Abstract:

The endemic diplommatinid land snails (Caenogastropoda: Mollusca) of Belau (Republic of Palau, Micronesia) are an exceptionally diverse group of largely undescribed species distributed among rock and leaf litter habitats on most of Belau's 586 islands. Diplommatinid shell morphology (e.g. shell sculpture) reflects habitat type. In this study, I analyzed a subset of the 90 diplommatinid species representing a broad geographical spread of islands in order to reveal the species' phylogenetic relationships and biogeography within the Belau archipelago. Diplommatinid species from the islands of Yap, Pohnpei, Kosrae and Guam are also included in the analysis. One nuclear (28S rRNA) and two mitochondrial (16S rRNA, COI) gene regions, comprising 1906 bp were used for phylogenetic reconstruction. Results show that there is little evidence for in situ radiation on individual islands. Instead, different colonists resulted in two clades of diplommatinids, one of which contains rock-dwelling species, and the other that contains leaf litter dwelling species. Leaf litter dwelling species have ribbed shell morphologies and rock dwellers have lamellate (spined) or heavily calcified shell morphologies and subtle morphological differences within these groups are sufficient for species delineation. Species within each of these clades may be ecologically equivalent and therefore weakly interacting. Species within each clade have potentially diversified non-adaptively and allopatrically, by isolation on the many islands of Belau. Although discovering the relationship between Belau diplommatinids and other diplommatinids in the region would require extensive sampling throughout the Pacific and tropical Asia, it is clear that Yap and Guam species must be included as part of a single Belau/Yap/Guam radiation. Pohnpei, Kosrae (Caroline Islands) species and New Guinea and Borneo species are distantly related to Belau diplommatinids. This work is an important addition to the small number of detailed evolutionary studies in existence for Pacific island land snails. This study also suggests that island radiations may come in different forms and the details of species' ecology and islands' geological history may, perhaps unsurprisingly, dramatically impact evolutionary patterns.