Four-eyed fish, Anableps anableps, eye detail. 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. Four-eyed fish spend most of their time at the surface of the water. Their diet mostly consists of terrestrial insects which are readily available at the surface. Aquarium, Portugal