In a widespread species, a matching of phenotypic traits to local environmental optima is generally attributed to site-specific adaptation. However, the same matching can occur via adaptive plasticity, without requiring genetic differences among populations. Adult sea kraits (Laticauda saintgironsi) are highly philopatric to small islands, but the entire population within the Neo-Caledonian lagoon is genetically homogenous because females migrate to the mainland to lay their eggs at communal sites; recruits disperse before settling, mixing up alleles. Consequently, any adaptive matching between local environments (e.g., prey sizes) and snake phenotypes (e.g., body sizes and relative jaw sizes) must be achieved via phenotypic plasticity rather than spatial heterogeneity in gene frequencies. We sampled 13 snake colonies spread along a ~200km northwest-southeast gradient (N>4,500 individuals) to measure two morphological features that affect maximum ingestible prey size in gape-limited predators: body size and relative jaw size. As proxies of habitat quality, we used protection status, fishing pressure and lagoon characteristics (lagoon width and distance of islands to the barrier reef). In both sexes, spatial variation in body sizes and relative jaw sizes was linked to habitat quality; albeit in different ways, consistent with sex-based divergences in foraging ecology. Strong spatial divergence in morphology among snake colonies, despite genetic homogeneity, supports the idea that phenotypic plasticity can facilitate speciation by creating multiple phenotypically distinct sub-populations shaped by their environment.

In the field during long term mark recapture study (2002-2002) on sea snakes. Initiated and supervisedby XB.

The data set provided is extracted from the main data base. Further details can be obtained by contacting XB.