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Sometimes, no camouflage is the best camouflage, and colorful is not as good as transparent and colorless. Many of the creatures that live in the sea are almost transparent, which makes it difficult for predators to spot them. However, it is impossible to be transparent all over the body, such as the eyes, which require dark pigments that are indispensable for vision, which increases the risk of their discovery and becomes the "soft underbelly" of aquatic organisms.
Larvae and eye structures of several crustaceans. Image credit: Integr. Comp. Biol. [1]
Of course, aquatic organisms have also evolved various methods to solve this problem, including shrinking the size of the eyes and retinas. Crustaceans, especially larvae before acquiring adult hard carapaces, are masters of hiding themselves. Studies have found that the larvae of a mantis shrimp (Neogonodactylus oerstedii) can even scatter three different wavelengths of ultraviolet light to interfere with the photosensitive system of natural enemies, which makes it look like a shadow against the background of open water, taking the "invisible" ability to the extreme.
"Invisibility" of crustaceans. Image source: Science [2]
Recently, researchers such as Johannes Haataja of Cambridge University in the United Kingdom and Benjamin A. Palmer of Ben-Gurion University in Israel published a paper in the journal Science to discover a new secret that crustacean larvae make eyes invisible - an adjustable reflector covered on opaque eye pigment. This reflector is constructed of photonic glass containing numerous crystalline isoxantrexate nanospheres. By changing the size and arrangement order of the nanospheres, the reflectivity from dark blue to yellow-green can be adjusted to match their color to the background to achieve the effect of camouflage. In addition, reflectors can also enhance eyesight by acting as light screens between photoreceptors to enhance the acuity or sensitivity of the eyes. This versatile reflector provides inspiration for how to build tunable artificial photonic materials from biocompatible organic molecules. The work was selected for the cover of the current issue of Science magazine.
Current issue cover. Image source: Science
Reflectors do not need to reflect all wavelengths of light, which is related to the living environment of crustacean larvae. The larvae of many crustaceans such as shrimp and crabs have this "camouflage", typical of which is the larvae of the Macrobrachium rosenbergii.
The transparent body and reflective eyes of crustacean larvae such as shrimp and crab. Image source: Science
The eyes of Roche swamp shrimp larvae appear to be made up of hexagonal units, which are observed by cryoscanning electron microscopy, each containing a protein lens ("cc" below) in the upper part, which directs light to the retinal cells ("Ret" below) and photoreceptor cells ("P" below) behind. However, between the photoreceptor cell and the lens, there is also a layer of reflector, composed of crystalline isoxanthrexate nanospheres, and the anisotropy and refractive index of the crystal enhance the scattering efficiency. The reflective layer even passes through the photoreceptor cells and extends all the way around the striated muscle ("Rh" below) below. Since the larvae usually swim on the surface of the water, when the predator looks from below, the reflector will obviously mask part of the eye pigment, which not only plays the role of "invisibility", but also improves the larvae's own visual sensitivity in dim light environments.
Ultrastructure of the eyes of the larvae of Roche marsh shrimp. Image source: Science
Interestingly, the crystalline nanospheres of these eye reflectors are somewhat adjustable. Larvae living in pale blue water differ slightly in size and arrangement in the reflector compared to larvae living in pale green water, resulting in a change in the wavelength of reflection. For example, increasing the size of nanospheres from ~250 nm to ~400 nm can change the eye color of these larvae from blue to yellow-green.
Different shrimp larvae reflect the reflectivity and structural relationships of cells. Image source: Science
In eye reflectors, the packing density of crystal nanospheres is usually between 50%~70%, which is much lower than the tightly packed photonic crystals we usually think. The researchers used molecular dynamics and time-domain finite difference methods to simulate the relationship between nanosphere size and packing density and reflectance spectrum, which was consistent with the experimental data. It is worth noting that if an ordered photonic crystal structure is used as a reflective layer, iridescent colors will be produced, that is, the color changes with the viewing angle, which is not conducive to one's own vision and camouflage. This photonic glass formed by the accumulation of crystalline isoxanthrexate nanospheres can produce efficient, angle-independent colors while meeting the requirements of transparency and compactness.
Optical modeling of eye reflectors. Image source: Science
"This highly adaptable eye structure helps larvae go unnoticed in the ocean, and at night, crustaceans migrate to shallow waters to feed and dive back into deep water when the sun rises," commented Heather Bracken-Grissom, an evolutionary biologist at Florida International University, "an ingenious way to camouflage themselves" [3]. By mimicking nature, humans may be able to develop and optimize better photonic materials for use in solar energy, communications, remote sensing and other related technologies.
A tunable reflector enabling crustaceans to see but not be seen
Keshet Shavit, Avital Wagner, Lukas Schertel, Viviana Farstey, Derya Akkaynak, Gan Zhang, Alexander Upcher, Amir Sagi, Venkata Jayasurya Yallapragada, Johannes Haataja, Benjamin A. Palmer
Science, 2023, 379, 695-700. DOI: 10.1126/science.add4099
Bibliography:
[1] T. W. Cronin, et al., Crustacean larvae-vision in the plankton. Integr. Comp. Biol. 2017, 57, 1139-1150. DOI: 10.1093/icb/icx007
[2] K. Feller & M. Porter, Photonic tinkering in the open ocean. Science 2023, 379, 643-644. DOI: 10.1126/science.adf2062
[3] Glassy eyes may help young crustaceans hide from predators in plain sight
https://www.sciencenews.org/article/glassy-eyes-young-crustaceans-predators
(This article is contributed by Xiaoxi)