| Summary: | Throughout the Earth’s history climate change has led to a proliferation and disappearance of species thus playing a major role in shaping the structure of marine ecosystems. Over the next century, rising atmospheric carbon dioxide (CO2) levels will cause a rise in the partial pressure of CO2 (pCO2) in the surface ocean leading to a reduction in surface ocean pH from 8.1 to 7.7 units and a reduction in the concentration of surface ocean carbonate ions (in a process known as ‘ocean acidification’) vital in the construction of calcium carbonate (CaCO3) shells and skeletons. At the same time, this rise in atmospheric CO2 levels will lead to an increase in sea‐surface temperatures (SSTs). Of growing concern are the potentially serious implications that ocean acidification and rising SSTs may have on marine organisms and ecosystems and the potential risk to marine aquaculture internationally. Recent studies, which have mimicked ocean acidification, have reported adverse consequences of elevated pCO2, including reduced calcification and growth, and increased rates of abnormality and mortality in larvae and adults of a range of marine organisms. Yet, we know comparatively little of the synergistic consequences of elevated pCO2 and temperature nor the potential for species to acclimate through genetic adaptation, at the population and ecosystem level. This lack of knowledge limits our ability to create models that accurately predict the consequences of future climate change. Among the organisms most likely to be affected in an acidifying and warming ocean are marine calcifying organisms which construct shells and skeletons of CaCO3. Studies on these organisms, to date, have focused predominately on adults, with comparatively few considering the sensitive early life history stages. This study compared the synergistic effects of elevated pCO2 and temperature on the early life history stages of two ecologically and economically important oysters: the Sydney rock oyster, Saccostrea glomerata and the Pacific oyster, Crassostrea gigas. Gametes, embryos, larvae and spat were exposed to four pCO2 (375, 600, 750, 1000 ppm) and four temperature (18, 22, 26, 30 °C) levels in acute exposure experiments. At elevated pCO2 and suboptimal temperatures there was a reduction in the fertilisation success of gametes, a reduction in the development of embryos and size of larvae and spat and an increase in abnormal morphology of larvae. These effects varied between species with S. glomerata having greater sensitivity than C. gigas. Combined, elevations in pCO2 of 750–1000 ppm and a temperature of 30 °C resulted in 100% mortality of D‐veliger larvae of S. glomerata. In contrast, the same pCO2 and temperature combination resulted in only 26% mortality of D‐veliger larvae of C. gigas. A comparison between two commonly used methods for manipulating the seawater carbonate system (HCl‐acidification and CO2‐acidification) showed that there was no significant difference in the responses of S. glomerata and C. gigas to the two methods. This indicates that both HCl‐acidification and CO2‐acidification are adequate methods for determining the effects of elevated pCO2 for these species.
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