Thus, in where the MO2 and oxygen independence of shrimp under hypoxic conditions are represented together for each species, the low MO2 on the right (high oxygen independence) and left (low oxygen independence) extremes of the plot could be interpreted, respectively, as good and poor physiological fitness of individuals to these environmental conditions. Indeed, the relatively lower values of MO2 observed for C. crangon and P. longirostris on the extreme left of the plot occurred at a low salinity (S = 1), environmental conditions found within the estuarine stretch inwhich the two species are usually less abundant. Furthermore, at such a low salinity level C. crangon showed osmoregulator difficulty and the partial mortality of individuals. Conversely, the relatively low MO2 value found for P. macrodactylus at a moderate temperature (20 °C) and low salinity (S = 1 and 12) corresponds to environmental conditions in which the studied population is usually abundant. Under the assumption that estuarine species are more tolerant to environmental stress than fully marine or freshwater species as a result of evolutionary selection, it seems reasonable to predict that inhabitants of areas with fluctuating salinity would have higher phenotypic plasticity. In this study, high variability in the oxygen regulatory capacity between individuals of the same species was associated with severely stressful conditions for the species, as observed for individuals of the marine species C. crangon and P. monacanthus at the bottom limit of their respective salinity tolerances. Moreover, under such stressful conditions most survivors of these two species showed low MO2 values and oxygen independence, suggesting poor physiological fitness. When organisms were exposed to unusual environmental changes, new conditions may be: a) physiologically tolerable, leading to physiological changes in individuals (acclimation) and/or increased abundance of tolerant genotypes over generations (adaptation); b) physiologically intolerable, leading to migration or death. According to the results of this study, we hypothesize that the high intraspecific variability in the oxygen regulatory capacity of individuals fromthe same population could be interpreted as a symptom of stress; that is, individuals are being exposed to environmental conditions which are unusual for the species, and consequently the process of acclimation (short-term changes) and/or adaptation (log-term changes) have not yet occurred. Nevertheless, due to the wide range of experimental stress conditions and shrimp’s size, this interpretation of the observed intraspecific variation should be considered with caution. Intuitively, species whose respiratory regulation remains stable at all salinity levels represent the highest adaptation to environments with fluctuating salinities like estuaries, especially in hypoxic conditions. When salinitytemperature related changes in MO2 and the percentage of oxygen independence were analysed together for each species, a better tolerance to the fluctuating salinity conditions (low changes in MO2 and oxygen independence) was observed for C. crangon and P. longirostris, except in very diluted seawater (salinity = 1), as well as for P. macrodactylus, except at salinities ≤ 12 in warm water (25 °C). This suggests that this non-indigenous species is well adapted to the oligohaline zone provided that the temperature remains moderate. Non-indigenous species often occupy stressful empty niches in aquatic systems, where limited predation pressure favours them becoming successfully established.