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Extraordinary Vision: Polychromacity, Polarization in Mantis Shrimp

You may have heard of mantis shrimp. They are members of the order Stomatopoda, a group of marine crustaceans with some extraordinary properties. They’re hard-hitting bastards, but the most interesting property about them is their magnificent vision. Their vision varies between different species, but some species are capable of seeing light that is invisible to us, distinguishing more shades of visible light than us, and detecting properties of light impossible for humans to comprehend.

In order to understand the mantis shrimp’s vision, we need to understand some fundamental properties of light. Humans, of course, can distinguish millions of colors, but these are all detected by three different kinds of photosensitive cells in the eye, cone cells which have a peak sensitivity at three different wavelengths. Sophisticated visual processing in the brain then combines the varying intensities of these three types of cells to distinguish the millions of shades we see. Our cone cells can only see a very small sliver of the entire spectrum of light; most of the spectrum is invisible to us, either because the wavelengths are too short (ultraviolet) or too long (infrared), and on either side of that are even more extreme kinds of light, like X-rays. In addition to that, we also have rod cells, which are more sensitive to light than cones, but which can’t discriminate between different colors. These are mainly responsible for low-light vision and for detecting movement; this is why all cats are gray in the night (we rely for night vision on cells that cannot discriminate color).

So, we have three different kinds of cones and one kind of rod cell, for a total of four different photosensitive cells. Some women may be tetrachromats, that is, they have four different kinds of cones and can distinguish more colors than everyone else. That doesn’t even begin to compare to mantis shrimp. Some species have as many as sixteen different kinds of photosensitive cells! Here is a diagram:

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Not only can this species see ultraviolet light that is invisible to us, it can also distinguish much better between shades of visible light than us! However, this is kind of misleading. It’s true that the eyes of mantis shrimp have much more sophisictated color vision than human eyes; at the same time, however, they have much smaller brains. The visual center in the human brain is much more sophisticated than the mantis shrimp’s. As a result, it’s hard to compare color vision directly. What we can say for sure is that the raw input from the eyes is much better for color vision than the raw input from human eyes.

But that’s not all. Mantis shrimp can also detect polarization of light, which is a property that is completely invisible to us, as difficult to imagine as color would be to someone with monochromatic vision. To understand polarization, we need to understand a few things about the wave nature of light. Waves can be transverse or longitudinal. Here is a diagram:

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Sound waves propagating through air are longitudinal. The air compresses and expands, carrying energy along. Light waves, on the other hand, are transverse. To simplify, they move “up and down.” Longitudinal waves, by definition, are moving “left and right” along the direction of the wave. The “up and down” movement of transverse waves, however, can go in different directions. This means that transverse waves, such as electromagnetic (light) waves, can be polarized.

Unpolarized waves basically move up and down in random directions. Polarized light, however, “waves” in a specific pattern, for lack of a better description. Light coming from the Sun is unpolarized, but becomes partially polarized when it passes through the atmosphere. Thus most light we see is partially polarized, but we are completely blind to this. Light of different polarizations looks the same to us, all else being equal. Some animals can distinguish different polarizations, however.

It’s been well known for some time that some animals can perceive linear polarization. Here is an illustration of linear polarization (look at the blue line):

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The waving is confined to a given plane along the direction the wave is traveling. However, this is not the only way for light to be polarized. There is also circular polarization. Until recently, it was believed that no animal could perceive circular polarization. Here is a circularly polarized wave that goes clockwise from our vantage point:

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And here is one that goes the opposite way:

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Stokes parameters are a mathematical construction that completely describes a state of polarization. If you know these parameters, you basically know all there is to know about the kind of polarization (or non-polarization) that characterizes whatever light you’re looking at. Scientists recently demonstrated that the Gonodactylus smithii species of mantis shrimp can perceive all the Stokes parameters, or, in other words, they have perfect polarization vision.

Why would this be useful? Well, polarization information can be used to navigate, or it can be used to identify prey that would be transparent to humans (the light doesn’t change intensity or color when it passes through, but it may change polarization). It could also be used to communicate in a way that other species couldn’t detect. Below you see a signalling structure (viewed through filters to make the direction of circular polarization visible to humans) on the mantis shrimp:

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Fun fact: in organic chemistry, one way of naming different enantiomers (molecules that are mirror images of each other) is by the direction they rotate polarized light.