Fig. 4. V-H⁺-ATPase-rich cell numbers in the gills of freshwater (FW), seawater-acclimated (SWA) and seawater (SW) Atlantic stingrays. (A) ‘Sum’ number of V-H⁺-ATPase-rich cells (per 100 µm of lamella + per interlamellar region); (B) number of V-H⁺-ATPase-rich cells per 100 µm of lamella; (C) number of V-H⁺-ATPase-rich cells per interlamellar region. Values are means + 1 S.E.M. Lower case letters indicate statistical categorization of the means as determined by a Student–Newman–Keuls post-hoc test (P<0.05). Note that both mean and S.E.M. for the number of V-H⁺-ATPase-rich cells per 100 µm of lamella = 0 in SW stingrays.