‘Disco Eye-Glitter’ Makes Baby Crustaceans Invisible
Reading Time: 4 minutesShrimp Larvae Have a Bizarre Secret Way to Hide From Predators, New research explains how shrimp larvae hide from predators—and what we can learn from their tricks., Shrimp larvae use ‘disco eye-glitter’ to be invisible to predators.
Floating around in the open ocean, shrimp larvae are alone and exposed. Like jellyfish and other other drifters, their best bet for survival is to be as inconspicuous as possible. Transparent bodies go a long way, making larvae look like little more than a trick of the light.
For many animals, like glass catfish and baby eels, a big barrier to true invisibility is the messy business of having eyes. The conundrum being: if eyes are transparent, you can’t see out of them. Shrimp larvae, it turns out, have solved for that. They can almost completely camouflage their eyeballs, using a glittery ‘eyeshine’ that they tune to blend in with different shades of blue-green ocean water.
This eyeshine adaptation was first discovered in mantis shrimp larvae in 2014. Now, a new paper in Science found that larvae from many species of crustaceans use this trick to make their eyeballs functionally invisible. It also discovered the mechanism at play, at least in shrimp specifically: Eyeball-shielding invisibility cloaks made of glittering ‘disco eye-crystals,’ which, the researchers hope, could eventually inspire advances in nanoparticle engineering.
I talked with Keshet Shavit, the master’s student who discovered the disco eye crystals, Benjamin Palmer, whose chemistry lab at Ben-Gurion University of the Negev in Israel decided to look into the optics of crustacean eyes, and Johannes Haataja, a computational researcher from the University of Cambridge who created a model to simulate how they work. Our conversation has been lightly edited and condensed for clarity.
Meg Duff: After reading this paper, the image I can’t get out of my mind is being surrounded by invisible baby lobsters—which some of your findings also apply to—while I’m swimming.
Keshet Shavit: If you open your mouth you’d probably eat them. They’re super small. Less than a millimeter.
Benjamin Palmer: You could just about see them with your naked eye? But not easily.
Shavit: If you want to be transparent in the sea, the darker pigments kind of ruin it. So the larvae shrimp and larvae prawns and many other larval crustaceans just covered the dark eye pigments with crystals that make their eyeshine color the same as the water color in their native habitat. And then they’re completely transparent.
Palmer: There are many strategies that organisms use to be transparent. There’s a recent paper in Science about glass frogs, for example, where they take all of their blood at certain times and concentrate it in small spots in their bodies to appear transparent. What we found here is a reflective device in the eyes of many crustaceans. It’s a very broad finding, from stomatopods, to shrimp, to lobsters, to crabs…probably much wider than we were able to sample.
What is actually going on in crustacean larvae eyes to make this reflective strategy work?
Palmer: The reflector covers the eye pigments. It’s a sheet of material, but there are holes. Then in the adult, the reflector migrates behind the retina. It’s camouflaging the larvae, and then increasing the intensity of light in the adult. In the adult, there is a backscattering mirror under the retina; it’s like what you have in cats and dogs. We were surprised to find the same crystalline material in the larval animal, but in a very different anatomical position and with a very different optical function.
I am so impressed by shrimp! What is this reflector made of, exactly?
Shavit: Basically, disco balls made of crystalline isoxanthopterin. Think: many flat features creating something with a core inside. These small nanospheres, or small disco balls, are really efficient in reflecting back light.
Johannes Haataja: The reflector is a few micrometers of these optical spheres. By tuning the diameter of those particles, but also how densely you pack them, you get a certain color response. These days, these ‘structural colors’ are a very hot topic. But it’s difficult to predict what type of structures we need to have in order to have a certain color response.
Palmer: What Johannes did, which was a very great piece of work, is that he was able to recreate this biological cell in a computer and then computed how light travels through it. When you have all these particles packed very closely together in an ordered arrangement, that’s called a photonic crystal. It causes iridescence, like when you look at peacocks and from one angle it’s purple and another angle is green.
Here, though, we have photonic glass: where you relax the particles a little bit, you don’t jam pack them all together. And this means you don’t have iridessence: the color is the same from all angles. That’s critical for this camouflage function.
I have an even more basic question now, which is: what is glass? I always thought glass was made of sand. Are these disco eye-crystals made of sand?
Palmer: Glass is a much wider thing than just sand. It’s a phase like liquid, solid, gas. The material the Keshet found and characterized is an organic molecule.
Shavit: It’s two rings of carbon with a little bit of nitrogen and hydrogen and oxygen and it has a really high reflective index.
If we humans want to become invisible—obviously the frog’s strategy of pooling their blood is obviously not going to work for us. Is this our breakthrough? We cover ourselves in layers of tiny disco balls?
Palmer: No.
No?
Palmer: We published a paper a few years ago about scallops and everyone was like, ‘Can we make our eyes like the scallop eyes?’ My opinion is that we’re doing OK. I think the applications are not based on invisibility, but on the other properties of these particles.
Haataja: I’m sure it will have important implications for the development of new systems. If you create miniature cameras, for example, it’s important to understand how you can maximize efficiency.
Keshet, is there an application you’re excited to think about?
Shavit: Nail polish! You wouldn’t need a thick layer of paint, you’d just need a really thin layer of these nano disco balls and it would be a consistent color.
Reference: https://slate.com/technology/2023/02/shrimp-larvae-eye-glitter-nanotechnology.html
Ref: slate
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