A sensory investment syndrome hypothesis: Personality and predictability are linked to sensory capacity in the hermit crab Pagurus bernhardus

Correlated phenotypic traits (i.e., syndromes) may manifest as associations between different behavioural types or between behavioural and non-behavioural phenotypes. While syndromes research is extensive, correlations involving behavioural type and sensory morphology have yet to be investigated. Sensation is essential in decision-making and should be correlated with behavioural phenotypes involved in risk response, including boldness. We investigated correlations between boldness and sensory capacity in Pagurus bernhardus hermit crabs, taking repeat measures of startle response durations to assess hermit crab personality and predictability. The correlation between startle response and the sensillar density (i.e., number of sensilla per unit surface area) of both chelipeds was assessed using Bayesian-fitted double hierarchical general linear models. Negative correlations between these traits support the existence of a syndrome linking sensory capacity and behavioural type, hereafter distinguished as a ‘sensory investment syndrome’. Increasing sensillar density on the major claw also corresponded with reduced within-individual variation, or predictability, in startle response duration. By revealing a correlation between sensory and behavioural phenotypes, our results demonstrate the importance of considering sensory morphology and performance in behavioural ecology and show how sensory investment syndromes might support behavioural strategies that help to maximise fitness.