Here’s a 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 we’re talking about shrimp. They haven’t told me much during our occasional encounters.
A quick primer for those out of the loop. We see colors because the retina of our eye has special light-sensitive cells, namely “rods” and “cones”. “Rods” provide vision in low light conditions, such as at night, and have very high-sensitivity to light but not to color. There are 130 million of them. Cones are responsible for color sensation; there are 7 million, and their light sensitivity is 100 times less than that of rods. We have three types of cones: long (L), medium (M), and short (S). They correspond to yellow-red (570 nm), green-yellow (544 nm), and violet-blue (443 nm) colors respectively. The combination of activations of these cells is perceived by the brain as color. For instance, a combination of blue and red creates indigo, which isn’t in the rainbow. Excitation of all three yields white. Beyond the 400-700 nm range, we can’t see because our three types of cones are tuned to the three peaks (see above), and color sensitivity rapidly decreases to the left and right of them. Below 400 nm is ultraviolet, above 700 nm is infrared radiation.
There are people who see the world in black and white — monochromats. Among animals, all pinnipeds, such as seals and walruses, sharks, whales, octopuses are like this. Dogs don’t have long cones, meaning their color vision is incomplete, but saying they can’t see red is incorrect. They see it as grey only when there are absolutely no shades of yellow and blue in it, which is not the case in the real world. However, this makes red on their color wheel equivalent to green.
Moreover, all primates are dichromats. Only humans have evolved vision similar to the vision of birds. But birds are even more advanced – they are tetrachromats. They have cells sensitive to ultraviolet light. The ancestors of modern mammals had lenses that let through ultraviolet light and had a photoreceptor sensitive to soft ultraviolet light. However, in some primates, particularly in humans, the lens evolved to block photons shorter than 400 nm, rendering this receptor obsolete.
So, it turns out that there’s a woman currently living in the United Kingdom with a genetic disorder, making her a tetrachromat. She sees the world with an additional color dimension — ultraviolet. Interestingly, Claude Monet, presumably after cataract surgery, perceived ultraviolet. I specifically looked for his paintings from this period in Chicago, but didn’t find any. He was old and had poor vision overall. He painted lilies abundantly during that period, it seems. By the way, for tetrachromats, the concept of “white” is different — it requires full ultraviolet presence. Without it, white looks different to them.
Apparently, this is precisely a dimension, not just an “expanded rainbow”. That is, imagine a color wheel and mentally draw an axis perpendicular to it for the saturation of ultraviolet from “none” to “a lot”. In other words, if indigo is a mix of red and blue, there is another color which we’ll call rurple, that represents indigo with ultraviolet. And this color can have different saturations.
And now, the most interesting part. There’s a kind of shrimp — mantis shrimp. It has the fastest strike in nature — an acceleration of 10,000 Gs. When hunting, its limbs develop speed up to 80-100 kilometers an hour, which is 50 times faster than the blink of a human eye, and comparable to the impact force of a .22 caliber bullet fired from a gun. The force of the strike is about one and a half thousand newtons, enough to break the hard shells of mollusks. The speed of the strike causes the formation of cavitation bubbles. When these bubbles implode, they release a large amount of heat, temporarily raising the temperature to very high levels and further weakening the armor of its prey. It was found that their “claws” consist of the mineral hydroxyapatite, and the impact part consists of nanoparticles that absorb and disperse the energy of high-magnitude impacts. Nanometer-sized spherical particles are arranged in a “Christmas-tree” pattern into a continuous sequence, similar to fish scales, which allows the impact force to be evenly distributed over the surface.
So, about those shrimp eyes, they have the most unusual eyes in the world. There, they have 12 different types of cones from ultraviolet to infrared, with four just for UV. Interestingly, their brain uses these differently: they cannot distinguish colors closer than 12-25 nm, while humans with three types of cones can distinguish a difference of 4 nm. Likely, these shrimp have “digital” color vision – red shades aren’t very important to them, and there’s simply one receptor for red. And it operates in a crude yes-no fashion. But that’s just because their brains haven’t fully developed yet. When shrimp take over the world, they will fully utilize their hardware.
They have three pseudopupils. These organs are stacked one over the other. They also have tens of thousands of clusters of photoreceptor neurons. We, for example, have just one cluster. The cells form ommatidia, making the eyes of mantis shrimp similar in structure to the compound eyes of flies.
By the way, you know that we actually see only a small circle in front of us due to the structure of the eye. The entire “world” is completed by the brain and the system of micro-movements of the eye (socalled saccades), moving this circle left, right, up, and assembling the picture into a whole. In other words, we don’t see simultaneously to the left and right. Actually, milliseconds pass between “snapshots” (from tens to hundreds).
Beyond that, shrimp eyes move independently, and there are mechanisms to determine distance with one eye. But the most mind-blowing thing is their ability not just to see the polarization of color, which is already unique, but to perceive, for example, circular polarization. First, what is polarization? Light is an electromagnetic wave. If you move a rope tied to a doorknob up and down, the plane in which that rope moves is polarization. Ordinary sunlight is unpolarized, meaning its waves vibrate in all possible directions perpendicular to the direction of propagation. However, when sunlight is reflected from surfaces like water, glass, or wet roads, it can become partially polarized. This is what polaroid sunglasses filter out, helping the driver. That’s why they also darken, because only part of the light, polarized in a certain direction, passes through. So, shrimp have the ability to see polarization as a “separate color”. This is for planar polarization. There’s also circular polarization — this is when the direction of oscillation rotates in a circle. Ultimately, it looks like a spiral or spring, where the wave moves forward, but its oscillations rotate around this direction. Shrimp can separate this type of polarization too. Moreover, they have coloring that, for shrimp that understand circular polarization, offers a richer visual picture.
It’s some kind of obscure shrimp that might not have caught our attention, but essentially, it’s like an alien. If you really look at the ocean’s inhabitants, you can find anything at all. I think real aliens will be less surprising.
Hi, I'm Rauf Aliev. 