Spatial ecology, habitat and speciation in the Porto Santan land snail genus Heterostoma

The Madeiran land snail genus Heterastoma expresses two forms of genital anatomy. One form (hemiphallic) lacks the distal male organs while the other (euphallic) has fully developed hermaphroditic genitalia. Recent evidence suggests this genital variation characterizes two sibling or incipient species. The spatial ecology and habitat associations of these taxa are described in the light of recent ideas on the role of ecology in speciation. Hemiphallic snails are found at a lower density and show a tendency to be more separated from each other compared to euphallic snails. Both taxa are more likely to be closer to other members of the same taxon. The genus as a whole is not found in the presence of sand or pale rock. However, when only areas actually containing snails are examined, hemiphallics show a positive association with sand and rock while euphallics continue to show a negative association. The differences in population density and spacing may suggest an adaptive explanation for reduction of male organs in hemiphallics since this taxon would be expected to have significantly fewer mating encounters. The observed differences in habitat association may suggest a mechanism for sympatric or parapatric divergence in keeping with current models of habitat-driven adaptive speciation. Both of these factors (selection on the mating system in a marginal environment and small scale separation based on habitat) may have been significant in the divergence of hemiphallic and euphallic taxa.