Should I stay or should I go? Causes and dynamics of host desertion by a parasitic crab living on echinoids

In some long-living symbiotic species, movements between hosts are not limited to offspring since adult parasites can move from one individual host to another one. Host-switching may be driven by different parameters such as (1) mating strategies of symbionts, (2) foraging for resources or (3) avoiding overcrowded or diseased/dead host. Symbiotic marine crustaceans are suitable models to understand what underlies host-switching behavior. In this study, we investigated host desertion by the parasitic pea crab Dissodactylus primitivus associated with the echinoid host Meoma ventricosa. Mark-recapture field experiments, during which crabs were almost always found on their host in heterosexual combinations, suggest that host desertion occurs less frequently when 2 crabs (compared to 3) share the same host. During laboratory experiments with high crab density, the proportion of crabs leaving an echinoid was low when the 2 genders of crabs were present on the host, compared to 1 gender only (males or females). This suggests that host desertion is mostly driven by intersex selection and the search for a mate and, to a lesser extent, by competition between crabs. However, both field and laboratory experiments showed evidence that when they switch host, most crabs remained for a while in the sediment underneath their host. We propose that this behavior, associated with the aggregative behavior of their hosts, would allow the crabs to solve the trade-off between staying on their hosts (therefore suffering overcrowding and sub-optimal mate search) and moving too far from the host (therefore suffering loss of food source and high predation risk).