As white as... a shrimp

A scientific quest for safer artificial white materials got a boost recently, with scientists unravelling how a species of shrimp found in the Pacific Ocean gets its white body stripes. The team discovered that nano-sized spheres of the biological molecule isoxanthopterin are responsible for white colouration in the Pacific cleaner shrimp (Lysmata amboinensis). The work, published in Nature Photonics (go.nature.com/42nd1aW), can inspire the development of ultra-thin white coatings to replace inorganic chemicals associated with health concerns.

To produce artificial white materials, industry has so far relied on inorganic materials with a high refractive index, such as titanium dioxide, zinc sulphide and zinc oxide. However, in 2022, the European Union banned the use of titanium dioxide (TiO₂) as a food additive, following which the European Medicines Agency emphasised the need for an alternative to TiO₂ in medicines.

Apart from food and medicines, such white coatings are essential for making sensors, solar cells and displays. Though scientists have looked to nature for organic alternatives, they have had little success until the team of researchers, including Venkata Jayasurya Yallapragada of the Indian Institute of Technology Kanpur, discovered the physics behind the extremely bright white stripe found on the shell of the Pacific cleaner shrimp, the white pattern on its tail, and its white antennae and feeding appendages.

Combining microscopy, optical measurements and simulations, the scientists found that the white colouration is caused by ultra-thin layers of densely packed isoxanthopterin nanospheres, which are a few millionths of a metre thick. When the spheres are packed together in thin films, the light scatters less due to a phenomenon called 'optical crowding', they found.

"Many species of shrimp contain brilliant white chromatophore cells, and these can be arranged either in stripes, spots (Lysmata debelius) or more diffusely distributed across the body (Crangon crangon). The white colours are particularly striking in the Pacific cleaner shrimp due to the fact (that) the cells are packed very densely together, forming a uniform, brilliant white stripe," says lead author Benjamin Palmer, a chemist at Ben-Gurion University of the Negev, Israel.

According to G.V. Pavan Kumar, a physicist specialising in nanophotonics at the Indian Institute of Science Education and Research Pune, the work is significant in that the team identified a biological system and correlated an important optical property to the structure of the system. "This adds a lot of value in design – and plenty of wonder," remarks Kumar, who wasn't involved in the study.