Up or down? Explaining the variability of communication signals in cleaning symbioses

In this study, we used cleaning symbioses among coral reef fishes as a model system to investigate the form and function of signalling in interspecific mutualisms. More specifically, we examined the causes and significance of inter‐specific and inter‐individual variation in the design of client solicitation poses in cleaning interactions with Caribbean cleaning gobies ( Elacatinus spp). Using empirical data collected during field observations at cleaning stations, we tested three hypotheses: (1) The form of client incitation poses depends on species‐specific features; (2) Intraspecific variability in the form of client poses reflects the strength of each cleaner‐client relationship; and (3) Client individuals that deviate from a clear species‐specific form of pose will receive a different cleaning service. As predicted by (1), we found that the type of incitation pose was related to client body size, with small species performing mainly head‐down displays which may facilitate retreat into coral cavities upon predator approach. There was nevertheless some intraspecific variation in the type of display performed by clients, which was negatively related to client ectoparasite load, as predicted by (2). Hence, species with higher ectoparasite loads, and thus with a greater need to be cleaned, were less variable in display form than clients with fewer ectoparasites. Finally, cleaning gobies inspected for longer those individual clients that performed the species‐specific solicitation poses (3). We conclude that solicitation poses function to minimize uncertainty about a client's need to be cleaned and that their design has been partly determined by the risk of predation on posing clients.