Effects of varying acidic levels on dissolution, strength, organic content ands urface texture of Pacific oysters (Crassostrea gigas) shells

Abstract

Marine coastal organisms are exposed to periodic fluctuations in sea water pH driven By biological carbon dioxide (CO2) production which may in the future be further exacerbated by the ocean acidification associated with the global rise in CO2. There is widespread concern that these changes have direct impact on coastal organisms and alter the habitats severely. However, little or no attention has been given to the effects of the anticipated decrease in coastal pH on farmed oysters within the Namibian coastal waters. In this investigation, shells of the Pacific oysters, Crassostrea gigas were exposed to varying acidic levels under laboratory conditions; pH level 6.5 represented extreme hypercarpnia condition, 7.0 and 7.5 representing future predicted coastal pH levels. Shell dissolution rate, strength, organic content and surface texture were assessed after a two-week exposure period. Significant loss (p < 0.05) in weight and diameter were observed in shells exposed to 6.5,7.0 and 7.5 pH levels compared to shells in the control groups (pH8.1-8.2). With regard to organic content of the shell, significant reduction (p < 0.05) was only observed in shells exposed to 6.5 and 7.0 pH levels. Microscopic examination of the shell surface revealed reduced nacreous layer while the organic layer of the shells was sheared in acidic conditions. Visual inspection of the nacre region of shells exposed to 6.5 ,7.0 and 7.5 pH showed straight edged tablets, with the nacre regions characterised by sparse with irregular shaped tablets within a reduced organic matrix. Ocean acidification can impact potential changes in morphometry and shell structure of pacific oysters during culture.