Effects of potential future CO₂ levels in seawater on emerging behaviour and respiration of Manila clams, Venerupis philippinarum

High atmospheric CO₂ dissolves into the surface of the ocean and lowers the pH of seawater and is thus expected to pose a potential threat to various marine organisms. We investigated the physiological and behavioural responses of adult Manila clams, Venerupis philippinarum (n=96, shell length 25.3261.66mm and total wet weight 3.1060.54 g), to three levels (400, 700, and 900 μatm) of CO₂ partial pressure (pCO₂) for 48 days. There were no significant differences in mortality, growth, respiration rate, or emergence from the sediment between the three levels, indicating that near future atmospheric levels of CO₂ do not seem to have a serious effect on the physiology and behaviour of adult Manila clams. However, Manila clams could be exposed to notably higher pCO₂ and lower pH levels at local conditions due to the other issues, including eutrophication. Thus, the younger clams (n=240, shell length 16.7160.96mm and total wet weight 0.7060.13 g) were exposed to pCO₂ levels of 900 μatm (pH 7.8) and higher, such as 1300 and 2300 μatm (pH 7.7 and 7.5, respectively), for 39 days. Although mortality and growth were not significantly different between treatments, the emergence rates at the two higher pCO₂ levels were higher than that at the lowest level during the last 10 days of the experiment. The oxygen consumption rate (OCR) was reduced after 39 days of exposure to 2300 latm of pCO₂. The increase in emerging behaviour and the decrease in the rate of oxygen consumption indicated worse physiological conditions of the clams; the population may be negatively influenced due to worse conditions or increased probability of predation.