Phenotypic variation contrasts with genetic homogeneity across scattered sea snake colonies

AimIn philopatric species distributed across wide geographical areas, each population is influenced by local factors. We assessed the expected spatial genetic and phenotypic structure across a set of colonies of a highly philopatric sea snake. LocationSouth Lagoon of New Caledonia, Western Pacific Ocean. MethodsWe used body size (N=4915 individuals) to assess phenotypic variations across 11 islet colonies of the amphibious sea kraits (L. saintgironsi) spread throughout the South Lagoon of New Caledonia. We also used 11 microsatellite markers in a subsample of 302 individuals to examine the demographic history and genetic structure. ResultsA major colonization event and rapid population expansion occurred during the mid-Pleistocene. Mean body size increased significantly along the north-south gradient investigated, suggesting that each colony is relatively independent and responds to local conditions. However, our data revealed a total lack of genetic structure despite a substantial genetic diversity. Main conclusionsA peculiar reproductive system likely underlies this pattern that combines a marked phenotypic gradient with a lack of geographical genetic structure. Gravid females from colonies scattered throughout the South Lagoon converge towards few communal coastal nesting sites to lay their eggs. Hatchlings disperse randomly in the lagoon and supply islets with recruits. This process homogenizes the overall genetic structure. However, because when settled snakes are philopatric, the mean body size of each colony presumably reflects the influence of local conditions. This system resembles those documented in various organisms (e.g. fish) but was not previously described in any amniotic lineage.