Fig. 3. Scanning electron micrographs from the 2nd gill arch of crucian carp kept in normoxic or hypoxic water. (a) In normoxia, the gill filaments have no protruding lamellae. (b) The morphology has already changed after 1 day of hypoxia exposure (0.75±0.15 mg O₂ l^-1). (c,d) The change progresses for up to 7 days in hypoxia, but (e) there were no further changes with subsequent exposure. (f) When the fish were moved to normoxic water, the morphological changes were reversed within 7 days. Scale bar, 50 µm.