Whether or not organisms become infected by parasites is likely to be a complex interplay between host and parasite genotypes, as well as the physiological condition of both species. Details of this interplay are very important because physiology‐driven susceptibility has the potential to confound genetic coevolutionary responses. Here we concentrate on how physiological aspects of infection may interfere with genetic‐based infectivity in a snail–trematode (Potamopyrgus antipodarum/Microphallus sp.) interaction by asking: (1) how does host condition affect susceptibility to infection? and (2) how does host condition affect the survival of infected individuals? We manipulated host condition by experimentally varying resources. Contrary to our expectation, host condition did not affect susceptibility to infection, suggesting that genetics are more important than physiology in this regard. However, hosts in poor condition had higher parasite‐induced mortality than hosts in good condition. Taken together, these results suggest that coevolutionary interactions with parasites may depend on host condition, not by altering susceptibility, but rather by affecting the likelihood of parasite transmission.

Understanding the mechanisms that species use to succeed in new environments is vital to predicting the extent of invasive species impacts. Food quality is potentially important because it can affect population dynamics by affecting life history traits. The New Zealand mudsnail, Potamopyrgus antipodarum, is a worldwide invader. We examined how mudsnail growth rate and fecundity responded to the C:P ratio of algal food in laboratory conditions. Mudsnails fed low-P algae (C:P 1,119) grew more slowly, matured later, produced smaller offspring, and grew to a smaller adult size than snails reared on algae with high levels of P. A relatively small increase in algal C:P (203-270) significantly increased mudsnail age at maturity. We suggest that the relatively high body P requirements of mudsnails make them susceptible to allocation trade-offs between growth and reproduction under P-limited conditions. The elemental composition of algae varies greatly in nature, and over half of the rock biofilms in streams surveyed within the introduced range of mudsnails in the Greater Yellowstone Area had C:P ratios above which could potentially pose P limitation of life history traits. High growth rate and fecundity are common traits of many species that become invasive and are also associated with high-P demands. Therefore, fast-growing consumers with high P demands, such as mudsnails, are potentially more sensitive to P limitation suggesting that limitation of growth and reproduction by food quality is an important factor in understanding the resource demands of invasive species.

Evolutionary and ecological factors that explain natural variation in ploidy level remain poorly understood. One intriguing possibility is that nutrient costs associated with higher per-cell nucleic acid content could differentially influence the fitness of different ploidy levels. Here, we test this hypothesis by determining whether access to phosphorus (P), a main component of nucleic acids, differentially affects growth rate in asexual freshwater snails (Potamopyrgus antipodarum) that differ in ploidy. As expected if larger genomes generate higher dietary P requirements, tetraploid P. antipodarum experienced a more than twofold greater reduction in growth rate in low-P versus high-P conditions relative to triploids. Mirroring these results, tetraploid P. antipodarum also had a significant reduction in body P content under low P relative to high P, whereas triploid body P content was unaffected. Taken together, these results set the stage for the possibility that P availability could influence the distribution and relative frequency of P. antipodarum of different ploidy levels. These findings could be applicable to many other animal taxa featuring ploidy-level variation, which includes many mixed sexual/asexual taxa.

Abstract. This study was undertaken to determine whether intraspecific variation in shell morphology of the freshwater snail Elimia (=Goniobasis) livescens is caused by predator‐induced morphological changes. Juvenile snails from 3 populations were grown in the presence of effluent from predatory crayfish feeding on conspecific snails or in effluent from conspecific snails only. Snails from one population, Clear Creek, exhibited a predator‐induced morphology; they grew a narrow body whorl when exposed to the effluent from crayfish. Experimental feeding trials with crayfish and snails from Clear Creek were conducted to determine whether a narrow body whorl reduced predation in the presence of the crayfish Orconectes propinquus. In the feeding trials, snails with a narrow body whorl were eaten less frequently than snails with wider shells. However, there was no difference in overall size (length) between snails that were eaten and those that were not. Thus, juvenile snails from Clear Creek exposed to the presence of crayfish were induced to develop relatively narrow shells, which reduce the risk of successful attack by these crayfish. Hence the induced shell morphology is probably a defense against predation by crayfish.

Parasitism can affect size in gastropods by altering the host's growth rate, but other morphological effects of parasitism have rarely been examined. In this study, the relationship between variation in host morphology and parasitism was examined in a population of the freshwater snail Elimia livescens. Differences were found in the morphology of snails infected with the digenean Proterometra macrostoma and uninfected snails. In order to differentiate between 2 hypotheses to explain these differences in morphology, snails were experimentally infected in the laboratory and several morphological traits were measured after 180 days. One hypothesis suggests that parasite-induced changes in shell development explain differences in morphology between infected and uninfected snails. The other hypothesis suggests that selective mortality of infected hosts explains the difference. In the experiment, differences were found between infected snails and uninfected snails in overall size but not in any measurements of shape. The short duration of the experiment relative to the duration of most infections may account for why field-infected snails differed in shape but experimentally infected snails did not. Parasite-induced changes in growth rate are the most likely explanation for the larger size of infected snails relative to uninfected snails.