Under a scenario of increased hypoxic stress in aquatic habitats (eutrophication and climatewarming), the ability tomaintain theMO2 independently of declining DO values may become an essential adaptation for organisms to inhabit the resulting oxygen-depleted environments. Thus, some native species could become more vulnerable to competitive displacement by less oxygen-sensitive invader species.
However, although P. macrodactylus seems to have a more efficient metabolism (low MO2) and a higher tolerance to hypoxic conditions in brackish water at a moderate temperature, the results of this study suggest that this competitive advantage over its native counterpart P. longirostris may disappear under a climate warming scenario. Conversely, the native P. varians showed relatively high oxygen independence irrespective of the temperature in brackish water, the salinity conditions in which the species was usually collected in this study. Furthermore, it is expected that the strongly euryhaline P. varians, a typical inhabitant of more isolated and stressful estuarine habitats, would have higher phenotypic plasticity, and consequently individuals from populations adapted to more saline waters may show high oxygen independence irrespective of temperature at high salinities. Thus, for more tolerant but less metabolically efficient species, such as P. varians, a potential scenario of climate warming and eutrophication could represent a clear adaptive advantage. In conclusion, results of this study corroborated the hypothesis that, within each higher taxa, the level of mobility/activity of species is among the main factors determining the MO2 of aquatic organisms. However, the different level of oxygen regulation displayed by studied species was more related with environmental characteristics of their common habitats than with their metabolic resting rates and/or mobility. Namely, this study evidence that estuarine species, and that of associated habitats, are more tolerant of environmental stress, as salinity changes and hypoxia, than are fully marine or freshwater species. In order to overcome the difficult interpretation of decreased MO2 within individuals of a same species, it is suggested that a simultaneous evaluation of respiration rates and their dependence on DO could be an integrative way of determining the potential effects of increased hypoxia (warming and eutrophication) on aquatic species fromdifferent ecological guilds within estuaries and associated habitats.