Stunning shades to speedy signals – what science says about the mantis shrimp

Last week I discovered, via Discover Magazine in fact, that the marvellous mantis shrimp was not quite the creature I thought it to be. Mantis shrimp - or Stomatopods - as we science types like to call them - are spectacularly colourful marine shrimp found in the tropics and more temperate waters, but they are most famous, not for their colours, but for their ability to see colour. It's this particular feature of mantis shrimp that's not quite what I - and a good many others - thought it to be.

Within their mesmerising eyes, some species poses an astounding 12 different types of photoreceptors - cells specifically tailored for processing light and making sense of the images in front of them. For comparison, we mere mortals have 3 different sorts of photoreceptors: red, green, and blue. Our three receptors permit us distinguish between colours that are about 1 nanometre apart and make the vital decision of whether to choose snowdrop white or magnesium white when redecorating at home.

While the reason for the shrimp's many receptors has remained a bit of a mystery, it made sense that having a lot of receptors would mean being able to see a lot of colours. After all, dogs only have two types and can only see in shades of blue and yellow and butterflies have 5 sorts of receptors and can see light like UV, which remains invisible to the rest of us. However, a recent study, published in Science, has shown that this is not the case at all. Rather than seeing more colours than we can possibly distinguish, the mantis shrimp sees far fewer.

The tedious task of choosing what shade of white paint to go for is open to us only because we can distinguish wavelengths of light that are 1 nanometre apart. Mantis shrimp can only see colours that are 25 nanometres apart and would have a much easier time choosing paint with its partner if it ever engaged in such a thing.

The way the scientists figured this out was rather cool. By training the shrimp (Haptosquilla trispinosa) to snap its claws on seeing a particular colour, they could tell what shades it could and couldn't separate. The process follows the same principle as Pavlov and his dogs. First, they shone a particular shade of light at the shrimp and rewarded it for a feisty snap of its claws. They did this again and again until the shrimp knew what trick to perform for a treat. The shrimp soon learned that snapping and, say, blue light meant food and would snap whenever that colour was revealed. By mixing blue light with yellow light of various strengths - and keeping tabs on the claw snapping - the researchers could work out what colours it could distinguish between and led them to find 25 nanometres was the key separation between shades.

So why so many receptor cells? One reason could be that the shrimp skips out the brain step in the process. We receive a lot of information via our eyes that is sent to the brain to make sense of, and then we have the ability to say something is lime green, or Grinch green etc. Having an abundance of photoreceptors means that the mantis shrimp may not need it at all. Instead, the many receptors can say "it's a Grinch - attack!" in an instant, allowing it to react much faster in the event of danger. Such a power could well be the difference between life and death. This is a far cooler trait (I think) than being able to see the world as a thermonuclear bomb of light and beauty.

The face of science is constantly changing as studies like this break through new frontiers in our understanding and add another brick to the building of previous discoveries. Yes, we've known for decades that these colourful creatures have a lot of light processing cells, but not what they were for or just how different they are from other visual systems scientists have studied! There could be other critters out there capable of deciphering things without the need for a brilliant brain, the mantis shrimp may have other exciting secrets to tell and the receptors could be for more than a fast reaction!

References

Deriso, D. Invisible Colors, The Artful Brain, Scitable (22 May, 2011)

Draxler, B. Mantis Shrimp's Bizarre Eyesight Finally Figured Out, D-brief, Discover Magazine (23 January, 2014)

Thoen, H., H., How, M. J., Chiou, T-H. and Marshall, J. A Different Form of Colour Vision in Mantis Shrimp. Science 343 411-413 (2014)

Images

These images are all of the peacock mantis shrimp, (Odontodactylus scyllarus) rather than the one used in the study. You can find some great shots of Haptosquilla trispinosa here.

1) Portrait of a mantis shrimp. Credit: Wikimedia Commons user prilfish

2) Another lovely mantis shrimp. Credit: Alexander Vasenin via Wikimedia Commons

3) Did I get a bit carried away? They are pretty stunning shrimp - the last lovely mantis shrimp. Credit: Nazir Amin via Wikimedia Commons

There are more awesome photos of mantis shrimp in combat over at NeuroDojo.

2 Comments

January 27, 2014 | 07:15 PM

Posted By: Sara Mynott

Thanks Sedeer! They can indeed tell the difference between linear and circular polarized light. Within their eyes they have a central band made up of specialised optical units. These are arranged in rows and the first 4 are devoted to making sense of colours. The last two are to detect polarized light.

The study doesn't delve in to the shrimp's other abilities much more than that, but Michael Bok has written a great blog post explaining how they see circularly polarized light at http://arthropoda.southernfriedscience.com/?p=2964

While I'd love it if they could see an incredible range of colours, their vision is still pretty cool - they can see UV light, which is invisible to the human eye, and the closest we come to seeing polarized light is a barely detectable phenomenon called Haidinger’s Brush! Here's how to spot it: http://bit.ly/1iDPvNV.

January 27, 2014 | 02:49 PM

Posted By: Sedeer el-Showk

I had seen something about this online but hadn't really followed up on it. Thanks for the excellent write-up about it, Sara!

If I remember correctly, mantis shrimp are also supposed to be able to distinguish the polarization of light, right? Did the study say anything about that?

It's a nice piece of research, but I have to admit that my world is slightly poorer without the mantis shrimp's incredible vision in it. Though using the receptors for faster processing is a pretty cool trick, too, so maybe it's OK... ;)