Birefringent nanospheres help give a shrimp its bright white stripe

Creating artificial white material is a difficult task. Previously, chemists relied on titanium dioxide to create the white pigment in sunscreen, medicinal pills, most toothpastes, and other necessities. But because the European Union banned the chemical compound as a food additive in 2022, the European Medicines Agency has scientists looking for solutions elsewhere. One place is under the sea.

Pacific cleaner shrimp (Lysmata amboinensis) are small crustaceans that attract other marine life by using the bright white stripe along the back of their shell. That also attracted the attention of Tali Lemcoff of Ben-Gurion University of the Negev and colleagues. Whatever physics was causing the white to display so brightly in even a thin shrimp shell had the potential to be adapted for use in ultrathin manufactured materials.

Humans see white when photons of all visible wavelengths are scattered so many times that all directional information is lost, which produces broadband, angle-independent reflectance. That process is easy to achieve in a thick layer of a material; a thin layer often doesn’t create enough scattering—the resultant color won’t be pure white—and the layer may be translucent rather than fully opaque. Manufacturers generally want thin layers of color to reduce the weight of final products, and the same is true of animals.

The cleaner shrimp isn’t the only example of brilliant whiteness in nature. Some beetles and butterflies also display the color, but that effect is achieved in air, whose lower refractive index means it’s easier to generate the needed refractive-index contrasts. A higher contrast between indices will result in more effective scattering. The researchers hoped that understanding what makes the shrimp’s white color so brilliant despite being in water would translate to other materials in aqueous environments.

Using cryo-scanning electron microscopy, the scientists studied the incredibly thin (3–7 µm) pigment cells in the shrimp’s shell. They found very densely packed nanospheres within the cells, which provide the mechanism for the white color. Those spheres are birefringent: Their index of refraction depends on the direction that light hits the spheres. Unlike many other birefringent materials, where the multiple refractive indices are associated with different crystallographic directions, the indices in the cleaner shrimp are due to a highly polarizable material: isoxanthopterin.

That material is familiar to biologists as a reflector in crustacean eyes. There, it is in a crystalline structure. In the cleaner shrimp, however, it does not exhibit three-dimensional periodicity. The particles are in 1D ordered assemblies, reminiscent of spokes of a wheel. That creates very different indices of refraction, depending on the angle of incidence.

The unusual structure also means that the nanospheres’ high packing density contributes to the scattering instead of hampering it. As such, the shrimp display a brilliant whiteness despite only a thin layer of particles.

Studying the shrimp’s color may provide an avenue for creating new artificial, ultrawhite, ultrathin layers. Coincidentally, birefringent nanospheres would also be biodegradable and organic; thanks to the cleaner shrimp, researchers can start working with cleaner materials. (T. Lemcoff et al., Nat. Photonics, 2023, doi:10.1038/s41566-023-01182-4 .)