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Predictions of the International Panel for Climate Changes on sea level rise foresee that the number of coastal regions impacted with salinization will increase in a near future. The present work intended to evaluate the sensitivity to salinization of two freshwater vertebrate species (the frog Pelophylax perezi and the fish Lepomis gibbosus) and their ability to acclimate to this stressor. For this, three specific objectives were targeted: (i) to assess if NaCl may be used as a safe surrogate for risk assessment of seawater (SW) intrusion for freshwater vertebrates; (ii) to evaluate the sensitivity of two freshwater vertebrate models to increased salinity (both due to NaCl or SW); (iii) to determine the capacity of the studied species to acclimate to low levels of salinization. To assess specific objectives (i) and (ii), organisms were exposed to serial concentrations of NaCl or SW dilutions. To assess the capacity of acclimation of both species to salinization, organisms were exposed to low serial concentrations of NaCl during the embryonic development or for a period of two months, respectively, and their sensitivity to NaCl was re-evaluated after this period.

Results showed that fish juveniles were more tolerant (96-h LC50 of 21.3 mS cm(-1) for NaCl and 23.6 mS cm(-1) for SW) than frog embryos (96-h LC50 of 10.7 mS cm(-1) for NaCl and 10.7 mS cm(-1) for SW) and tadpoles (96-h LC50 of 19.4 mS cm(-1) for NaCl and 8.72 mS cm(-1) for SW). The fish was able to cope with conductivities of almost one third of SW conductivity, while effect conductivities computed for the amphibian were much lower than SW conductivity (approximate to 52 mS cm(-1)). The two-fold difference between the sensitivity of the two tested species reinforces the idea that ecological risk assessment for amphibians based on fish toxicity data may underestimate the risk to the former. Acclimation to low levels of salinity caused an increase in tolerance to salinization in P. perezi tadpoles but not in fish.