Life on the beach: Movements and growth of a coastal amphibian vary with distance to the sea

Proximity to the seashore is a critical structuring factor of coastal wetlands due to its influence on varying sources of nutrients (marine and terrestrial) and on the spatial gradient of salinity (higher salinity closer to the seashore). The spatial gradient of salinity may impact organisms because most organisms need to maintain an osmotic balance. Osmoregulation is energetically costly, and exposure to salinity should induce a trade-off in energetic allocation between osmoregulation and other competing functions such as growth rates and movement patterns. In this study, we used a capture–mark–recapture design during 3 consecutive years to investigate how distance to the sea influences growth rates and foraging movements (outside reproduction) in the western spadefoot toad (Pelobates cultripes), a typical coastal amphibian, in three populations from the French Atlantic coast. Growth rates were lower in larger individuals and in individuals living closer to the seashore. Distances travelled between captures were very limited (~20 m). Between years, these distances were larger for individuals located on the beach rather than inland, but were not influenced by body size or sex. Exposure to salinity and associated costs of osmoregulation may explain lower growth rates closer to the shoreline. The mechanisms underlying the effects of location on distances between captures remain to be identified, but may be related to foraging for abundant prey items on the wrack line. Our study confirms the remarkable terrestrial site fidelity in P. cultripes, which might be detrimental to coastal populations if localized perturbations affect coastal environments.