Direct and diet-mediated indirect effects of ocean acidification do not impact larval slipper limpet Crepidula onyx

Much attention has been given to the effects of ocean acidification (OA), a reduction in sea surface pH caused by the increased uptake of atmospheric carbon dioxide. OA is known to directly impact larval development and physiological processes of many marine organisms. In addition, OA is shown to affect the nutritional quality and algal palatability. This indirect effect of OA via trophic interactions, however, has not been fully explored. In this study, veligers of the slipper limpet Crepidula onyx were exposed for 14 days to different pH levels representing the ambient (as control) and projected near future pH values (pH 7.7 and pH 7.3), and were fed with the algae Isochrysis galbana cultured at these 3 respective pHs. Our results showed that larval mortality, growth rate, and respiration were not significantly affected by pH, food treatments, nor their interactions. Settlement rate did not differ significantly among pH treatments but showed an increasing trend towards decreasing pH. Shell morphology revealed minor but noticeable structural changes at low pH conditions. Larval size at day 14 and protoconch size also differed significantly among pH treatments. Our follow up experiment showed when exposed directly to reduced pH and/or fed with low-pH grown algae, larval clearance rates increased. This increase in food acquisition may serve as compensatory response to mitigate the impacts of OA and may explain the minimal effects observed in this study. Our work demonstrated that this non-native C. onyx population to be quite robust, if not, well adapted to future pH reductions and low algal nutritional value.