Allopatry and overlap in a clade of snails from mangroves and mud flats in the Indo-West Pacific and Mediterranean (Gastropoda: Potamididae: Cerithideopsilla)

Abstract

Cerithideopsilla is a genus of potamidid snails found in high abundance on sedimentary intertidal flats and beneath mangrove trees on continental shores in the tropical and subtropical Indo-West Pacific region and Mediterranean Sea. Taxonomic revisions have recognized four species, but recent molecular studies have hinted at a higher diversity. Here, we analyse 377 individuals sampled from across the known range and use a combination of molecular phylogenetic (mitochondrial COI and 16S rRNA, and nuclear 28S rRNA genes), statistical (generalized mixed Yule-coalescent GMYC method) and morphological (shell form) criteria to delimit 16 species. These form four species groups, corresponding with the traditionally recognized species C.alata, C. 'djadjariensis' (for which the valid name is C.incisa), C.cingulata and C.conica. Distribution maps were compiled using museum specimens identified by diagnostic shell characters. In combination with the molecular phylogenetic trees, these suggest an allopatric speciation mode, with diversification centred on the East Asian coastline and northern Australia, and a pronounced gap in the 'eastern Indonesian corridor', an area of low oceanic productivity. There is, however, frequently geographical overlap between sister species and we suggest from several sources of evidence (e.g. presence of C.conica in isolated saline lakes 900 km from the sea) that post-speciation transport by migratory birds has occurred. Nine of the 16 species occur between the Gulf of Tonkin and Hong Kong, so southern China is significant for both the evolution and conservation of Cerithideopsilla species.