Warming and acidification-mediated resilience to bacterial infection determine mortality of early Ostrea edulis life stages

The combined effects of temperature and seawater acidification were investigated across larval stages of the European flat oyster Ostrea edulis, from veliger sizes released by gravid individuals to spat. Simultaneous experiments were also conducted to investigate the potential effects of reduced pH levels on bacterial growth that could provide a more comprehensive understanding of the effects of seawater acidification on larval mortality. Larvae (veliger, umbonate, and pediveliger) and spat were exposed to 4 temperatures (18, 22, 26, and 30 degrees C) and 4 pH treatments (7.83-7.92 [ambient], low-reduced, medium-reduced, and high-reduced), and the 4 pH treatments were also used in bacterial experiments. Results showed increased larval mortalities at 30 degrees C (by ca. 49 and 44% in veliger and umbonate stages, respectively), although there was also a bottleneck in pediveligers from 22 to 30 degrees C and no effects on spat. In contrast, the survival of veligers increased with pH reductions by ca. 26%, and was marginally increased in pediveligers despite high mortality at this stage. No shell malformations were observed at any larval stage or in spat, and growth patterns tended to mirror those of survival. This coincided with lower bacterial growth, particularly of Vibrio spp., in the 2 lowest pH treatments, suggesting that seawater acidification may help to prevent bacterial pathogenicity in O. edulis larvae. Compared to available information on the vulnerability of other commercial bivalves to ocean acidification, our results suggest that O. edulis could be a more resilient species; however, further research is needed to investigate the potential effects on gravid females and sperm.