Oddly, the shrimp themselves are not great at color discrimination. Hanne Theon and colleagues trained mantis shrimp to attack squares of certain colors to get

Oddly, the shrimp themselves are not great at color discrimination. Hanne Theon and colleagues trained mantis shrimp to attack squares of certain colors to get food rewards. Attacks on other “distractor” squares weren’t rewarded. By varying the color of those distractors, one can measure the spectral sensitivity of their visual system. Using similar methods, humans can distinguish between wavelengths a few nm apart; mantis shrimp are more like 20 nm. This is disappointing, but the reason is pretty wild. Most animals have “color opponency”: circuits early in the visual system convert the photoreceptors’ output into a differential representation: the intensity of red light here minus the intensity of green light in the surrounding neighborhood. This mechanism “normalizes” for the total amount of light, thereby letting you separately perceive hue and brightness, and has some other nice properties. Mantis Shrimp took a different path. There’s no obvious inhibitory interaction between the photoreceptors. They may therefore separately perceive redness, greenness, blueness, etc, but without combining these streams of information, their color acuity is poor. Interestingly, they do a “scanning” behavior that sort of recreates the opponency by dragging different types of photoreceptors across the same part of the visual world. Thoen, H. H., How, M. J., Chiou, T. H., & Marshall, J. (2014). A different form of color vision in mantis shrimp. Science, 343(6169), 411–413. http://dx.doi.org/ http://dx.doi.org/ 10.1126/science.1245824 https://www.researchgate.net/profile/Tsyr-Huei-Chiou/publication/259883148_A_Different_Form_of_Color_Vision_in_Mantis_Shrimp/links/53f168e40cf23733e815b97d/A-Different-Form-of-Color-Vision-in-Mantis-Shrimp.pdf https://www.researchgate.net/profile/Tsyr-Huei-Chiou/publica...