The effect of diet and reproductive maturity on the growth and reproduction of Helisoma anceps (Pulmonata) infected by Halipegus occidualis (Trematoda).

Inhibition of snail reproductive activity by larval trematodes often results in increased growth of the snail, termed gigantism, during which the snail is hypothesized to allocate excess energy normally used for reproduction to somatic growth. Using the planorbid snail Helisoma anceps and the hemiurid trematode Halipegus occidualis, this hypothesis was examined by raising snails on low and high quality diets under 3 infection conditions: uninfected (UNI) and infected prior to (IPR) and after (IAR) reproductive maturity. Snails exhibited gigantism as an increase in wet mass only in the IPR condition on a high quality diet; snails in the IAR condition were not significantly different in size from UNI controls. Throughout the experiment, snails raised on a high quality diet were larger than those on the low quality diet, regardless of infection status, primarily because of increased growth rates prior to the onset of reproduction. Egg production by UNI snails on the high quality diet was 20 times higher than snails on the low diet, even though diet quality only differed by a factor of 4, and infected snails (both IPR and IAR) showed significantly reduced reproductive activity compared to UNI controls. The number of cercariae shed by each snail on a daily basis was not significantly different between IPR and IAR conditions when each diet quality was examined separately; more cercariae were produced by snails raised on the high quality diet. The prevalence of infection at the end of the experiment was lowest among snails raised on the low quality diet, suggesting the diet, i.e., energy intake, influences the loss of infections. Snail mortality did not differ among the various treatment conditions. These data suggest that gigantism is dependent upon snail reproductive maturity at the time of infection and food quality, and comparison to growth rates and fecundity data from previous field studies indicates that H. anceps in the field are probably protein deprived. Gigantism does not appear to be adaptive for the snail because survival was not increased, or for the parasite because cercariae production was the same for snails exhibiting gigantism or not. These results also stress the need for caution when examining the fecundity and growth rates of both uninfected and infected snails in the laboratory, especially when comparing these data to other laboratory or field studies.