The most complex eye in the world
On the Great Barrier Reef, a group of crustaceans called stomatopods, or mantis shrimps, have developed what is the most complex visual system in the animal kingdom. Their two eyes, mounted on mobile stalks, can move independently of each other, rotating in all three axes. Each compound eye is made of tens of thousands of individual units called ommatidia.
The eye's structure is unique, divided into two hemispheres and a central "midband". This physical separation gives each eye trinocular vision, meaning a single eye can perceive depth on its own, unlike the binocular vision of humans which requires two eyes. The upper and lower hemispheres are primarily used to see form and motion. The midband, made of six parallel rows of specialized ommatidia, handles the unusual aspects of their vision.
A spectrum beyond human perception
Humans have three types of photoreceptor cells (cones) for color vision. Mantis shrimps have between 12 and 16 types of photoreceptors. Twelve of these are dedicated to color analysis, allowing them to see a spectrum from deep-ultraviolet (300 nm) to far-red light (720 nm). Six of these photoreceptor types are specifically tuned for seeing different bands of ultraviolet light.
They are also can see polarized light—light waves that vibrate in a single plane. They can detect both linearly polarized light and circularly polarized light, an ability not found in any other animal. This is achieved with specialized photoreceptors in rows 5 and 6 of the midband. Some species, like Gonodactylus smithii, can measure all four Stokes parameters, giving them what is believed to be optimal polarization vision. This ability may be used for a secret communication channel, as patterns on their bodies reflect circularly polarized light, making them visible to each other but not to most predators.
Initial assumptions were that having 12 color channels would give mantis shrimp superior color discrimination. However, behavioral studies revealed they are actually worse than humans at telling apart similar shades of color. The current understanding is that they do not compare inputs from their different receptors in the brain as humans do. Instead, their system performs processing directly in the eye. It scans the midband across an object and instantly recognizes a color, a "barcode-like" system that prioritizes speed of identification over nuanced discrimination. This rapid color recognition is an efficient adaptation for the bright and complex visual environment of a coral reef.