Fish Eye

A fish eye works differently from ours. Underwater, the cornea does little optical work because its refractive index closely matches water. Focus is entirely up to the lens - which is spherical, dense, and highly refractive. Reef fish routinely see UV light (invisible to us). Some fish are tetrachromatic - four colour receptors versus our three. Others detect polarised light for navigation and communication. Deep-sea fish evolve tubular eyes that concentrate scarce photons; cave fish lose their eyes entirely.

Spherical lens (not lens-shaped like in humans), fixed pupil in most species, retina packed with rods and multiple cone types, tapetum lucidum for low-light vision in many. Focus is achieved not by lens shape change but by moving the lens forward or backward - like an underwater camera zoom.

Colour vision (often 3-4 cone types), UV sensitivity (many reef fish), polarised light detection (salmon, some tuna for navigation), high acuity in some species (mackerel, sailfish rival raptors), extreme low-light sensitivity in deep-sea and nocturnal species.

Deep-sea tubular eyes (spookfish, hatchetfish). Cave fish reduced eyes (Astyanax mexicanus). Flounder migrating eye (both eyes on one side). Four-eyed fish (Anableps) - single eye divided horizontally for simultaneous above/below-water vision.

Popeye (exophthalmia), cataracts, cornea damage from ammonia/high-pH, congenital eye malformation in inbred lines.

The four-eyed fish (Anableps anableps) has each eye divided horizontally - the upper half sees in air, the lower half sees in water, both at the same time.