Why do you get transparent animals in the sea but not land?

Chris Smith put this question to marine biologist Kate Feller...

Kate - There are so many. The majority of invisible animals on the planet are found in what’s called a pelagic zone, or the pelagic animals which means open ocean, open water. It has evolved many many times so you can find worms, crustaceans, jellyfish, fish, so many molluscs. There are certain squid that can make themselves totally transparent. And the reason why there are so many in the ocean and not so much on land - there are a few examples, like there are certain butterflies, these glasswing butterflies have transparent wings. They are absolutely stunning if you’ve never seen one, find one at the Zoology Museum. But the reason that transparency is a lot more difficult on land is because of this thing called refractive indices. Refractive index is just a fancy word to describe the property of light when it enters some sort of material, how it slows down. The refractive index of air is very low, whereas the refractive index of materials like glass - glass is a good one, windows are clear -  is very high, so you have a very big difference. And so when you have a very big difference in your refractive index then light is going to bend a lot when it enters that glass. So what happens on a sunny day when you look at a window from the wrong angle? You get a really bright glare, which is not good if you’re trying to be invisible. Water has a much higher refractive index, so does the material that the animals are made of so you can really make that difference in how light moves through the materials very minimal; therefore you can’t see them. Even a piece of glass underwater is very difficult to see because that glare just doesn’t happen.

Chris - So it’s easier for them to become invisible underwater? So, therefore, given that it’s a lot easier to do they’re more likely to do it?

Kate - Correct, yes.

Chris - So some animals do resort to that and it enables them to escape from being eaten?

Kate - Actually, one of my favourite solutions that terrestrial animals have come up with to avoid this anti-glare is a structure called a nipple array.

Chris - Tell us more!

Kate - It’s an array of literally little nipples that will be on the surface of your transparent material. You find this on butterfly wings or moth wings, on a lot of insect corneas so that they don’t give off that glare. Essentially the way that the structure is kind of disrupts the light’s ability to produce glare, and it’s called the nipple array. Let’s have a giggle about that!

Chris - Sounds nearly as good as I heard this week about a beaver deceiver, which is when you want to introduce beavers to an area and you don’t want them to flood the area, you have a way of tapping the water off. But the beavers don’t block it because they don’t know it’s there, but you can nonetheless accommodate the beavers, and have the benefits of the beavers without them actually flooding you out of house and home.

Matt?

Matt - I just wanted to ask about invisible animals. If a marine animal is invisible, that just means the photons are all just passing straight through it. Is it then blind because it can’t capture and of the photons used for vision?

Kate - This is actually what I did my PhD thesis on, or as an aspect of it. A lot of crustacean larvae, for instance, like mantis shrimp larvae, use transparency as a way to disguise themselves in the open ocean because otherwise they’d get eaten very readily. However, they want to see because they’re little predators themselves and, as you were hinting at, you cannot actually see without having some sort of opaque screening pigment within your eye to isolate each photoreceptor from one another because otherwise you’ll muddle all of the pixels of the image that you’re generating. So what they have evolved, a lot of crustacean larvae but I studied the mantis shrimp, is this really beautiful material that I, unfortunately, published under the name of “eyeshine.” I would rather rename it as eye glitter because it is this beautiful blue/green glittery material that is over the surface of this black part of their eye that reflects the wavelength that are behind them perfectly. It is a perfect match and so they have this reflective camouflage strategy to enhance their visibility in the open ocean because you can’t have a truly transparent eye.