What's love got to do with it? Ontogenetic changes in drivers of dispersal in a marine ectoparasite

Sex-biased dispersal is common in nature and can influence the way in which organisms are distributed throughout the environment with consequences at the individual, population, community, and species level. Much of our understanding of what drives sex-biased dispersal stems from work on birds and mammals where dispersal tends to be female and male biased, respectively. Here, we draw on this large body of empirical and theoretical work on vertebrates to investigate what drives breeding dispersal in an ectoparasite, the salmon louse, Lepeophtheirus salmonis. We manipulated the density, sex, and developmental stage of lice on pairs of juvenile pink salmon (Oncorhynchus gorbuscha) hosts and show that the probability of leaving a host is density dependent at the preadult I stage and dependent on the presence of the opposite sex at preadult II and adult stages. Experiments in which louse movement was observed in groups of 25 individually infected hosts supported findings from individual experiments. Lice appeared to account for predation risk as they were 3 times more likely to disperse in the dark, when susceptibility to predation was low, than in the light. Our results support the hypothesis that asymmetry in reproductive investment shapes patterns of sex-biased dispersal and highlight the potential for drivers of dispersal to change with ontogeny. These findings are the first to establish what drives dispersal in the ecologically and economically important salmon louse and highlight the generality of the role mate competition plays in driving sex-biased dispersal across the animal kingdom.