Branchial localization of acid (A‐type) and base (B‐type) excreting ionocytes in Amazonian stenohaline freshwater <scp><i>Potamotrygon</i></scp> ray

The branchial epithelium of Potamotrygon-a member of the only strictly freshwater elasmobranch family Potamotrygonidae-was observed via immunohistochemistry, and two distinct forms of ionocytes were identified. The acid (A-type) and base (B-type) secreting cells with respective basolateral-apical localizations of Na⁺/K⁺-ATPase-Na⁺/H⁺ exchanger 3 in A-type and V-type H⁺-ATPase-pendrin Cl^-/HCO₃ ^- exchanger in B-type were detected and morphometric changes in response to acid-base disturbances (3 mmol/kg body mass acid [HCl] or base load [NaHCO₃] by intraperitoneal injection) were characterized. A-type cells were found in distinct follicular clusters in the interlamellar region, while B-type cells were found individually in both the filament and lamellar epithelia. Lamellar B-type cells were more abundant and larger, with greater fluorescence than their filament counterparts. The B-type cells responded to the base load by increasing in number in the filament and showed a more peripheral H⁺-ATPase distribution, indicative of activation by translocation to the basolateral membrane labyrinth. A-type cells decreased in size with the base load. In contrast, no changes in A-type or B-type cells were observed with the acid load compared to the sham group. Together, these results indicate that the stenohaline Potamotrygon rays have branchial ion uptake cells that allow them to effectively ionoregulate under the challenging ion-poor water conditions of the Amazon.