Size and survival of two freshwater snail species in relation to shedding of cercariae of castrating Echinostoma spp

Trematode-induced castration of snails is widespread and can lead to other life history changes of snails such as changes in trajectories of size and growth or survival. The changes produced likely depend on whether the parasite or host controls allocation of host resources remaining after partial or complete cessation of host current reproduction by castrating trematodes. Documenting host life history changes, like changes in host size in response to castration, is a first step in assessing whether these changes are beneficial to the parasite (increasing transmission success) or to the host (outliving the infection) or to neither. Herein, we test for differences in size and survival among individuals of two snail species in relation to infection by Echinostoma spp. trematodes. Active shedding of Echinostoma spp. was associated with castration of all Stagnicola elodes snails from a site in Eastern Ontario. Snails actively shedding cercariae were not different in size from non-shedding, egg-laying snails but had a higher mortality than egg-laying snails. Active shedding of Echinostoma spp. cercariae was also associated with castration of nearly all Helisoma trivolvis monitored, from a site in Southwestern Ontario. Actively shedding, non-laying H. trivolvis hosts were smaller on average than non-shedding egg-laying hosts, but both non-laying and egg-laying snails survived equally well. We discuss these results in light of what is known about effects of castration on snail hosts in terms of growth and survival for these and other trematode species and speculate on whether changes in size or survival benefits parasite or host.

Host life history and host-parasite syntopy predict behavioural resistance and tolerance of parasites

There is growing interest in the role that life-history traits of hosts, such as their 'pace-of-life', play in the evolution of resistance and tolerance to parasites. Theory suggests that, relative to host species that have high syntopy (local spatial and temporal overlap) with parasites, host species with low syntopy should have lower selection pressures for more constitutive (always present) and costly defences, such as tolerance, and greater reliance on more inducible and cheaper defences, such as behaviour. Consequently, we postulated that the degree of host-parasite syntopy, which is negatively correlated with host pace-of-life (an axis reflecting the developmental rate of tadpoles and the inverse of their size at metamorphosis) in our tadpole-parasitic cercarial (trematode) system, would be a negative and positive predictor of behavioural resistance and tolerance, respectively. To test these hypotheses, we exposed seven tadpole species to a range of parasite (cercarial) doses crossed with anaesthesia treatments that controlled for anti-parasite behaviour. We quantified host behaviour, successful and unsuccessful infections, and each species' reaction norm for behavioural resistance and tolerance, defined as the slope between cercarial exposure (or attempted infections) and anti-cercarial behaviours and mass change, respectively. Hence, tolerance is capturing any cost of parasite exposure. As hypothesized, tadpole pace-of-life was a significant positive predictor of behavioural resistance and negative predictor of tolerance, a result that is consistent with a trade-off between behavioural resistance and tolerance across species that warrants further investigation. Moreover, these results were robust to considerations of phylogeny, all possible re-orderings of the three fastest or slowest paced species, and various measurements of tolerance. These results suggest that host pace-of-life and host-parasite syntopy are powerful drivers of both the strength and type of host defence strategies against parasites. Future research should evaluate how often and how strongly host pace-of-life and host-parasite syntopy are correlated and which is the better predictor of the strength and type of host investments in anti-parasite defences.