Egg size variation as a function of environmental variability in parasitic trematodes

Abstract

Environmental predictability is often assumed to select for variability in egg size or offspring size, since a greater variance in offspring size may insure that, on average, a greater proportion of offspring will survive whatever the conditions experienced. In a comparative analysis across 747 species of parasitic trematodes, we examined the relationship between intraspecific variability in egg size and three predictors of environmental stability: the type of definitive host, the type of habitat in which parasite eggs are released, and latitude. Although trematode species using ectothermic hosts and (or) releasing eggs in terrestrial habitats tended to have more variable egg sizes than species using endothermic hosts and (or) releasing eggs in water, the trend was not significant, even when controlling for phylogenetic influences. Latitude correlated strongly and negatively with variability in egg sizes among trematode species releasing their eggs in terrestrial habitats, whether or not phylogenetic effects were removed. No relationship was found among species releasing their eggs in water. We propose that spatial heterogeneity of the external environment, and not its temporal stability, has played an important role in shaping intraspecific variability in egg sizes in parasitic trematodes.