Fig. 5. Representative scanning electron micrographs of the gills of northern (A,C) and southern (B,D) killifish before (A,B) and 8 days after transfer from brackish water (10 g l^-1) to freshwater (C,D). Apical crypts (arrow), freshwater-type mitochondria-rich cells (arrowhead) and cells with intermediate morphology (asterisk) are apparent (scale bar, 10 µm). (E) Enlarged view of freshwater-type mitochondria-rich cell (scale bar, 1 µm).