Impossible White for Cleaner Shrimp

Such as workers’ uniforms equipped with reflective strips to be visible on construction sites, cleaner Pacific shrimp (lysmata amboinensis), a small, detritus-eating crustacean that lives in symbiosis with many species that dispose of their own organic waste, has long white stripes on its antennae and tail. These signs will serve as a signal to the animals that need to be cleaned. But the intense color of these white bands seems to defy the laws of optics. In fact, white can only be obtained if the light is refracted many times by an unordered structure, as in a large mass such as a cloud, but is difficult to obtain in a very fine structure, such as the shell of a shrimp. Chemists Tali Lemkoff and Benjamin Palmer of Ben-Gurion University of the Negev in Israel and their colleagues solved this mystery: The white bands of cleaner shrimp in the Pacific contain a compact arrangement of nanospheres.

Albedo results from diffuse reflection of light, that is, when it bounces off in many directions. Snow, milk, or a piece of sugar gets its white color from its ability to scatter light from different angles. The thickness of these materials provides enough space for the particles to occupy random locations and thus scatter light in all directions.

But to get a very bright white color, you also need a lot of diffusing particles. “Square sugar is definitely white, but powdered sugar is whiter because it has more reflective particles for the same size,” explains Tali Lemkoff. Therefore, to obtain a bright white color, an unstructured structure and many diffuse particles are required.

There are two conditions that are difficult to bring together in very thin materials. The reduced area limits the number of scattered particles and tends to make them adopt an ordered arrangement. Furthermore, when particles accumulate in this way, they scatter less light, even if they are numerous: the refractive efficiency begins to decline once the scattered particles occupy more than 20% of the area. After this threshold, it is impossible to obtain white.

By analyzing several white areas of cleaner shrimp in the Pacific Ocean using various microscopy techniques, the researchers identified very fine layers (maximum 7 micrometers) of particles dispersed at a high density: their occupancy rate exceeds 50%. The threshold is usually prohibitive for getting white!

But these diffusing particles are actually nanoshells of isoxanthopterin. This small aromatic molecule scatters light differently from the particles that usually form white substances, which refract light symmetrically, that is, symmetrically regardless of its angle of incidence. On the other hand, Isoxanthopterin nanospheres refract light anisotropicly. Their refractive index ranges from 1.40 to 1.96 depending on whether the light arrives in a radial or tangential direction with respect to the nanoparticles. This is referred to as the “refraction phenomenon”. Because of this strong refractive index difference, the interaction of light scattered by the isoxanthopterin nanosphere with its neighbors is weak… as if the density of the nanoparticles were lower! Even tightly packed into a surface as thin as the shell of a Pacific cleaner shrimp, the refractive nanospheres produce a brilliant white color.