For us humans, sunscreen is something we use on holiday to avoid looking like angry lobsters with sensitive skin. But for some members of the animal kingdom who can’t just pop into the local pharmacy, making their own sunscreen is essential to protect them from the golden orb’s scorching rays.

But a new study by scientists from the US has found that the light-absorbing chemicals normally used by some creatures to protect them from the sun is used by mantis shrimps to enhance their vision.

Mantis shrimps are fearsome predators of the shallows, and are sometimes referred to as ‘thumb-splitters‘ because of the fast-moving, powerful claws which they use to kill or stun prey. As predators, they need excellent vision to locate and catch their prey, and as a result have evolved ‘one of the most elaborate visual systems ever discovered.’

This study has cast new light (no pun intended) on how these incredible creatures manipulate the light coming into their eyes to gain as much useful information about their surroundings as possible.

So, what’s the point?

Making their own sun protection is something many organisms have to do, for instance, hippos manufacture a red mucous-like substance that they ‘sweat’ over their bodies to shield them from the intense African sun.

A less well-known type of UV-protection are mycosporine-like amino acids (MAAs), which are simple molecules made by certain types of bacteria. MAAs absorb light so that it doesn’t damage important chemicals and structures inside the bacterial cells.

The authors claim that eukaryotes (higher organisms such as plants and animals) can’t make their own MAAs (except in one unusual case), so the most likely way for the shrimp to get them is through their diet.

The fact that MAAs can absorb certain wavelengths of light means that creatures can use them as a filter. Although they are normally used for protection, this appears to be an interesting case where their light-filtering properties have been used for a very different purpose.

Shrimp (like other arthropods) have compound eyes known as ommatidia. These organs work by allowing light to stimulate chemical dyes housed in specialist cells. These dyes then change shape, kickstarting a chain of biochemical reactions that ultimately sends a signal to the brain.

Light of different energies (wavelengths) are bounced off of surrounding objects and into the ommatidia, where they are interpreted by the shrimp’s brain as different colours to allow them to differentiate between the objects. Using the filters in tandem with the receptor cells allow the shrimp to distinguish between a wider range of colours in the UV range.

Figuring out how the shrimp see can help us to understand more about vision in general and how creatures utilise naturally-occurring chemicals to gain an advantage over competitors.

What did they do?

From previous work, the scientists suspected that the mantis shrimp has an ability to see certain wavelengths of UV light, and so set out to figure out precisely which chemicals within its photoreceptor cells were responsible for this.

The cells were known to respond electrophysiologically to 5 different UV wavelengths, but analysis of the light-sensitive proteins (opsins) present in the photoreceptor cells found only 3 of these proteins that could respond to UV light (usually arthropods have more opsins than light wavelengths that they can see).

This led them to believe that some kind of filtering system was at work, since sets of multiple colored filters within a single eye have been described in various vertebrates, butterflies and stomatopods’, which could explain how the shrimps’ eyes can respond to more UV wavelengths than their opsins should allow.

So they shined a special UV light through the eye of a mantis shrimp, and saw a range of different fluorescent colours emitted by some of the individual ommatidia. From looking a a cross-section of the eye they could see that the fluorescence was coming specifically from some the cone section at the front of the ommatidia in rows 3-6 (see picture below).

This fluorescence is an indication that the UV light has been absorbed (and then re-emitted as fluorescence) by chemicals in the cones of the ommatidia. So the researchers analysed the cones to see if they could figure out what these chemicals were and modelled how they could potentially work in tandem to explain how the shrimp could see the 5 different UV wavelengths.

Did they prove anything?

The researchers found 4 different pigments the the cones of the ommatidia, which were each capable of absorbing a specific frequency of UV light. These pigments were found to be MAAs.

Because each row of ommatidia had different amounts of each of these MAAs, different wavelengths of UV light will be allowed through by each one to then interact with the dyes in the photoreceptor cells.

In reality, both the filters and the dyes absorb a distribution of wavelengths. But if each filter blocks out some of the wavelengths that would ordinarily be absorbed by the dye, then the range of the dye is effectively narrowed. So an ommatidium with a dye that can normally respond to UV light across a range of 300-375nm, can be made to pick up more specific wavelength, such as 310 or 350nm (see diagram below).

Imagine if there were two objects, 1 and 2, which reflected light at 310 and 350 nm, respectively. If the shrimp didn’t have the filters, its brain would receive the same signals from row 4 as it would from rows 5 and 6, which it would interpret as being the same colour.

But with the filters, the ommatidia are now ‘tuned’ to different colours, so if light coming from direction of object 1 is picked up by row 4, and light coming from the direction of object 2 is picked up by rows 5 and 6, the shrimp can see them as different colours and tell them apart more easily.

So, what does it mean?

This study has found evidence of an ingenious method to improve vision without needing to change the fundamental chemical make-up of the photoreceptor cells: By using naturally-occurring sunscreens (MAAs) to filter out certain wavelengths of light before they reach the ‘sensing’ part of the eye.

Not only does this appear to be a rather elegant method of re-purposing a group of chemicals manufactured by bacteria for a new use, but it also raises the possibility of other creatures using similar systems for their vision.

What is also pretty cool is that many marine animals can’t see in the UV range, so the authors suggest that the shrimp could signal to each other covertly by using movements that reflect UV light in a particular way that they can see, but other animals can’t.

From a conservation standpoint, the fact that the shrimp are likely to obtain the MAAs from their diet rather than making them themselves raises the possibility that an animal’s vision may be compromised by malnutrition – i.e. if it doesn’t eat enough of the right food, it can’t get hold of the MAAs it needs to see properly.

While the mantis shrimp itself is not endangered, there may be other creatures which are at risk that are reliant on their diet to provide certain chemicals that are central to their vision. This means that changes to the food chain could affect them in ways which were previously not considered.

Human parents will sometimes tell their kids that eating carrots will help them to see in the dark (which is true to an extent), so maybe mantis shrimp secretly signal to their younglings using their special UV-vision, that they need to eat all their bacteria-containing dinner. In my head that’s how it works.

Original article in Current Biology Jul 2014

All images are open-source/Creative Commons licence.Credit: prilfish (First); P Maritz (Second); S Campbell annotated by TSIC(Third); R Emperley (Fourth); M J Bok et al. Edited and annotated by TSIC(Fifth and Sixth); TSIC (Seventh)



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