Lucía de la Canal

The sandy beach isopod Excirolana armata is a widespread intertidal species in the Southwestern Atlantic, occurring across a wide range of environmental conditions. While ecological and genetic evidence suggest population-level divergence along the Uruguayan coast, differences in tolerance to combined environmental stressors remain largely unexplored. This study evaluated the acute survival of E. armata populations from two contrasting sites: an estuarine beach and an oceanic beach. A fully factorial laboratory experiment was conducted, exposing individuals to six salinity levels (0 to 50) and two temperatures (20 °C and 30 °C) during 5 days. Survival probability was modeled using binomial logistic regression with bias correction for complete separation. Our results show that salinity is the primary determinant of survival for both populations, with temperature affecting survival only near optimal salinity levels. Estuarine populations exhibited higher tolerance to low salinity, whereas oceanic populations performed better under high, stable salinities but showed reduced plasticity to low salinities. The observed survival patterns align with the species’ ecological distribution and previously documented genetic structuring, suggesting population-level differentiation associated with local salinity regimes. Salinity acts as a dominant stressor that masks thermal effects, which only emerge under optimal osmotic conditions. Our results underscore the need to account for hierarchical interactions and population-specific responses to accurately assess coastal macrofauna vulnerability to climate change.