There is a widespread meme about "shrimp colors" which hinges on the idea that mantis shimp can see way more colors than us. It turns out this is not true! Mantis shrimp can see ehhh wigglehand, roughly as many colors as humans can.
However, figuring this out required some very interesting science into the mechanics of seeing color, and it paths into the resolution of a second animal-vision myth: cuttlefish are not colorblind, and the explanation for that is even cooler.
Super tl;dr: human vision uses three types of color receptors and make your brain do math to average them out. Mantis shrimp have twelve types of color receptors but don't brain-math so they see slightly fewer colors faster. Cuttlefish don't have color receptors at all and instead use the physics of light refraction (prisms!) to figure out its wavelengths, ie color.
Human (and a lot of other vertebrate) eyes use the "rods & cones" system to see: rods handle non-color vision and mostly kick in when it's too dark, cones come in three types that are each specialized to different wavelengths, ie colors. Your big fancy mammal brain checks the info cones give it and does the physics equivalent of math to arrive at what color something is: if the L-cones are going nuts while the M- and S-cones don't activate much, "it's red". If the M- and S-cones are both moderately activated and L-cones are asleep, average it out for "it's teal". And so on. This works pretty well as most humans with no color-blindness will attest! However it's regarded as pretty brain-intense, because your brain is running these calculations all the time and fast enough that you don't notice any interruptions in "knowing what color something I'm looking at is" which... yeah, that's not a free action!
Mantis shrimp do something different (currently thought unique to stomatopods!) so it took researchers a bit to figure out what was going on. Mantis shrimp have twelve different color receptors in their eyes, so initially everyone thought that if it worked like humans do where the brain averages all of them out to arrive at the color, they must be able to see tons! However someone did some clever testing where they had mantis shimp pick "the correct color" of two colors for food and then made the colors progressively more similar. Finding: the shrimp could tell the difference up to the point where the colors were 50-100 nm apart in wavelength, but around 12-25 nm difference they couldn't anymore. In comparison, humans can distinguish colors 1-5 nm apart! So if anything it seems mantis shrimp probably can't see quite as many colors as humans can (although obviously they're still very good at it).
But science did not stop there. See, the thing about those 12 different color receptors is they're arranged unusually. Those of us with cones and rods, the receptors are a big jumbled ballpit in the back of the eyeball. Mantis shrimp, like a lot of "buggy" things, instead have compound eyes, where each individual unit is small but adds up to a total eye. Someone noticed the mantis shrimp color receptor units are specially arranged in a strip across the eye.
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I can't make it any sillier than this article: mantis shimp appear to be barcode-scanning their surroundings! (Kinda.) They can just roll their gaze across something and the receptors do all the work for them to arrive at a color answer, no need to wait for brain calculations! WILD.
(Why would they do that? How did it evolve? Science says "give us another 20 years of grant money and maybe we'll have an answer for you!")
So alright, mantis shrimp eyes seem to work rather differently than a fox or a toad or a human, but everyone more or less knew they could see SOMETHING color-wise because they had color receptors in their eyes. Very interesting but fundamentally not too weird.
This brings us to cuttlefish (and lots of other cephalopods).
Cuttlefish (and some octopi) are pretty damn famous for their lickety-split color-changing skills. A lot of them can even tweak their shape and texture on the fly to more convincingly go invisible! They are, in short, phenomenal mimics, and it has stumped the hell out of science for quite some time that if you dissect an eyeball you will not find any color receptors at all. How on Earth do they know the exact seaweed color to turn into if they can't see the seaweed's color?!
There's a phenomenon known as "chromatic abberation" which is a source of petty annoyance for people working with microscopes: when light passes through a prism and refracts into different wavelengths (ie colors) each wavelength is actually angled a little differently. This is how a rainbow: the light is scattered by the rain such that each color forms a band slightly to the side of its neighbors, because each wavelength is angled just a little off from the rest. This means that technically they're out of focus from one another, which is not much of a problem for looking at a rainbow on the horizon, but much more annoying for someone peering through a microscope. The solution for annoyed human scientists is a second lens in the microscope that tries to bounce the disparate angles again so they all reconvene at the same spot (in your eyeball).
It appears cephalopods went the opposite way: their unusual eye shape and corneas would seem to increase chromatic abberation. Which is terrible if you need all the light to be in focus together to get a good view... but very useful if, say, you're a creature with a massive fancy brain who wants to measure the different focal points of arriving light to figure out how much of what wavelengths you're seeing. So someone tried modeling that! Some computer sims confirmed yep, that would theoretically work. It hasn't been 100% proved in actual animals yet but it's by far the leading theory, by which I mean basically the only one I know of right now, because everyone's been going "no color receptors... but color camouflage... wuh" for 20+ years.
There's some thought that this would be EVEN MORE brainpower-heavy than human-style vision. But do you know what sea creatures are absolutely legendary for how smart and big-brained they are? Cephalopods! So this is pretty plausible as hypotheses go (imo) and a much better guess than assuming the little gals matching their tank environs in a quarter second are faking it somehow.
🌈And now you know!✨ Next time someone jokes about shrimp colors feel free to tell them the reality is so much more wild.
Further Reading, Links, IDK What Else:
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Australian Academy of Science: "All Eyes on the Reef" (science.org.au)
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EDIT: Special thanks to @widr for their comment inadvertently explaining how chromatic abberation vision should work! I had very much misunderstood that it's measuring focal points instead of arrival delay, whoops! Post has been edited to be Even More Full Of Facts. Thank you!
