Intertidal oysters reach their physiological limit in a future high-CO world.

Sessile marine molluscs living in the intertidal zone experience periods of internal acidosis when exposed to air (emersion) during low tide. Relative to other marine organisms, molluscs have been identified as vulnerable to future ocean acidification; however, paradoxically it has also been shown that molluscs exposed to high CO environments are more resilient compared with those molluscs naive to CO exposure. Two competing hypotheses were tested using a novel experimental design incorporating tidal simulations to predict the future intertidal limit of oysters in a high-CO world; either high-shore oysters will be more tolerant of elevated because of their regular acidosis, or elevated  will cause high-shore oysters to reach their limit. Sydney rock oysters, , were collected from the high-intertidal and subtidal areas of the shore and exposed in an orthogonal design to either an intertidal or a subtidal treatment at ambient or elevated , and physiological variables were measured. The combined treatment of tidal emersion and elevated  interacted synergistically to reduce the haemolymph pH (pH) of oysters, and increase the  in the haemolymph ( ) and standard metabolic rate. Oysters in the intertidal treatment also had lower condition and growth. Oysters showed a high degree of plasticity, and little evidence was found that intertidal oysters were more resilient than subtidal oysters. It is concluded that in a high-CO world the upper vertical limit of oyster distribution on the shore may be reduced. These results suggest that previous studies on intertidal organisms that lacked tidal simulations may have underestimated the effects of elevated .