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Theory predicts that 2 to 4 color receptors are ideal for the best color reception. Theory never met mantis shrimp.
Mantis Shrimp Vision
Mantis shrimp are awfully fierce predators considering the seeming serenity of the clear, aquamarine tropical waters in which they live. They have what are widely considered the most complex eyes. Each eye has 3 separate sections, mounted on a flexible stalk that can be swiveled any which way.
Humans have 4 types of light-sensitive pigment cells, including 3 for color. Mantis shrimp have 16 kinds of photocells, 12 of which sense color.
A mantis shrimp eye can perceive a broad color palette in a frequency range from ultraviolet to infrared, including polarized light. The characteristics of ambient light vary in seawater depending upon depth.
Mantis shrimp vision is tunable. Different individuals can express different light filters appropriate to the depth at which the shrimp lives. Received images are clear, with excellent depth perception befitting their predatory lifestyle.
Not surprisingly, mantis shrimp are dappled with fluorescent markings attuned to maximize conveyance to other shrimp, affording clear signaling of the postural gesturing that the shrimp use in mock combat between males, as well as more subtle courtship rituals.