Cephalopods — including squid, octopuses, and cuttlefish — manipulate their body coloration for camouflage and communication through a combination of controllable pigmentation and structural light scattering. The structural color that emerges from coherent light scattering is due to photonically active nanostructures within the cephalopod’s reflective skin cells. In particular, the reflectin proteins help produce iridescent colors by forming an intracellular Bragg reflector. A team of researchers from the University of California, Santa Barbara investigated reflectins and published their results in APL Materials.

The authors performed in vitro analyses of purified recombinant reflectins from Pacific squid (Doryteuthis opalescens). With increasing pH, used as a surrogate for the phosphorylation that triggers neutralization in vivo, the proteins condensed and self-assembled into monodisperse spherical nanoparticles of progressively larger sizes. The sizes were measured with dynamic light scattering and verified visually with electron microscopy.

The reflectins can be thought of as a signal-controlled molecular machine that controls an osmotic motor. The configuration of reflectins regulates the osmotic shrinking or swelling of the Bragg lamellae that lead to adaptive iridescence. Although reflectins remain the key driver of the iridescent effect, the study has found that the tunable nature of cephalopod’s structural color is dependent on the larger cell structure and not just the proteins themselves. The membrane-bound, reflectin-filled Bragg lamellae of the iridocytes possess the controllable dimensions and refractive indices responsible for the changing brightness or color of reflected light.

According to lead author Robert Levenson, the findings move the field towards a better understanding of reflectin behavior and could enable the creation of a tunable, bio-inspired photonic device.

Source: “Molecular mechanism of reflectin’s tunable biophotonic control: Opportunities and limitations for new optoelectronics,” by Robert Levenson, Daniel G. DeMartini, and Daniel E. Morse, APL Materials (2017). The article can be accessed at https://doi.org/10.1063/1.4985758.