Crustacean chicks have eyes like the sea, a characteristic that could help them hide from predators.
Young shrimp, crab or lobster larvae already remain in the rock, their bodies almost transparent from view. But the dark pigments of the eye, essential for vision, are exposed to the danger of animals anyway.
Some see-through-the-ocean animals rely on iridescent or tiny spied eyes to avoid detection. Juvenile shrimp and prawns, on the other hand, camouflage their dark pigments behind the light glass reflected from the tiny, crystalline balls, the researchers report on Dec. 17. Science.
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Variations in the size and placement of the orbs allow the crustacean’s eyes to emit light that matches the color of the surrounding water, possibly rendering them invisible to predators on the hunt for food.
Technologies that mimic the structure of nanospheres could one day inspire more efficient solar energy or bio-friendly paint, scientists say.
“I’ve often wondered what’s going on when [these animals’] eyeshine,” says evolutionary biologist Heather Bracken-Grissom of Florida International University in Miami, who was not involved in the study. She and her colleagues often collect crustaceans from the deep sea, and identify them as “blue arthropods” or “eye-eyed, unlucky shrimp” because the grown-up creatures do not resemble their forms. Now, he says, eye color makes sense.
In the lab, chemist Keshet Shavit and colleagues used an electron microscope to peer into the eyes of lab-raised and uncultivated crustaceans. Inside shrimp and prawn eyes, a group of crystalline nanospheres were found made of isoxanthopterin, a molecule that reflects light.
The spheres look like balls, with highly reflective outer surfaces, says study coauthor Benjamin Palmer, a chemist at Ben-Gurion University of the Negev in Beer-Sheva, Israel. Each sphere is made of thin lamellae, isoxanthopterin, which stick together to form spheres that range in size from about 250 to 400 nanometers in diameter.
These globules are arranged in clusters at the base of the protein-dense cones, which place light on the animal’s light-sensing nerves, and form a protective layer on the pigmented cells. But the crustacean larvae can still see because there are small holes in the glass, says Palmer. “It usually allows light to fall on the retina at certain specific angles, but it reflects light at other angles.”
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The size and arrangement of the orbs appear to influence the color of the reflected light, the team’s observations and computer simulations show.
“The relationship between the particle size and the color of the colors above is amazing,” says Shavit, also at Ben-Gurion University. The size of the nanosphere appears to help the animals’ eyes match the color of their native habitat, helping the critters blend into the background.
Blue shrimp, which inhabit the clear blue waters of the Gulf of Aqaba off the coast of Israel, have shells that are about 250 to 325 nanometers in diameter. 400 nanometer-sphere freshwater prawnMacrobrachium rosenbergii) they shine yellow and green, imitating the muddy waters found in the salt estuaries where they live.
The wrong eye also seems to be able to reflect different colors in different environments. Individuals exposed to the lab alone for four hours had silvery yellow eyes, perhaps from nanospheres arranged in disorderly confusion. But private people had green eyes to spend the night in the dark. Their nanospheres are arranged in layers — yet the spheres within each layer are disordered, says Palmer.
Such adaptive eyes can help the larvae hide in different parts of the ocean, as changing light levels change the color of the water, says Bracken-Grissom. At night, young crustaceans migrate to shallow waters to feed and return when the sun rises. “If you’re actually using it as camouflage, cleverly camouflage it.”
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