Carbon dioxide, pH and bicarbonate (CO2/pH/HCO3–) are essential for life because they affect chemical kinetics and protein structure and function; they also are substrates or products of respiration, photosynthesis and calcification. Although regulating these crucial processes surely relies on the ability to sense CO2/pH/HCO3– levels, the specific mechanisms remain practically unknown in aquatic organisms. Our research seeks to provide a link between metabolic and environmental acid/base stress and downstream physiological responses.