Fig. 2, Scanning electron micrographs of gill filaments in killifish adapted to defined freshwater with low- (A,B), mid- (C,D) and high- (E,F) NaCl. The apical membrane of mitochondria-rich (MR) cells is flat or slightly projecting, and equipped with microvilli (arrowheads) in low- and mid-NaCl groups, whereas the apical membrane of MR cells forms an apical pit (arrows) in the high-NaCl group. pv, pavement cell. Scale bars, 10 µm.