Effect of starvation on parasite-induced mortality in a freshwater snail (Potamopyrgus antipodarum)

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 o n their intermediate snail host, Potamopyrgus antipodarum. One of the trema tode species,Microphallus sp., encysts in the snail host and the encysted l arvae "hatch" following ingestion of infected snails by birds. The other sp ecies, Notocotylus gippyensis, by contrast, releases swimming larvae; inges tion 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 l aboratory, and followed the survival of infected and uninfected snails unde r two conditions: not fed and fed ad libitum. Mortality of the infected hos ts was higher than mortality of the uninfected ones, but the response to st arvation treatment was parasite species specific. N. gippyensis induced sig nificantly 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 t he probability of transmission to the final host.