Ocean acidification alters the predator-prey relationship between the oyster Crassostrea gigas and the whelk Morula marginalba

It is predicted that ocean acidification will alter predator-prey relationships of molluscs. Some molluscs, including oysters, have the ability to respond to predation risk by increasing their shell size. Also in the presence... [ view full abstract ]

It is predicted that ocean acidification will alter predator-prey relationships of molluscs. Some molluscs, including oysters, have the ability to respond to predation risk by increasing their shell size. Also in the presence of predators, molluscs have been found to reduce their metabolism to avoid detection. In an acidifying ocean, where shell calcification is predicted to decrease and energy demand to increase, these defensive mechanisms may be impaired.
This study set out to investigate whether defences of the Pacific oyster, Crassostrea gigas to the predatory whelk Morula marginalba were altered under elevated CO2. Juvenile C. gigas and wild M. marginalba were simultaneously exposed to elevated (1000 ppm) or ambient (395 ppm) CO2 for eight weeks. Following this acclimation predation risk was manipulated. Whelks exposed to elevated and ambient CO2 were placed in tanks with oysters at elevated and ambient CO2 using a fully orthogonal design. Shell size and standard metabolic rate (SMR) of oysters with and without whelks was then measured over 17 days. The consumption of oysters and SMR of whelks was also measured. In the absence of the whelk there was no significant difference in size or SMR of C. gigas at elevated or ambient CO2. In the presence of the whelk there was a significant decrease in size and SMR of C. gigas at ambient compared to elevated CO2.
When whelks are present and CO2 is at ambient levels, C. gigas may have the ability to enter a state of low metabolic activity and reduce the output of metabolites to prevent being detected by whelks. When whelks are absent, the metabolic rate of C. gigas was maintained to meet the energy demand required to sustain homeostasis. Further, when M. marginalba were exposed to elevated CO2 they initially increased their consumption of C. gigas and their SMR. This response of whelks may exacerbate the vulnerability of oysters in a climate changed ocean.