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Four-Eyed fish Wikipedia page

Physical characteristics
The Four-eyed fish eye. 1.Underwater retina 2.Lens 3. Air pupil 4. Tissue band 5. Iris 6. Underwater pupil 7. Air retina 8. Optic nerve

The maximum length of four-eyed fishes is up to 32 cm TL in A. microlepis, making this species the largest in the order Cyprinidontiformes.

Four-eyed fish have only two eyes, but the eyes are specially adapted for their surface-dwelling lifestyle. In early development, the four-eyed fish’s frontal bone expands dorsally allowing the eyes to be positioned on top of their head and appear bulging. This allows the fish to simultaneously see above and below the water as it floats at the surface. The eyes are divided into dorsal and ventral halves, separated by a pigmented strip of tissue. The name is given to these species because each eye has two pupils and two corneas filtering light onto one lens, refracting onto separate hemiretinas and processed through one optic disc. The upper (ventral) half of the eye is adapted for vision in air, the lower (dorsal) 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 ventral hemiretina is characterized by thicker cell layers containing more sensory neurons and an increased visual acuity compared to the dorsal hemiretina. Four-eyed fish are livebearers. Along with their sister genus Jenynsia they mate on one side only, right-"handed" males with left-"handed" females and vice versa. The male has specialized anal rays which are greatly elongated and fused into a tube called a gonopodium associated with the sperm duct which he uses as an intromittent organ to deliver sperm to the female.