## 4 Discussion  
By the end of this century, climate models predict an increase in average sea surface temperatures by up to 3 \({}^{\circ}\)C, accompanied by a significant drop in ocean pH of approximately 0.31 units [44]. These changes in ocean conditions are anticipated to profoundly impact marine organisms, especially affecting the growth and physiology of bivalves [17; 19; 23; 24; 25; 26]. Consequently, this study investigated the effects of an increase in sea temperature by 2 \({}^{\circ}\)C, a reduced pH of 0.2 units, and a combination of both, as projected under near-future climate change scenarios, on the growth, survival, and physiological indices of the queen scallop _A. opercularis_. Figure 9: Adductor index (AI), gonadosomatic index (GSI), and hepatosomatic index (HPI) of queen scallops kept in controlled conditions (Con) (mean \(\pm\) SD), and exposed to decreased pH (\(\Delta\)pH), increased temperature (\(\Delta\)T), and combined effect of temperature and pH changes (\(\Delta\)pH) for a month. The means labeled with a star (*) are significantly different from the control (Mann–Whitney test, \(p<0.05\)). The survival rate of the queen scallops in the experimental tanks can be compared with the results of the study with a reduced pH (7.65) in _Mytilus galloprovincialis_ [PERSON] 1819 and _Xenostrobus securis_ [PERSON] 1819 [25]. The two organisms share the same habitat along the shores in the inner part of the Galician Rias Baixas (NW Spain). In the aforementioned study, the mortality of the mussels was recorded to be only 6.52 \(\pm\) 0.95%, and the same low mortality was also noted in this study. While ocean acidification has been shown to cause mortality in bivalves [31; 42], lethal effects are not universally observed [21]. Mortality typically occurs after prolonged exposure to severely reduced pH levels [13]. However, even when their survival is not immediately threatened, acidification frequently disrupts physiological processes, leading to suppressed metabolic activity, impaired ion regulation, and diminished protein synthesis. These physiological disruptions can significantly slow the growth rates of bivalves due to reduced energy availability, which hampers the organisms' ability to maintain their normal growth functions [24; 25; 44]. The results of this study demonstrate a significant impact of a pH decrease on the growth and physiological indices of queen scallops. The decreased pH conditions led to a decrease in weight growth rate and length growth rate, highlighting the detrimental effects of acidification on shell formation. This finding aligns with previous studies indicating that a lower pH disrupts calcification processes in marine organisms, leading to reduced shell growth [22; 23; 45]. Furthermore, these results can be compared with the growth rates of the mussels from the Galician Rias Baixas (NW Spain). [PERSON] et al. [15] found a significant effect of pH on the growth rates of _M. galloprovincialis_, with a 50% reduction in growth under reduced pH (7.65) treatments, whereas no effect of the experimental conditions was detected on the growth rates of the invader _Xenostrobus securis_. Furthermore, the condition index of _M. galloprovincialis_ was noted to be higher with an increase in acidity (pH of 7.65) at 16 \({}^{\circ}\)C, as was noted in this study. The enhanced somatic growth observed under hypercapic conditions, characterized by elevated CO\({}_{2}\) levels, may be attributed to these organisms' ability to prioritize tissue development over shell formation. Under these acidic conditions, soft tissue growth tends to be less affected than shell accretion, which becomes more challenging as pH decreases [15]. Similar patterns have been reported in other marine species, such as the keystone sea star, where researchers found that reduced pH levels stimulated its somatic growth [33; 34; 35; 36; 37], while its calcification processes were significantly inhibited [22; 23].