Evolutionary implications of hemiphally in the land snail Zonitoides nitidus (Gastropoda: Stylommatophora: Zonitidae).

Abstract

Zonitoides nitidus has been known to consist of mixed populations of euphallic and hemiphallic individuals since 1888 in England and 1928 in North America. English populations show high frequencies of euphallics in the fall and spring and high frequencies of hemiphallics during the summer, which is the breeding season. The high proportion of hemiphallics during the breeding season has lead researchers to speculate that it reproduces by self-fertilization. Zonitoides nitidus is also a predator on Fossaria truncatula, the intermediate host of sheep liver fluke, a characteristic which has attracted intense scrutiny to its natural history in Europe. The natural history data have not been interpreted in light of the possibility of the snail selfing. Neither have comparable data on its life history and effects of hemiphally during the breeding season been collected from North American populations. Zonitoides nitidus has one generation per year. Each generation is nearly non-overlapping, hatching between mid July and late August of one year and dying during July and August of the next. The hemiphallics mature before the euphallics, reaching the adult shell diameter of 5 mm as early as November. Euphallics mature during May and June, when they apparently grow much more quickly, becoming on average a full millimeter greater in diameter than the average hemiphallic by late June. During the breeding season, 12.25% of the adults are euphallic; the remaining 87.75% are hemiphallic. A comparison of the ratios of male investment, as measured by volumes of penis, epiphallus, and dart sac, over female investment, as measured by volumes of the albumen and oviduct glands, shows that euphallics invest approximately ten times as much into male function as do hemiphallics. It is inferred from these data that Zonitoides nitidus has a mixed breeding system of selfing and outcrossing. The hemiphallic snails are predicted to produce 89.9% of their offspring by selfing and to outcross as females with euphallics to fertilize the remaining 10.1%. The euphallic snails are predicted to reproduce entirely by outcrossing, apparently as both males and females with each other and as males with the hemiphallics. A review of selfing among mollusks indicates that selfing for reproductive assurance is the best-supported hypothesis to explain selfing within the phylum. However, Zonitoides nitidus lives at densities too high for this hypothesis to be likely. Hypotheses that may explain the likelihood of selfing are that it is an adaptation for long-distance dispersal, locally adapted offspring, or colonization of disturbed habitats.