<br> Feral individuals of Pacific oysters (<br> Magallana gigas<br> Thunberg, 1793) in natural habitats are increasingly being reported from the Bulgarian coast. Studies on the interactions of the Pacific oyster with native species in local ecosystems are contradictory and it is not yet definitely established to what extent ecological conditions in the Black Sea are favourable for the Pacific oyster growth and reproduction.<br> <br> For the first time in this study, an assessment has been made of the adaptive capacity and resilience of the alien Pacific oyster allowing its development in the ecological conditions of the Bulgarian Black Sea ecosystems and the risk of a future mass invasion.<br> Oysters were gathered manually by SCUBA diving from natural habitats or obtained from shellfish farms. A set of major oxidative stress (OS) indicators: lipid peroxidation, protein oxidation, glutathione level and activity of anti-oxidant enzymes: superoxide dismutase, catalase, glutathione peroxidase, glutathione reductase and glutathione-S-transferase, were measured spectrophotometrically, using commercially available kits. The adaptive capacity and resilience of the Pacific oysters were assessed by the integral Specific Oxidative Stress (SOS) index.<br> <br> The results indicate that<br> M. gigas<br> from more polluted localities have higher OS, but it was effectively compensated by their anti-oxidant system. A comparison was also made between<br> M. gigas<br> OS and that of the native species<br> Mytilus galloprovincialis<br> Lamarck, 1819, using the SOS index. In some localities, Pacific oysters had an even lower SOS index than the native black mussels.<br> <br> The level of adaptive capacity of the oysters, as indicated by the SOS index, was compared to the phases of Holling’s adaptive cycle theoretical model and showed that the Bulgarian Black Sea oyster population is in an initial growth stage (resource exploitation), which confirms its high adaptive potential and resilience.<br> In conclusion, this study confirms that the Pacific oyster possesses the capacity to cope with the marine environment and the native species, which allows further expansion of the oyster population in the Bulgarian Black Sea.<br> <br> More research and monitoring of the<br> M. gigas<br> population, along with assessment of their impact on biodiversity and the local ecosystems, are urgently needed for taking adequate management decisions.<br>
