An eye built for speed
In the bright coral reefs of the Visayan Sea, the Peacock Mantis Shrimp (Odontodactylus scyllarus) views the world through arguably the most complex visual system in the animal kingdom. Humans perceive color using three types of photoreceptor cells, but the mantis shrimp has between 12 and 16. Some species can even tune the sensitivity of their long-wavelength color vision to adapt to their environment. Their eyes are mounted on stalks that move independently, and each eye has trinocular vision, meaning it can perceive depth with just one eye.
The eye is divided into two hemispheres and a central "midband" of six parallel rows of specialized, enlarged optical units called ommatidia. The top four rows of this midband are responsible for color vision. Within each ommatidium is a light-guiding structure called a rhabdom, which contains the photoreceptor cells. In these four rows, the rhabdoms are divided into three tiers, each sensitive to different wavelengths. This tiered structure, combined with color filters between the layers, allows each individual ommatidium to act as a multi-channel color sensor. Rather than the brain comparing signals from different photoreceptors to determine color, as humans do, the mantis shrimp's eye pre-processes this information. The data sent to its small brain is already refined, allowing for incredibly fast color recognition rather than fine discrimination. This system favors speed and efficiency over perceptual depth.
Seeing the invisible
Besides its color processing, the mantis shrimp's vision extends into spectra invisible to humans. It can see deep-ultraviolet light, using at least five different frequency bands, and also far-red light. The bottom two rows of the midband, rows 5 and 6, are specialized for seeing polarized light. Mantis shrimp are among the only known animals that can detect circularly polarized light, where the light wave travels in a rotating helix.
This is accomplished by a remarkable biological structure within the rhabdoms of rows 5 and 6. An upper cell, called R8, acts as a quarter-wave plate, a type of filter that converts incoming circularly polarized light into linearly polarized light. The seven cells below (R1-R7) are then able to detect this converted linear light. Different cells are sensitive to left-handed or right-handed circularly polarized light, allowing the shrimp to distinguish between them. Some male mantis shrimp have body parts that reflect circularly polarized light, which they use as a secret communication channel for mating that is invisible to predators and most other species.