Color vision is a complex and interesting subject to learn more about because of how significant it is in our understanding and perception of the world and how diverse color vision is in our world. Human color vision is determined by three different types of photoreceptors, making our vision trichromatic. These three types are “green”, “blue”, and “red” receptors, which give us vision of the visible spectrum of light. Humans also have an incredibly precise and accurate ability to distinguish and identify between colors. Animals on the other hand see the world in a whole different way and the mantis shrimp can be able to see colour than is invisible to us humans.
The eyes of a mantis shrimp
So how are they able to do this? Mantis shrimp have unusual eyes. Mostly famously, they have 16 color receptors, compared to a human’s three. Oddly, they are not that good at distinguishing between colors, this may be because the cones in human eyes have intricate mechanisms which allow them to work together to detect differences in color based on the differences of the cone responses as well as a network of neural computations to process that sensory information. In contrast, cones in mantis shrimp eyes “work independently of each other, without complicated neural computations but they can detect another property of light invisible to humans: polarization.
While polarization can be difficult to understand, you can imagine light waves as a bunch of different strings that each have one end attached to a wall. If you shake them randomly, they will vibrate in every direction; that is how non-polarized light behaves. If you only the shake the strings up and down, restricting vibrations to one direction, then that is like vertically polarized light. Light can be polarized in different directions. Polarized sunglasses take advantage of this phenomenon: They reduce glare by filtering out horizontally polarized light that bounces off a road or water surface.
In the ocean, light can be also be polarized when it bounces off molecules in water. Mantis shrimp can see up to six types of polarization: horizontal, vertical, two diagonals, and two types of circular polarization, in which a light wave spirals clockwise or counterclockwise. (They are the only animals known to see circularly polarized light.)
Mantis shrimp use only three colour receptors to detect colour. The question then begs, what are the rest of the photoreceptors used for? Like most animals, mantis shrimp require a form of gaze stabilization. Gaze stabilization is the phenomenon when an organism’s eyes correct for errors made in visual processing by accounting for existential factors, such as blur, so that the organism can have a clear view of the world (Daly, How, Partridge, Temple, Marshall, Cronin, & Roberts, 2016). In fact, different animals have various strategies to stabilize their gaze including “using eye, head, and body movements to avoid motion blur” (Daly et al., 2016). The most prominent forms of gaze stabilization in mantis shrimp incorporate three types of eye rotation known as “pitch, yaw, and torsional(roll)” (Daly et al., 2016). These three rotations form an intricate gaze stabilization system that has a large range of scanning eye movements, allowing the mantis shrimp to be very discerning and analytical of any movements in their vision. Using this scanning technique coupled with the photoreceptor modalities, mantis shrimp vision allows for rapid color recognition without the need-to-know which colour it really is. This unique technique gives mantis shrimp an evolutionary advantage as a predator, to quickly attack prey without them being aware, and as a prey, to notice signs of danger and make a rapid escape.
How else do they use polarized light
Scientists have also uncovered that mantis shrimp can use polarization as a compass. For instance, a shrimp venturing out from its burrow could track the distance it has traveled and turn around until the light-polarization pattern has changed by 180 degrees to find its way back home. This could be especially helpful in the featureless, muddy plains where some species live.
There are, in fact, animals on land that navigate using polarization. (Light bouncing off molecules in the atmosphere can become polarized, too.) Honeybees do this. As do dung beetles rolling their balls of dung to polarized light from the moon and even Milky Way.
Killer punch
It would be an injustice to the mantis shrimp to go without mentioning its killer punch.
A smasher mantis shrimp’s punch has the same acceleration as a 22-caliber bullet, delivering a blow of 15,000 newtons, a force equal to more than 2,500 times the shrimp’s weight. That power is pretty impressive, especially for such a little guy (most are in the 6-inch range, but they can measure between 4 and 15 inches). Their punch can easily break quarter-inch glass (they’ve been known to smash aquarium tanks).How does such a tiny package deliver such a big punch? In layman’s terms, the shrimp’s arms are hinged and folded away under its head, ready to unfurl via a springlike latch. When unsuspecting prey happens by, the shrimp releases its latch, launching its lower arm forward at a rapid speed.
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