In the ocean, the Pacific cleaner shrimp stands out from the crowd, with vibrant white coloration on its back and its whisker-like antennae. An international team of scientists recently revealed the intricate biological engineering behind this brilliant white, publishing their findings in the journal Photonics of nature.
As the name suggests, Pacific cleaner shrimp “clean” fish, eating parasites and dead tissue from their skin. The crustacean’s intense white coloration serves as an eye-catching advertisement, attracting fish to participate in its mutually beneficial cleaning services.
The vibrant color of the Pacific cleaner shrimp is doubly impressive considering it occurs in a light-challenged medium. Water greatly limits the distance that particles of light (photons) can travel. Within the first 10 meters, water absorbs more than 50% of visible light energy. Above 200 meters, there is rarely any significant light.
Objects appear white when they reflect all light. But with fewer light particles to reflect in the ocean, it’s hard to have such a brilliant white. This means that everything in the cuticle (skin) of the Pacific cleaner shrimp must be highly reflective.
It also has to be extremely refractive: deflect light. Brilliant white requires an object to bend light much more than the medium in which it exists. Air isn’t very refractive, but water is. That’s why when you immerse a white object in water, its color fades, appearing almost gray.
To uncover the inner workings of the Pacific cleaner shrimp’s white cuticle, the team of scientists — led by Dr. Ben Palmer, an assistant professor in the Department of Chemistry at Ben-Gurion University of the Negev, and his student Tali Lemcoff — used cryo-scanning electron microscopy. This involved freezing the shrimp’s white tissues and then blasting them with focused electron beams. The bombardment produces signals by interacting with atoms in the sample that can be distilled into a remarkably sharp nanoscale image.
What they saw in those images was truly remarkable bioengineering. Inside the shrimp’s cells were spherical particles about 300 nanometers wide, each containing stacks of molecules arranged in spokes almost like a bicycle. The placement overcomes a phenomenon called optical crowding, in which light reflection decreases when light-scattering structures are too dense. If you were trying to diffuse light (thus creating white), it’s hard to conceive of a more efficient design.
“The shrimp has overcome a seemingly fundamental hurdle in optics by creating particles with this special arrangement of molecules,” said Dr. Palmer in a statement. “Now the question is, how can we replicate this effect to create new materials that we could use as food additives in white bread, or white paint, and other applications?”
Palmer plans to replace titanium dioxide, a bleaching agent, with an organic compound inspired by Pacific cleaner shrimp. Titanium dioxide is used in a wide variety of food, cosmetic and paint products. In August 2022, it was banned in the European Union over fears it could cause cancer, but the FDA still allows it in the United States. Scientific evidence suggests that titanium dioxide causes no harm at the extremely low amounts it is used at.
Diederik S. Wiersma, a physicist specializing in spectroscopy at the University of Florence, offered another potential use in a viewpoint published alongside the recent study.
“One could imagine developing a new sunscreen for men that protects the skin from UV rays while providing a pleasant cooling effect.”
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