Light-manipulating material in their “eyeglitter” hides larval crustacean from view

Light-manipulating reflectors covering the dark eye pigments of otherwise transparent larval crustaceans allow these creatures to fully blend into their backgrounds, researchers report. The findings – which reveal a particularly compact and tunable photonic glass in the “eyeglitter” structures of these animals – could be used to inspire the design of various materials. “By mimicking nature’s solutions, humans can optimize and develop better photonic materials for solar energy, commu­nications, remote sensing, and other light-dependent technologies,” write Kate Feller and Megan Porter in a related Perspective. Many ocean creatures – from fish to zooplankton – appear transparent underwater to camouflage themselves, but total body transparency is impossible for creatures that need to see as vision requires the use of opaque eye pigments. As a result, transparent organisms have evolved various strategies to balance the trade-off between the ability to see and not be seen. Some larval decapod crustaceans use a reflector that covers dark eye pigments and produces a distinctive eyeshine that reflects light matched to the color of the surrounding water. The nature of the photonic structure of this reflector, however, remains unknown. To better understand it, Keshet Shavit and colleagues used optical and cryogenic scanning electron microscopy to investigate the eyeshine reflector in several larval crustaceans, including Machrobrachium rosenbergi, a model species of freshwater prawn. Shavit et al. found that the eyeshine is produced by highly reflective cells made from a photonic glass comprised of crystalline isoxanthopterin nanospheres, which reside on the interior surface of the eye. “A distinguishing feature of the eyeshine reflector described here is its tunability and compactness,” say the authors. Further investigation across different larval crustacean species showed that their eyeshine color is modulated – from deep blue to yellow – by tuning the size and ordering of the nanospheres. “The diversity of optical solutions discovered in just a few recently investigated pelagic crustaceans spotlights the untapped potential for photonic innovation yet to be found in animals living in the open ocean,” write Feller and Porter, discussing the broader research field. They say such exciting photonic discoveries are only being made now because “the vastness of the pelagic realm, and the relatively sparse distribution of animals within it, makes this habitat difficult to access and sample effectively.”