Dispersal rates of the freshwater snail symbiont, Chaetogaster limnaei, depend on host contact and host characteristics

Background/Question/Methods

By dispersing to new hosts, ectosymbionts may escape the suboptimal fitness conditions imposed by environments with low resource availability, strong intraspecific competition, or low mate availability and inbreeding.  However, dispersal is often an energy-intensive and risky undertaking.  Symbionts may balance risks with future fitness benefits by waiting to disperse until risks are low and by preferentially dispersing to hosts with the best resource conditions.  In a series of laboratory experiments, we evaluated the conditions under which the symbiotic annelid Chaetogaster limnaei limnaei would disperse among freshwater snail hosts and determined whether Chaetogaster preferentially disperse to/from hosts with certain characteristics: high symbiont density, infection with echinostome trematodes, and large body size.  Chaetogaster is a symbiont of particular interest because these annelids consume larval trematode parasites and thus reduce trematode transmission.  By developing a mechanistic understanding of Chaetogaster dispersal among snails, we facilitate future research regarding the tripartite interactions among Chaetogaster, trematodes, and their snail hosts and identify snail characteristics that might lead to high Chaetogaster densities and low rates of trematode infection.

Results/Conclusions

Chaetogaster only dispersed from one snail host to another if the two hosts could directly contact or if the current host died.  Therefore, Chaetogaster appear to avoid dispersal across open environments unless absolutely necessary.  When the two hosts varied in size and presumably available space for Chaetogaster, dispersal rates were higher from large snails to small than from small snails to large. This could be an adaptation for the spring dispersal event in which Chaetogaster disperse from the previous year’s cohort of large snails to the newly-hatched cohort. Finally, even though Chaetogaster eat larval trematodes and asexually reproduce rapidly on infected snails shedding trematode cercariae, dispersal rates were lower from uninfected snails to trematode-infected snails than they were from infected to uninfected snails. Understanding Chaetogaster dispersal can help us to understand Chaetogaster population dynamics.  In addition, these findings can aid the understanding of population and dispersal dynamics in other ectosymbiont systems.