Fig. 3: Chemical and structural properties of the reflective nanospheres.

a, Raman spectra from the white stripe of the cleaner shrimp, overlaid with the spectra of crystalline biogenic and synthetic isoxanthopterin. Inset: molecular structure of isoxanthopterin. Right: low-frequency region of the cleaner shrimp spectrum. The broad peak at ~1,080 cm⁻¹ corresponds to amorphous calcium carbonate from the cuticle⁵⁵. The peak at ~1,600–1,700 cm⁻¹ is probably an amide I band, and the sharp peak at ~1,000 cm⁻¹ is a phenylalanine aromatic ring stretch, both associated with proteins⁵⁶. There are several other peaks whose chemical origin is not yet identified. b, In situ WAXS diffraction pattern from the maxilliped (obtained by radial integration of the 2D scattering pattern; inset). c, TEM images and corresponding SAED of two particles with d spacing ~3.2 Å. The reflection angle changes when moving along the dihedral angles of the particle, indicating that the stacking axis projects radially away from the centre of the sphere. d, A TEM image of three extracted particles exposing the internal spoke-like structure. e, TEM micrograph of the nanospheres in an ultra-thin tissue section (~100 nm), showing the spoke-like, spherulitic structure. The white arrow denotes the nanosphere membrane. f, Schematic of a nanosphere showing the spherulitic arrangement of stacked isoxanthopterin molecules. Radial (n_r) and tangential (n_t) refractive index vectors on the nanosphere surface illustrate the birefringence produced from the spherulitic arrangement. Dark-blue trace in panel a adapted with permission from ref. ³⁷, AAAS.