Most people have three types of light-detecting cells, or photoreceptors, which are sensitive to red, green and blue light. But the mantis shrimp has anywhere from 12 to 16 different photoreceptors in its midband. Most people assume that they must therefore be really good at seeing a wide range of colours—a “thermonuclear bomb of light and beauty”, as the Oatmeal put it. But last year, Hanna Thoen from the University of Queensland found that they’re much worse at discriminating between colours than most other animals! They seem to use their dozen-plus receptors to recognise colours in a unique way that’s very different to other animals but oddly similar to some satellites.
Thoen focused on the receptors that detect colours from red to violet—the same rainbow we can see. But these ultra-violent animals can also see ultraviolet (UV). The rock mantis shrimp, for example, has six photoreceptors dedicated to this part of the spectrum, each one tuned to a different wavelength. That’s the most complex UV-detecting system found in nature. Michael Bok from the University of Maryland wanted to know how it works.
Like us, mantis shrimps see colour with the help of light-sensitive proteins called opsins. These form the basis of visual pigments that react to different wavelengths of light, allowing us to see different colours. If a mantis shrimp has six UV receptors, it should have at least six opsins that are sensitive to different flavours of UV.
Except it doesn’t. Bok could only find two.
To which: huh?
How could there possibly be six types of photoreceptors with only two opsins? There was one possibility. Something could be filtering the light hitting the different receptors.
That’s exactly what happens in the mantis shrimp’s eye. When light enters the units in its eye, it must first pass through a crystalline cone, which lies over the receptors. Bok found that these cones contain UV-blocking substances called MAAs (or mycosporine-like amino acids, in full). There are four, possibly five, of these, which block slightly different wavelengths of UV. Combine these filters with the two underlying opsins, and you get six different classes of UV receptor.
Many marine animals have one or two MAAs. They use these as sunscreens to block UV from reaching their skin and eyes, and causing damage that could eventually lead to cancer. The mantis shrimps also use MAAs to block UV but for a unique purpose: to turn their eyes into incredibly sophisticated UV detectors.
Where do the MAAs come from? It’s not clear. No animal can make these chemicals themselves, so they must get them from their environment, possibly from their diet or from microbes. But two of the MAAs that Bok discovered have never been seen before, so it’s possible that the mantis shrimps can somehow change any incoming MAAs into five different types.
“We presented these results last summer at a big vision conference and one of my colleagues said: Now, you’ve solved all the problems. What are you going to do next?” says Tom Cronin, who led the study. “We sort of feel that way. The big problem now is: What does this all have to do with vision?” Why do mantis shrimps have such ridiculously complicated eyes? That’s the big question, and no one really knows.
The team are now trying to study how mantis shrimps react to different UV signals. For example, they find some short wavelengths of UV so repulsive that they’ll avoid food that’s paired with those wavelengths. Maybe this has something to do with aggressive signals? Mantis shrimps have rich social lives and they might communicate with ultraviolet patterns reflecting off their bodies.
“That’s the leading hypothesis but it has its own problems,” says Cronin. “Signals don’t evolve unless you have the visual system to see them. So you generally don’t have a system in place to see signals unless it’s there to see something else.” So the team are also looking at the patterns of UV light in the places where mantis shrimps live. But even if that line of research pans out, many animals share the same waters, and none of them have such a complex eye. So why does the mantis shrimp?
When I spoke to Marshall last year, he said that the mantis shrimp’s style of vision might help it to process images very quickly without much contribution from its brain. That might be useful to a predator that uses some of the fastest strikes in the animal kingdom. But of course, that’s still a hypothesis.
And there’s another baffling layer of complexity: the receptors that detect red to violet colours are connected to different nerves than the ones that detect UV, and both streams lead to different parts of the brain. The mantis shrimp didn’t just evolve an absurdly over-engineered way of seeing, it did it twice.
source: nationalgeographic.com by Ed Yong