Snail size as a determinant of prepatent period duration in Helisoma anceps infected with Halipegus occidualis (Digenea : Hemiuridae)

Pulmonate snails (Helisoma anceps) of varying sizes were exposed to 2, 4, 8 , or 16 eggs of Halipegus occidualis and observed over several months to de termine the duration of the prepatent period. infection probability was pos itively correlated with number of eggs ingested. The number of eggs ingeste d did not, however, have a significant effect on the duration of the prepat ent period, presumably because low infectivity of the eggs (15%) dictated t hat most patent infections arose from a single miracidium. There was a sign ificant, positive correlation between snail length (measured at time of exp osure) and duration of prepatent period, suggesting that density-dependent effects are, in part, responsible for the initiation of cercariae productio n in H. occidualis.