Ecological Correlates of Male Outcrossing Ability in a Simultaneous Hermaphrodite Snail

Abstract

Phally, a genital dimorphism found in some species of self-compatible simultaneous hermaphrodites, presents an opportunity to examine factors maintaining outcrossing within an animal species that can self-fertilize. Both aphallics and euphallics can self-fertilize, but aphailics cannot donate sperm because they do not develop a functional prostate and penis. In this field study of Nigerian populations of the freshwater gastropod, Bulinus truncatus (Mollusca: Pulmonata), we evaluate ecological correlates of euphally to test hypotheses predicting a selectire advantage of outcrossing due to the production of genetically variable offspring. The prevalence of euphally across 49 populations ranged from 0% to 81%. We found no association between prevalence of euphally and any of the following: population density, snail density, mollusk species abundance, water pH, oxidation-reduction potential, dissolved oxygen content, habitat instability (as estimated by habitat type), human activity, vegetation density, desiccation rate, and water chemistry variation. There was a significant but weak correlation between conductivity and proportion of euphallics (r2 = 10%), with lower ion concentrations favoring higher levels of euphally. The prevalence of the most abundant trematode taxon (Xiphidiocercariae) correlated positively with proportion of euphallics, explaining 10% of the variation in proportion of euphallics after we controlled for mean snail age and time of year. Trematode richness (number of taxa per population) was not associated with proportion of euphallics across sites when we controlled for time of year. However, indexes of trematode diversity that incorporated both prevalence and richness did correlate significantly with proportion of euphallics. These results are consistent with hypotheses predicting an advantage of outcrossing due to temporal fluctuations in the biotic environment.