Impact of ocean acidification on the Patagonian scallop Zygochlamys patagonica, a key seafood species for the Southwest Atlantic Ocean

Ocean acidification (OA) is a global process decreasing seawater pH. It is a direct consequence of the increase in CO₂ emissions due to human activities, with consequences for marine species and ecosystems. The effects of long-term OA exposure (6 months) followed by 2 months of depuration were analyzed. Mortality, shell growth, shell mass, adductor muscle mass and, gonadal mass condition indexes (CI) were measured in the Patagonian scallop Zygochlamys patagonica, an important seafood of the Southwest Atlantic Ocean. The scallops were exposed at three levels of pH_T, (1) high pH_T: 8.004 ± 0.028 (mean annual pH_T at natural beds), (2) medium pH_T: 7.800 ± 0.067 (minimum value of natural variability recorded at the sampling site) and (3) low pH_T level (acidification treatment): 7.520 ± 0.070 (0.3 below minimum pH_T value found at natural beds). Individuals were analyzed at the beginning of the experiment (T0), 3, 6 and 8 months of experimental time. Scallop mortality rates were different between pH levels and experimental time, with no interaction. Mortality was higher at low pH as compared to high pH treatment, it was also significantly lower during the depuration period as compared to the OA exposure times (p<0.05). Shell growths did not show differences between pH treatments. Shell mass condition indexes at high pH did not differ over time. At low pH_, these indexes were significantly lower after 6 and 8 months as compared to other pH treatments and times (p<0.05). The external shell surface showed a gradual dissolution of the external part of the valve between T0 and 3 and 6 months of exposure to low pH. Shell disarticulation due to ligament damage was also observed in 30% of the animals in the low pH treatments after 6 months of OA exposure, as compared to 0% in the high pH treatment, resulting in loss of swimming ability of scallops under OA. Gonadal and adductor muscle condition indices showed no differences between pH treatments. Gonadal CI increased over time regardless of treatments, showing the natural process of gonadal maturation over time. These results show the vulnerability of this species to future OA conditions with the possible implications for the species' biological interactions and the ecosystem services it provides.