brackish water and are only rarely coastal marine. They originate in lowlands in southern Mexico to Honduras and northern South America.
Four-eyed fish have only two eyes, but the eyes are specially adapted for their surface-dwelling lifestyle. The eyes are positioned on the top of the head, and the fish floats at the water surface with only the lower half of each eye underwater. The two halves are divided by a band of tissue and the eye has two pupils, connected by part of the iris. The upper half of the eye is adapted for vision in air, the lower half for vision in water. The lens of the eye also changes in thickness top to bottom to account for the difference in the refractive indices of air versus water. The refractive index or index of refraction of a material is a dimensionless number that describes how fast light propagates through the material.
More Info:
en.wikipedia.org
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