Gill ionocytes of the Lake Magadi tilapia (Alcolapia grahami), an extremophilic teleost native to a highly alkaline environment

The Magadi tilapia thrives in arguably the most extreme aquatic environment on earth for fish, the hot springs of Lake Magadi in Kenya with its severe water chemistry: pH 10, alkalinity 300 mEq·L-1. This fish is 100% ureotelic yet has an osmoregulation pattern typical of marine teleosts, although the dominant anion is HCO 3- rather than Cl-. The gills must actively export basic equivalents (HCO 3-+CO 3 2-) and Na + against strong electrochemical gradients, yet simultaneously take up Cl-, for which a hypothetical “Laurent model” based on ionocyte structure alone was proposed. This model has been tested using immunohistochemistry to characterize ionocyte types based on ion transport protein expression patterns [CFTR anion channel, Na + :K + :2Cl-cotransporter (NKCC)/ Na + :Cl-cotransporter (NCC), Na + :HCO 3-co-transporter (NBC), Na + /K +-ATPase (NKA), and urea transporter (UT)]. A typical “seawater ionocyte” (Type IV) with apical CFTR and basolateral NKCC1 and NKA, is present validating key elements of the model. A “freshwater ionocyte” (Type II) is also present (apical NCC, weaker NKA and strong NBC1 basolateral staining). A third Type I ionocyte with only strong NKA staining was also identified. An acid excreting Type III ionocyte (apical NHE3 and basolateral NKA) was not present. The Magadi tilapia is unusual in having co-expression of both Type IV and Type II ionocytes, which are typically associated with Cl-excretion and uptake, respectively. Instead, we propose Type IV ionocytes are involved in basic equivalent and Na + excretion and Type II ionocytes in Cl-uptake. In these ureotelic fishes, the UT occurs only in lamellar pavement cells.