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Dryad

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

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Aug 20, 2024 version files 18.58 KB

Abstract

To be effective, signals must be detectable; thus, signal theory predicts that 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 their 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 these species. We compared reflectance and antennae ultrastructure in three obligate cleaner species with one facultative cleaner and one non-cleaner in two families (Palaemonidae and Lysmatidae). Obligate cleaner antennae had a reflectance of 41-52% of human-visible light, compared to 35% in the facultative cleaner and 21% in the non-cleaner. Scanning electron microscopy (SEM) revealed the antennae in two obligate cleaner species contain a layer of densely packed, high refractive index spheres (300-400nm diameter), which optical models showed are effective at increasing reflectance. In the third obligate cleaner, SEM showed thick (~6µm) chitinous layers that may also increase reflectance. The facultative and non-cleaning species had no visible antennae ultrastructure beyond chitinous exoskeleton. Our results suggest that some obligate cleaners have evolved ultrastructural modifications that increase the conspicuousness of the antennae as signals.