How sea creatures become invisible

The Cambridge Science Festival is one of the UK’s largest and most successful and this year’s edition is coming to a close. To celebrate the final day, we’ve invited the winner of this year’s Cambridge regional FameLab - that’s the competition where researchers have to give a three-minute science talk - to come along for a chat. Chris Smith spoke with Kate Feller...

Kate - I am a postdoctoral researcher in the Physiology, Development and Neuroscience Department here at Cambridge and I study the eyes of Mantis Shrimp. Historically my PhD training was studying the eyes of these remarkable crustaceans and their babies. What’s so amazing about them is the adults at least can see beyond the spectrum that we can see. They can see all the colours of the rainbow plus they can see different colours of ultraviolet light, and they can see the different types of polarised light, so they just have these remarkable eyes. And they’ve got one of the fastest animal movements on the planet where they can strike their prey or something that’s bugging them with these awesome raptorial appendages, and they can do it as fast as a bullet out of a gun.

Chris - I thought that was my daughter asking me for pocket money - that’s pretty fast as well. Now, why did you decide to go for FameLab and what's FameLab for people who are not initiated?

Kate - FameLab is a science communication contest to try and encourage us researchers to, I guess, be more effective science communicators and I kind of did it on a whim. I was in the middle of my fieldwork working in Spain and I got an email and I said hey, this sounds like fun.

Chris - You must be good at it. We’re going to find out I think, aren't we?  We’re going to give you three minutes to strut your stuff and tell us in three minutes what you delivered as your talk for FameLab that won you the regional final…

Kate - Alright here we go.

Chris - Off you go Kate Feller.

Kate - There is a place on the Earth where animals can make themselves disappear. It’s called “the pelagic environment.” The pelagic environment is anywhere in the ocean where there is nothing but open water. There’s nowhere to hide; it’s just the same blue/green water in almost any direction.

And just like in Star Trek, many pelagic animals have cloaking devices that help them avoid being seen in open spaces. One cloaking device is to cover your body in mirrors. Have you ever noticed how silvery some fishes are? These silver mirrors reflect all of the colours of the visible spectrum equally and, since this light is the same in almost every direction, these angled mirrors on their sides reflect light that’s in front of them and it looks exactly like the light behind them totally hiding the fish underneath.

Though these mirrors are great, obviously the best way to disappear is Harry Potter’s invisibility cloak. But, since most pelagic animals are muggles, they have to accomplish basically the same thing by having totally transparent bodies. The larvae or babies of many crustaceans such as crabs, lobster, or mantis shrimp have bodies that are so clear they look like they’re made of glass. Even though you can’t see their bodies, there’s one thing that transparent animals can’t make despair, and that’s their eyes.

In order to work properly all eyes and we're talking all eyes, including humans, need to have dark, light absorbing pigments. These dark pigments surround the individual light sensing cells and isolate them from any stray light of their neighbours. If eyes didn’t have these dark screening pigments, it would be like drilling a bunch of holes into the body of a camera. The final image would be ruined!

So, transparent animals have a problem where they want to be totally invisible so they don’t get eaten, but they also need to be able to see so they can’t really have a transparent eye. But, as my research revealed, some crustacean larvae have the solution. Covering the dark part of a mantis shrimp larval eye is a blue/green eye shine that looks just like blue/green glitter on a chocolate cupcake.

Unlike the mirrors that we see in silvery fishes that reflect all of the wavelengths of the rainbow, this eye glitter only reflects the wavelengths of light that are behind them, which is blue/green as we covered earlier. So, the reflection that this eye glitter gives off is matched to the background which aids to disguise this opaque dark eye, and overall lend the animal the ability to disappear. And with that, I’m going to disappear.