mantis shrimp eyes
You may have heard before that mantis shrimp have amazing color vision. Their eyes are very uniquely constructed, and in that sense they are amazing. But it turns out they discern color variations worse than we do.
Chapter 3 of An Immense Worldends with an examination of mantis shrimp vision, and I want to provide a brief summary.
Mantis shrimp have twelve photoreceptor classes. Humans have three. We derive a spectrum of colors through comparisons between our three classes; this is called the opponent process or opponency. Mantis shrimp do not do this. They collapse the spectrum into just twelve colors.
Mantis shrimp were trained to attack one of two colored lights in exchange for a reward…Humans can distinguish colors whose wavelengths differ by 1 to 4 nanometers. But the mantis shrimp failed with colors that were [less than] 25 nanometers apart, which is roughly the gap between pure yellow and orange.
The funny thing is they are not really motivated by food since they have such low caloric requirements. The researchers could only test them once a day before the shrimp would lose interest.
Mantis shrimp also have to trade off whether, at a particular time, they perceive color or movement. We perceive depth by comparing images from our two eyes. Mantis shrimp do so with three zones of a single eye. This is useful to a species that routinely gets into fights with rivals in which they may lose one of their eyes. It also saves energy.
The upper and lower portions of its eye do black-and-white panoramic motion detection. The midband detects color. They move their eyes, independently, to point the midband at an object, and scan it with their color receptors. While the eyes are moving, black-and-white motion detection goes offline.
This wildly unique setup works with their very small brain which likely doesn’t construct a two-dimensional image. It’s more like a direct mapping of network switches to specific interpretations of their environment. If receptors 1, 5, 8, and 12 light up, that’s a predator. If receptors 2, 5, and 6 fire, that’s another shrimp, etc..
Mantis shrimp are the only known species to see circularly polarized light.
Humans are largely oblivious to polarization, but most insects, crustaceans, and cephalopods can see it…Polarized light usually oscillates in a fixed plane, but that plane can sometimes rotate, so light travels along a twisting helix.
Circularly polarized light is rare…The only thing in a mantis shrimp’s environment that reliably gives off circularly polarized light is [light reflected off mantis shrimp].
They have receptors tuned to it and they reflect it off specific parts of their body to signal other mantis shrimp for courtship and combat.
Hats off to these incredibly peculiar lookers.