Dynamics in brood chamber pH of the European flat oyster (Ostrea edulis) in response to ocean acidification

Abstract

Ocean acidification is posing a threat to marine bivalve species who struggle to deposit calcium carbonate in order to grow their shell. Some oyster species have developed a brooding reproductive strategy which might help them cope with this acidification stress. Brooding oysters have shown to be more resilient against ocean acidification than broadcast spawning oyster species. It is suspected that because the brood chamber is on top of the maternal gills, the mothers add carbon dioxide into the chamber from her respiration. This suggests larvae evolved to develop in a more acidic environment than the surrounding water column. Through exaptation the larvae may have coopted traits needed for development in the brood chamber which now enable them to be more resilient to ocean acidification. In this study, we measured the pH inside the brood chamber of Ostrea edulis under current and future predicted ocean conditions (i.e., elevated temperature and decreased seawater pH) to get a better understanding of the ambient-maternal relationship on brood chamber pH fluctuations under ocean acidification scenarios. The results suggested that maternal respiration indeed makes the brood chamber always a more acidified environment then the surrounding water. Elevated temperatures in the surrounding water slightly lower the pH as a result of increased maternal metabolism. Yet lowering the ambient pH causes a much larger and significant reduction of internal pH levels since the oyster is constantly filtering the overlaying water while the valves open. Additionally, there seems to be a positive relationship between shell gape and internal pH changes suggesting that the mothers behaviour may also influence how fast and to which level pH values can drop inside the brood chamber. These results give an indication of what conditions brooding oysters larvae will have to face in the future and helps determine possible winners and beneficial strategies in an acidified ocean.