Interactions of diet quality and pH in growth and mortality in the marine gastropod Crepidula fornicata
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Ocean acidification metabolically stresses marine organisms, especially those that must expend energy to deposit calcium carbonate shells and skeletons. Nutrition may interact with pH to exacerbate or counterbalance acidification effects in early developmental stages. Previous work had indicated that growth of larvae and juveniles of the caenogastropod Crepidula fornicata is resilient to acidification within the pH range of 7.6-8.0 when animals are given a high-quality diet of Isochrysis galbana (ISO). We therefore reared larvae and juveniles at two levels of pH (7.6 or 8.0) and on either a diet of ISO or a poorer-quality diet of Dunaliella tertiolecta (DUN), and measured larval and juvenile growth and mortality as well as juvenile shell strength. Larvae grew about 30% faster, and juveniles grew about 50% faster, on ISO than on DUN. However, larvae and juveniles grew at similar rates at pH 7.6 and 8.0 within each diet treatment. Survivorship of both larvae and juveniles was not affected by pH, but was higher on a diet of ISO than of DUN. We thus did not find evidence for interaction of diet quality and pH in larval or juvenile growth or mortality. Juvenile shell performance was tested by measuring crushing force after 20 d of postmetamorphic growth in each combination of diet and pH. Shells were 25% weaker at pH 7.6 than at 8.0 for individuals that were fed ISO, and 55% weaker at pH 7.6 than at 8.0 for individuals that were fed DUN, even though mean shell lengths did not differ between pH levels within each diet treatment. Rigorous assessment of the interaction of diet and pH on shell performance will require further study of how shell strength scales with age and size.