Nanoscale ultrastructures increase the visual conspicuousness of signalling traits in obligate cleaner shrimps.

Abstract

Signal theory predicts organisms should evolve signals that are conspicuous to intended receivers in natural signalling environments. Cleaner shrimps remove ectoparasites from reef fish clients and many signal their intent to clean by whipping long, white antennae. Since white is a reliably conspicuous colour in aquatic environments, we hypothesized that selection has acted to increase broad-spectrum antennal reflectance in cleaners. Using Scanning Electron Microscopy, optical models, and reflectance measurements, we found that the antennae in three obligate cleaner species from two families (Palaemonidae and Lysmatidae) had thick (∼6µm) chitinous layers or densely-packed high refractive index spheres (300-400nm diameter), which models show increase reflectance (400-700nm). Two facultative and non-cleaning species had no visible antennae ultrastructure beyond chitinous exoskeleton. Antennae reflectance was significantly higher in obligate cleaners than facultative and non-cleaning species. Our results suggest that some obligate cleaners may have evolved ultrastructures that increase the conspicuousness of the antennae as signals.