Thermal effluent exposure reveals early physiological constraints and the bioindicator potential of the invasive pearl oyster Pinctada radiata in a warming Mediterranean Sea.

Thermal pollution from coastal power stations creates persistent local hotspots that can serve as in situ analogues of future ocean warming scenarios. We investigated the physiological and metabolic responses of the invasive pearl oyster Pinctada radiata (Leach, 1814) along a thermal gradient (26.3 °C, 27.1 °C, 28.5 °C) generated by the Delimara Power Station (Malta) cooling-water effluent. Redox-related biomarkers (reactive oxygen species [ROS] content; lipid hydroperoxides [HPs] and protein carbonyls [CO]; susceptibility to oxidants [ΔHPs]; total antioxidant capacity [TAC]; glutathione peroxidase [GPX] and glutathione reductase [GR] activity) and aerobic (cytochrome c oxidase [COX] activity) and anaerobic (lactate dehydrogenase [LDH] activity) metabolism were evaluated in gills and adductor muscles. Gills exhibited no significant differences at 27.1 °C as compared to 26.3 °C. At 28.5 °C, reduced TAC, GPX, and GR, and increased HPs were indicative of oxidative damage. The adductor muscle exhibited a temperature-dependent response: at 27.1 °C, increased TAC, GPX, and GR, and reduced ΔHPs and CO, compared to 26.3 °C, were indicative of improved protection against oxidative damage; at 28.5 °C, the antioxidant profile was comparable to that at 26.3 °C. Interestingly, the increased ΔHPs and reduced TAC and GPX at 28.5 °C compared to 27.1 °C suggested a constraint of compensatory antioxidant mechanisms under higher thermal load. Metabolic analyses revealed unchanged COX activity in gills but reduced COX activity in the adductor muscle together with decreased LDH activity in both tissues, suggesting coordinated metabolic downregulation under prolonged thermal exposure. In conclusion, P. radiata exhibited thermal tolerance, although compensatory mechanisms became progressively constrained at the highest temperature. Further warming may therefore impair performance or shift distribution toward higher latitudes or deeper waters.