Distance to coastline modulates morphology and population structure in a coastal amphibian

Salinization due to sea-level rise and marine submersions is expected to strongly impact coastal ecosystems. Exposure to salinity can negatively impact biodiversity especially in coastal wetlands. To understand comprehensively the consequences of environmental salinization on coastal biodiversity, it is essential to document how coastal species currently respond to exposure to salinity. In this study, we investigated how variations of environmental salinity relative to the distance to the ocean influence population structure (age ratio and sex ratio), and individual hydro-mineral balance (osmolality) and morphology (size, mass, condition) in the western spadefoot toad (Pelobates cultripes) in two populations from the French Atlantic coast. We show that distance to coastline strongly influences exposure to salt on a small spatial scale. Some variables (e.g., abundances, osmolality, morphology) responded similarly in both sites and may be related to salt deposition due to landward sea-spray. Interestingly, other variables (sex ratio and age ratio) displayed site-specific responses and appeared to be linked to the salinity of breeding sites. Distance to the shoreline appears to be a critical population structuring factor in this coastal salt-tolerant species. Future studies should investigate how distance to shoreline-and thus environmental salinity-can ultimately affect individual performances and fitness.