Elevated seawater pco₂ differentially affects branchial acid-base transporters over the course of development in the cephalopod Sepia officinalis

The specific transporters involved in maintenance of blood pH homeostasis in cephalopod molluscs have not been identified to date. Using in situ hybridization and immunohistochemical methods, we demonstrate that Na⁺/K⁺-ATPase (soNKA), a V-type H⁺-ATPase (soV-HA), and Na⁺/HCO^-₃ cotransporter (soNBC) are colocalized in NKA-rich cells in the gills of Sepia officinalis. mRNA expression patterns of these transporters and selected metabolic genes were examined in response to moderately elevated seawater PCO₂ (0.16 and 0.35 kPa) over a time course of 6 wk in different ontogenetic stages. The applied CO₂ concentrations are relevant for ocean acidification scenarios projected for the coming decades. We determined strong expression changes in late-stage embryos and hatchlings, with one to three log2-fold reductions in soNKA, soNBCe, socCAII, and COX. In contrast, no hypercapnia- induced changes in mRNA expression were observed in juveniles during both short- and long-term exposure. However, a transiently increased ion regulatory demand was evident during the initial acclimation reaction to elevated seawater PCO₂. Gill Na⁺/K⁺-ATPase activity and protein concentration were increased by ∼15% during short (2-11 days) but not long-term (42-days) exposure. Our findings support the hypothesis that the energy budget of adult cephalopods is not significantly compromised during long-term exposure to moderate environmental hypercapnia. However, the downregulation of ion regulatory and metabolic genes in late-stage embryos, taken together with a significant reduction in somatic growth, indicates that cephalopod early life stages are challenged by elevated seawater PCO₂.