In carotid body glomus cells (Lahiri and Forster, 2003) and zebrafish gill NECs (Qin et al., 2010), the capacity to respond to hypercapnia is facilitated by the presence of cytosolic carbonic anhydrase (CA), owing to the catalysed dehydration of molecular CO2 to H+ and HCO3− (Haswell et al., 1980).