Reflector platelets within squid chromatophores reflect light because they are nanofabricated photonic structures composed of proteins called reflectins.
"The identification and characterization of the reflectins confirm that, although the majority of animal reflective tissues are composed of purine platelets, cephalopod reflector platelets are proteinaceous. Reflectins, a protein family with skewed amino acid compositions, repeating domains, and localized deposition, are thus far restricted to cephalopods. They represent a marked example of natural nanofabrication of photonic structures in these animals." (Crookes et al. 2004:237)
"A family of unusual proteins is deposited in flat, structural platelets in reflective tissues of the squid Euprymna scolopes. These proteins, which we have named reflectins, are encoded by at least six genes in three subfamilies and have no reported homologs outside of squids. Reflectins possess five repeating domains, which are highly conserved among members of the family. The proteins have a very unusual composition, with four relatively rare residues (tyrosine, methionine, arginine, and tryptophan) comprising 57% of a reflectin, and several common residues (alanine, isoleucine, leucine, and lysine) occurring in none of the family members. These protein-based reflectors in squids provide a marked example of nanofabrication in animal systems." (Crookes et al. 2004:235)
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