Imke M. Böök ^1,2* Erik C. Krieger ^1,2 Nicole E. Phillips ¹ Keith P. Michael ³ James J. Bell ¹ Wayne D. Dillon ⁴ Christopher E. Cornwall ¹

• ¹ Victoria University of Wellington, Wellington, Wellington, New Zealand
• ² King Abdullah University of Science and Technology, Thuwal, Saudi Arabia
• ³ National Institute of Water and Atmospheric Research (NIWA), Wellington, New Zealand
• ⁴ CSIRO Environment, Hobart, Australia

Ocean acidification is the process of increasing concentrations of surface seawater CO2 and decreasing pH due to increasing absorption of anthropogenic CO2 emissions. Ocean acidification can negatively affect a broad range of physiological processes in marine shelled molluscs. Marine bioeroding organisms could, in contrast, benefit from ocean acidification due to reduced energetic costs of bioerosion. Net shell accretion in molluscs could therefore be reduced due to negative effects of ocean acidification itself, increased bioerosion due to weakening of the mollusc shell and increased bioerosion due to energetic advantages of bioerosion under ocean acidification. Ocean acidification could thus exacerbate negative effects (e.g. reduced growth) of ocean acidification and shell borers on oysters. The aim of this study was to assess the impact of ocean acidification on the oyster Ostrea chilensis, the boring sponge Cliona sp., and their host-parasite relationship. We exposed three sets of organisms 1) O. chilensis, 2) Cliona sp., and 3) O. chilensis infested with Cliona sp. to pHT 8.03, 7.83, and 7.63. Reduced pH had no significant effect on calcification, respiration and clearance rate of uninfested O. chilensis. Low pH significantly reduced calcification leading to net dissolution of oyster shells at pHT 7.63 in sponge infested oysters. Net dissolution was likely caused by increased bioerosion by Cliona sp. at pHT 7.63. Additionally, declining pH and sponge infestation had a significant negative antagonistic effect (less negative than predicted additively) effect on clearance rate. This interaction suggests that sponge infested oysters increase clearance rates to cope with higher energy demand of increased shell repair resulting from higher boring activity of Cliona sp. at low seawater pH. O. chilensis body condition was unaffected by sponge infestation, pH, and the interaction of the two. The reduction in calcification rate suggests sponge infestation and ocean acidification together would Formatted: Not Highlight