Reading Ed Yong’s "An Immense World": The Mind-Blowing Eyes of Mantis Shrimp

Check out this new long read about something interesting. I’ve already made several posts while reading Ed Yong’s book An Immense World: How Animal Senses Reveal the Hidden Realms Around Us. Today, it’s about shrimps. They haven’t told me much during our occasional encounters.

Here’s a quick primer for those who might not know. We see colors because there are special light-sensitive cells on our retina, known as ‘rods’ and ‘cones.’ Rods provide vision in low light, such as at night, and have very high light sensitivity, but not color sensitivity. There are 130 million of them. Cones are responsible for color perception, numbering 7 million, with a light sensitivity 100 times less than rods. We have three types of cones: long (L), medium (M), and short (S). They respond to yellow-red (570nm), green-yellow (544nm), and violet-blue (443nm) colors, respectively. The combination of excitation of these cells is perceived by the brain as color. For example, a combination of blue and red gives indigo, which isn’t in the rainbow. Excitation of all three results in white. Beyond 400–700 nm, we can’t see because our three types of cones are tuned to three peaks (see above), and color sensitivity quickly drops off to the sides. Below 400 is ultraviolet, above 700 is infrared radiation.

There are people who see the world in black and white — monochromats. Among animals, all pinnipeds (like seals and walruses), sharks, whales, and octopuses are monochromats. Dogs don’t have long cones, meaning their color vision is incomplete, but it’s incorrect to say they don’t see red. They see it as gray only when there’s no hint of yellow or blue in red, which is rare in the real world. However, this makes red equivalent to green in their color spectrum.

Even primates are (almost) all dichromats. Among them, humans are barely only primates having vision close to that of birds through evolution. But birds are even better in that— they’re tetrachromats. They have cells sensitive to ultraviolet light in addition to RGB. The ancestors of modern mammals had a lens that transmitted ultraviolet light and had a photoreceptor sensitive to soft ultraviolet light. However, during evolution, in some primates, particularly humans, the lens stopped transmitting photons with wavelengths shorter than 400 nm, and this receptor became redundant.

So, it turns out that there’s a woman in the UK with a genetic disorder, and she’s a tetrachromat. She sees the world with an additional color dimension — ultraviolet. Interestingly, Claude Monet, after cataract surgery, presumably sensed ultraviolet. Recently, I specifically looked for his paintings from this period in Chicago but didn’t find any. He was already old and had poor vision overall. As I remember, he painted lilies abundantly during that period. By the way, for tetrachromats, the concept of ‘white’ is also different (like for dichromats) — it requires full ultraviolet. Without it, white appears different to them.

Interestingly, for tetrachromats, this is a new dimension; that’s not just an ‘extended rainbow.’ Imagine a color circle, and mentally draw a perpendicular axis for ultraviolet saturation from ‘none’ to ‘a lot.’ In other words, if indigo is a mix of red and blue, there’s another color, let’s call it ‘rurple,’ which is indigo with some amount of ultraviolet. And this color can have different continous levels of saturation.

Now, the most interesting part. There’s a shrimp called the mantis shrimp. It has the fastest strike in nature — an acceleration of 10,000G. When hunting, its limbs develop a speed of 80–100 kilometers per hour (50 MPH) almost instantly, which is 50 times faster than a human’s blink and comparable to the striking force of a bullet fired from a 22-caliber gun. The strike force is about 1,500 newtons, allowing it to break the hard shells of mollusks. The speed of the strike creates cavitation bubbles. When these bubbles burst, they release a large amount of heat, temporarily raising the temperature to a very high level, further weakening the armor of their prey. It was established that their ‘claws’ consist of the mineral hydroxyapatite, and the striking part is made of nanoparticles that absorb and scatter the energy of the large impact. Nanometer-sized spherical particles are arranged ‘like a Christmas tree’ in a continuous sequence, like fish scales, allowing the force of the strike to be evenly distributed over the surface.

So why I mentioned them. This shrimp also has the most unusual eyes in the world. There are 12 different types of cones in them (compare with our three), from ultraviolet to infrared. Just for ultraviolet alone, there are four. Interestingly, their small brain uses this differently: they cannot distinguish colors closer than 12–25 nm, although humans with their three types of cones can distinguish a difference of 4 nm. Most likely, these shrimps have ‘digital’ color vision — shades of red are not particularly important to them, but there is just one receptor for red which is sufficient. And it works in a rough yes-no manner. But this is simply because their brain has not yet evolved. When shrimps take over the world, they will fully utilize their hardware :-)

They have three pseudopupils. These organs are located on top of each other. They also have tens of thousands of clusters of photoreceptor neurons. For example, we have just one such cluster. The cells form ommatidia, making the eyes of mantis shrimps similar in structure to the compound eyes of flies.

By the way, did you know that we actually only see a small circle in front of us because of this eye structure? The ‘world’ is completed by the brain and the system of micro-movements of the eyes (saccades), moving this circle left, right, up, and compiling the picture into a whole. In other words, we don’t see simultaneously to the left and right from the central zone in front of us — even if we feel that we see. In fact, our eyes and our brain collect the information about the world and combine it to create a continous picture. And that picture is non-uniform at any given moment, some of its portions are newer than others. However, just milliseconds (tens to hundreds) pass between ‘snapshots.’

In addition, the eyes of the shrimp move individually, and there are mechanisms for determining distance with a single eye. But the most mind-blowing thing is their ability not just to see the polarization of color which is mind-blowing by itself, but to see circular polarization. So, in short, what is polarization. You know, light is an electromagnetic wave (if it is seen in this manner). When moving a rope tied to a doorknob up and down, the plane in which this rope moves is the polarization. So the light has such planes too. Ordinary sunlight is unpolarized, meaning its waves oscillate in all possible directions perpendicular to the direction of propagation. However, when sunlight reflects off surfaces such as water, glass, or wet roads, it can become partially polarized. Polaroid glasses subtract such reflected light, helping the driver.

So, shrimps have the ability to see polarization as a ‘separate color.’ There’s also circular polarization — when the direction of oscillation rotates in a circle. The result is similar to a spiral or spring, where the wave moves forward, but its oscillations rotate around this direction. And shrimps can also separate such polarization. Moreover, they have coloring that, for shrimps understanding circular polarization, provides a richer visual picture.

Overall, this shrimp seems like a dorky creature that might not have caught attention, but it’s essentially some kind of alien. If you look closely at the ocean’s inhabitants, you can find anything at all. I think real aliens might turn out to be less surprising :-)