Shiny shrimps with invisible eyes

Vulnerable juvenile marine creatures have an incredible strategy to escape being eaten before they grow sufficiently large to defend themselves: they resort to invisibility, by making themselves transparent. But they cannot work for the eye, because when you collect light in order to see it, you leave a dark spot, which predators can see. To get around the problem, some shrimp larvae cram their retinas with assemblages of tiny orbs, or nanospheres, which can bend light from the surroundings out into the direction of their gaze, making their eyes disappear. The optical properties of these nanospheres are tuned to the environments in which the animals live, as Will Tingle heard from Ben-Gurion University of the Negev’s Ben Palmer...

Ben - Many oceanic animals start their life as plankton. So these are small animals floating around in the open ocean, kind of at the whim of currents and that sort of thing. And they float around and they're largely defenceless organisms. But one of the strategies they've developed to avoid being eaten by predators is to become transparent. So there's almost no pigment in them at all. And they sort of appear almost glass like. But one of the places where you can't do away with pigment is in your eyes. So if you want to see, you have to absorb photons of light, which need to be captured by pigments. And if it wasn't for this reflector, then the animals would appear as two black dots on a transparent background. So the idea of this reflector is that it's a camouflage device which covers those absorbing pigments, those dark pigments in the eye. And the colour of this reflector is matched to the watercolour of the habitat that the organisms live in. So they are matched with the background and essentially appear invisible to predators.

Will - Is this a conscious choice or is this just how refraction works?

Ben - So this is definitely not a conscious choice. These organisms have this feature genetically hardwired into them. So what we found is that in different species of crustaceans, which occupy slightly different habitats in the water, then they have different eye colours which are matched to those water colours that they live in. So from deep blue all the way through to kind of yellow green. And the way that this reflector is tuned to match those colours is the size of those nanospheres. So in each species you find a different size of nanoparticle, which is optimised to camouflage it in the water that it lives in.

Will - Does this change if the individual is moved from one environment to the other?

Ben - We did find in one particular species that this organism did change its eye colour in the dark and the light. So this was found serendipitously by the student working on this project. She found, I think one morning when she came in, that the shrimp that had been in the dark had changed their eye colour from yellow to green. And this was a reversible change. So when you switch the light back on, they're able to change their eye colour, they maintain the size of the spheres. This seems to be something that's hardwired into each species. What they do is subtly change the arrangement of the spheres, and this changes the optical properties of the reflector.

Will - Do these crystals stick around when they become adults, and does that function change as they get older?

Ben - So this is a really interesting question actually. The decapod crustacean, so we're talking about shrimp, prawns, lobsters and so forth, they have a very interesting and complicated life cycle. So they begin as a kind of larval animal floating around the open ocean. And then they undergo this very dramatic metamorphosis event, which is akin to a caterpillar changing into a butterfly. And they completely change their behaviour, their habitat and anatomy and visual system. It's all altered. And actually, we first found this crystalline material in the adult crustaceans a few years ago. Adult crustaceans have a very amazing complicated eye called a reflective compound eye which, unlike our eyes, uses reflectors rather than lenses to do the optics and that was made of crystals that we discovered a few years ago. The reflector in the adult eyes is actually placed behind the retina. And the idea there is that light that gets transmitted through the retina gets a second chance to be absorbed. So it bounces back off this reflector, which is made of crystals, back to the retina to increase the photon capture of the eye.

Will - These crystals are organic. And do you think that the discovery of these crystals, or rather the discovery of how useful they are in their manipulation, might have an impact on our own material construction?

Ben - There is a growing field of bio-inspired materials and particularly bio-inspired optical materials. And there's growing interest in trying to find highly reflective materials which are organic as replacements for inorganic nanoparticles, for example, which are used currently in things like pigments, paints, cosmetics. So there is actually a growing interest and a growing sort of field in this area, trying to explore how we can take this information and translate it into real world applications of these sorts.

Will - But not quite an invisibility cloak just yet.

Ben - I think definitely we're some way off that. And I think there's many things we still have got to learn. So we're really trying to 1. Enjoy the pleasure of understanding these systems from a fundamental point of view, but also trying to learn what it is they're doing and how we can translate that into real world applications.