Comparative phylogeography and species boundaries in Echinolittorina snails in the central Indo-West Pacific

Aim This study aimed to test monophyly and geographical boundaries in five marine intertidal snail species from the central Indo‐West Pacific. We tested the prediction that phylogenetic breaks between the Indian and Pacific Ocean basins should be more pronounced in continental than oceanic settings, and sought common geographical patterns of interspecific boundaries and intraspecific phylogenetic breaks in the region. Location The tropical seas of the Indo‐West Pacific. Methods We sequenced over 1200 bp of the mitochondrial cytochrome oxidase subunit I gene (COI) from 18–92 individuals sampled from throughout the ranges of each of five species of Echinolittorina (Littorinidae): three members of the Echinolittorina trochoides species complex; Echinolittorina reticulata; and Echinolittorina vidua, together with sister species, in order to test species boundaries. In addition, 630 bp of the nuclear 28S rRNA gene were sequenced from E. reticulata and its sister Echinolittorina millegrana. Phylogenetic structure was assessed using neighbour‐joining and parsimony analyses. Results COI data confirmed species boundaries and geographical distributions for all species except the pair E. reticulata and E. millegrana, which were nevertheless reciprocally monophyletic for 28S rRNA. The species from ecologically ‘continental’ habitats (E. trochoides A and E. vidua, but not E. trochoides B) mostly showed strong interoceanic breaks (with age estimates 0.58–4.4 Ma), while the ecologically ‘oceanic’E. trochoides D and E. reticulata did not. The sister species E. trochoides A and B occupy the shores of the continental shelves of Southeast Asia and Australasia respectively; between them lies the oceanic ‘eastern Indonesian corridor’ occupied by E. trochoides D and E. reticulata. The widespread continental species E. vidua showed a complex pattern of deep division into six haplotype clades with apparently parapatric distributions. Main conclusions Our results show that ecological differences (in this case continental vs. oceanic habitat) influence both intraspecific phylogenetic structure and interspecific boundaries in these snails of intertidal rocky shores. Two of the three species restricted to continental shelves show phylogenetic breaks between the Indian and Pacific Oceans, consistent with vicariant separation during Plio‐Pleistocene low sea levels. The two oceanic species do not show breaks, suggesting that they maintained interoceanic connections through the eastern Indonesian corridor. The geographical location of the interspecific boundary between continental E. trochoides A and oceanic E. trochoides D mirrors intraspecific breaks reported in other species. The sister relationship of E. trochoides A and B in Asia and Australasia, respectively, is an example of a ‘marine Wallace's line’ distribution, and we suggest that it is the result of separation of two continental species by a barrier of unsuitable oceanic habitat.     

Speciation and dispersal along continental coastlines and island arcs in the Indo-West Pacific turbinid gastropod genus Lunella

Species trees were produced for the Indo-West Pacific (IWP) gastropod genus Lunella using MrBayes, BEAST, and *BEAST with sequence data from four genes. Three fossil records were used to calibrate a molecular clock. Eight cryptic species were recognized using statistical methods for species delimitation in combination with morphological differences. However, our results suggest caution in interpreting ESUs defined solely by the general mixed Yule Coalescent model in genera like Lunella, with lower dispersal abilities. Four almost entirely allopatric species groups were recovered that differ in ecology and distribution. Three groups occur predominantly along continental coastlines and one occurs on island arrays. Sympatric species occur only in the torquata and coronata groups along coastlines, whereas species in the cinerea group, distributed in two-dimensional island arrays, occur in complete allopatry. Dispersal along island arcs has been important in the maintenance of species distributions and gene flow among populations in the cinerea group. The emergence of new islands and their eventual subsidence over geological time has had important consequences for the isolation of populations and the eventual rise of new species in Lunella.