Shell shape and habitat use in the North-west Pacific land snail Mandarina polita from Hahajima, Ogasawara: Current adaptation or ghost of species past?

The endemic land snail genus Mandarina of the Ogasawara Islands provides an excellent model system to investigate adaptive radiation. Previously, it has been shown that coexisting species of the islands segregate by microhabitat, so that they are either predominantly found on the ground in relatively wet and sheltered sites, dry and exposed sites, or else are arboreal. Moreover, shell morphology correlates with microhabitat, so that species in wet and sheltered sites tend to have high-spired shells with a high aperture, and those in dry and exposed sites tend to have relatively low-spired shells with a wide aperture. We have now found that on Hahajima, Mandarina polita have variable shell morphology, and there is a correlation between morphology and the depth of leaf litter, as well as the presence/absence of other terrestrial species. Specifically, when high-spired terrestrial Mandarina ponderosa is present, M. polita tend to be low-spired and have a large aperture, indicative of character displacement. When M. ponderosa is absent, the shell shape of M. polita is much more variable, the overall spire is higher, individuals are found in deeper litter, and there is a strong correlation between litter depth and spire height. We argue that these patterns are due to local adaptation, but it remains possible that they are an artefact due to the 'ghost of species past'.