Seasonality and the Response of the Thecosome Pteropod Limacina retroversa to CO2 in the Gulf of Maine

Limacina retroversa is a thecosomatous pteropod found year round in the Gulf of Maine. Because carbonate chemistry within this shelf system is spatially variable and exhibits seasonal cycles, pteropods in this region may already be exposed to under-saturated, and hence corrosive, waters during certain seasons. To understand the implications of this variability, we have explored the physiological responses of L. retroversa at four time points over the course of a year to determine whether pteropods vary seasonally in their sensitivity to CO₂ exposure on time-scales relevant to acclimation responses. In the laboratory, these animals were exposed to CO₂ (ambient, 800, 1200 ppm) for 7-14 days and their response was assessed using an integrated set of metabolic, gene-expression and shell condition metrics. Similar to previous work with this species and others, pronounced changes in shell condition of exposed adults were discernible after less than 3 days of exposure, while changes to respiration rate were not consistently apparent. There were, however, seasonal variations in respiration rate indicative of an acclimation response. Differential expression analyses (RNAseq) revealed pronounced changes in gene expression among seasons, while laboratory CO₂ exposure resulted in a lower number of differentially expressed transcripts. These gene expression studies, together with both respiration rate and shell condition metrics provide an integrated picture of the seasonal effect of CO₂ on this sentinel species.