Microphallus

Abstract: Host-parasite interactions that result in host castration are evolutionarily similar to predator-prey interactions because both interactions terminate reproduction for the host or prey Yet, host-parasite interactions differ from predator-prey interactions in that infected hosts remain alive and potentially can make adjustments to their life-history strategy before castration is complete Here we exposed juvenile snails (Potamopyrgus antipodarum) to infection by a digenetic trematode (Microphallus sp) in order to determine whether: (1) pre-reproductive individuals could be infected, (2) individuals that were exposed to infection shifted resources to early reproduction (fecundity compensation), and (3) infected individuals exhibit altered growth rates relative to uninfected individuals We found that juveniles are susceptible to infection; hence P antipodarum could be selected for earlier maturation in populations where the risk of infection is high We also found that fecundity compensation does not occur in this snail Finally, we found that Microphallus-infected snails exhibit altered growth rates; individuals infected as juveniles have lower growth rates and are smaller than uninfected snails These results suggest that growth is altered by infection of a trematode parasite but reproduction in uninfected snails is not induced by exposure to trematode eggs

Abstract: The level of host exploitation is expected, under theory, to be selected to maximise (subject to constraints) the lifetime reproductive success of the parasite. Here we studied the effect of two castrating trematode species on their intermediate snail host, Potamopyrgus antipodarum. One of the trematode species, Microphallus sp., encysts in the snail host and the encysted larvae “hatch” following ingestion of infected snails by birds. The other species, Notocotylus gippyensis, by contrast, releases swimming larvae; ingestion of the snail host is not required for, and does not aid, transmission to the final host. We isolated field-collected snails for 3 months in the laboratory, and followed the survival of infected and uninfected snails under two conditions: not fed and fed ad libitum. Mortality of the infected hosts was higher than mortality of the uninfected ones, but the response to starvation treatment was parasite species specific. N. gippyensis induced significantly higher mortality in starved snails than did Microphallus. Based on these results, we suggest that host exploitation by different species of trematodes may depend on the type of transmission. Encysting in the snail host may select for a reduced rate of host exploitation so as to increase the probability of transmission to the final host.