High pCO2 affects body size, but not gene expression in larvae of the California mussel (Mytilus californianus)

Many studies have reported reductions in body size and calcification rates for marine larvae exposed to ocean acidification conditions. However, the physiological mechanisms driving these effects, and mechanisms underlying body size variation in general, are poorly understood. Here, we combine transcriptome sequencing with bulked segregant analysis to assess the physiological response to acidification in larvae of the California mussel, Mytilus californianus, and to explore physiological basis of variation in larval size. We reared three families of M. californianus larvae under ambient (similar to 350 mu atm, pH(total) 8.1) and high (similar to 1300 mu atm, pH(total) 7.6) pCO(2) conditions, then passed larvae through a mesh filter, separating each family x pCO(2) treatment into fractions of larvae with large vs. small body sizes. We sequenced larval mRNA for each family x treatment x body size combination, and assembled a de novo transcriptome. We then mapped reads from each library to this assembly to identify effects of high pCO(2) on gene expression, and to identify transcriptomic differences between small vs. large larvae of the same age class. Although larvae reared under elevated pCO(2) were smaller, we observed no consistent effect of elevated pCO(2) on gene expression. Nevertheless, 1225 transcripts, primarily related to metabolism, were differentially expressed between large vs. small larvae, regardless of CO2 treatment. We conclude that the observed reduction in larval body size under high CO2 may be driven by a direct effect of the environment on phenotype, unmediated by changes in gene expression. Because M. calfornianus has evolved in the context of seasonal upwelling, exposure to 1300 mu atm, pCO(2) may not produce the large stress-mediated effects on gene expression that might be expected for an organism exposed to conditions far outside those of its typical environment.