Chair: Ana Queiros

John M Wright⁽¹⁾, Laura M Parker⁽²⁾, Wayne A O’Connor⁽³⁾, Pauline M Ross⁽¹⁾

¹ School of Science and Health, Western Sydney University, NSW, 2753, Australia
^{2 S}chool of Biological Sciences, Centre for the Ecological Impacts for Coastal Cities, University of Sydney, Sydney, NSW, 2006, Australia
³ Port Stephens Fisheries Institute, Department of Primary Industry, Taylors Beach NSW, 2316, Australia

Background
Ocean acidification is anticipated to alter predator-prey relationships among molluscs because of the greater energy demand required to sustain homeostasis. Some molluscs, including oysters, have inducible defences and respond to predation risk by changing morphological characteristics or altering metabolism to avoid detection. Here we investigate the impact of elevated CO₂ on the energetic demand and predatory relationship of the oyster Crassostrea gigas and the whelk Tenguella marginalba. It was predicted that for both species the energetic demand would increase under elevated CO₂ and this may have consequences for standard metabolic rate (SMR) and inducible defences that could lead to increased predation of C. gigas.

Methods
We acclimatised C. gigas and T. marginalba in the laboratory to ambient (395 ppm) or elevated (1000 ppm) CO₂ for six weeks. We then introduced C. gigas into tanks with and without T. marginalba at ambient and elevated CO₂ for a further 17 days. We also exposed C. gigas to waterborne cues from conspecifics and T. marginalba and measured SMR.

Findings
The SMR of C. gigas was significantly greater at elevated than ambient CO₂. There was a significant decrease in SMR of C. gigas when exposed to waterborne cues released from T. marginalba at ambient but not at elevated CO₂. T. marginalba increased their SMR and consumption of C. gigas at elevated CO_2.

Conclusions
At ambient CO_2, C. gigas have the ability to reduce metabolic activity and the output of metabolites in the presence of T. marginalba, which may reduce the likelihood of detection and predation. But, under elevated CO₂ the ability to regulate metabolic rate was lost, possibly in favour of metabolic maintenance to meet the higher energy demand required to sustain homeostasis. Accordingly, in a high CO₂ world, C. gigas may be more vulnerable to an increasingly voracious T. marginalba.