Oysters Reach Their Intertidal Limit in a Future High CO2 World

Chair: Andrew McMinn

Elliot Scanes(1), Laura M. Parker(2), Wayne A. O’Connor(3), Pauline M. Ross(1)

1 School of Science and Health, Western Sydney University, NSW, 2751, Australia
2 School of Biological Sciences, Centre for the Ecological Impacts for Coastal Cities, The University of Sydney, Sydney, NSW, 2006, Australia
3 Port Stephens Fisheries Institute, Department of Primary Industries, Taylors Beach, NSW, 2316, Australia

Background
Paradoxically, while ocean acidification is predicted to be most severe for molluscs living at the limits of their physiology, it has also been shown that molluscs living in environments naturally high in CO2 are more resilient. For sessile molluscs, such as oysters, physiological stress varies across the intertidal distribution. Those oysters living high on the shore at the limits of their intertidal distribution experience longer periods of emersion and potentially greater metabolic acidosis than those oysters submerged in the subtidal. We tested two hypotheses to predict the future for the vertical distribution of oysters in a high CO2 world. Either intertidal oysters will be more tolerant of elevated CO2 because of their exposure to regular metabolic acidosis, or intertidal oysters will reach their physiological limit.

Methods
Oysters (Saccostrea glomerata) were collected from the intertidal and subtidal and taken to the laboratory. Using a fully orthogonal design, intertidal and subtidal oysters were exposed to either a subtidal (no tidal cycle) or intertidal (tidal cycle with 9h emersion, 3h immersion) regime at elevated (856µatm) or ambient (400µatm) pCO2. Physiological parameters including pHe, PeCO2 and standard metabolic rate (SMR) were measured.

Findings
Exposure to emersion or elevated pCO2 reduced the pHe of all oysters. This reduction in pHe was significantly greater when emersion and pCO2 treatments were combined. The combined treatment of emersion and elevated pCO2 also significantly increased PeCO2 and SMR. Intertidal oysters showed no greater capacity for tolerance of ocean acidification, compared to subtidal oysters irrespective of tidal regime.

Conclusions
Exposure to elevated pCO2 and emersion increases the impacts of elevated CO2 on oysters. Intertidal oysters living high on the shore are no more resilient than those living subtidally. It is thus concluded that the upper limit of the vertical distribution of oysters may contract in a high CO2 world.