Global diversification of mangrove fauna: a molecular phylogeny of Littoraria (Gastropoda: Littorinidae)

The genus Littoraria is one of very few molluscan groups that are closely associated with mangroves. We document its global evolutionary radiation and compare biogeographic patterns with those of mangrove plants, based on phylogenetic and fossil evidence. Using sequences from three genes (nuclear 28S rRNA, mitochondrial 12S rRNA and COI) we reconstruct a phylogeny of 37 of the 39 living morphospecies. Six monophyletic subgenera are defined (Bulimilittorina, Lamellilitorina, Littoraria, Palustorina, Protolittoraria, Littorinopsis) and we synonymize L. coccinea and L. glabrata. A deep division between Palustorina from the Indo-West Pacific and Littoraria from the Atlantic and Eastern Pacific is estimated by a Bayesian relaxed-clock method to be of Middle Eocene to Palaeocene age (43.2–62.7 Ma), which far predates the Early Miocene (18 Ma) closure of the Tethyan Seaway; this, as in mangrove plants, may reflect vicariance by climatic cooling, rather than tectonic processes. The age of Littoraria angulifera in the Atlantic is, however, consistent with Early Miocene vicariance of a Tethyan ancestor. We infer that speciation events are mainly of Miocene or older age, and that diversification has not been driven by depletion of mangrove habitats during recent glacial intervals. Parsimonious reconstruction of ancestral habitats suggests that the genus has inhabited mangrove or wood substrates since its origin, while the rock-dwelling habit of the four members of Protolittoraria is derived. Three species span the Eastern Pacific Barrier, and one is amphi-Atlantic, consistent with a long larval phase of up to 10 weeks. Allopatric speciation is inferred, but usually with subsequent range overlap. Ovoviviparity (interpreted as an adaptation to life in mangroves) has arisen twice.