Fig. 7. Ultrastructure of the filament epithelium surface viewed by scanning electron microscopy (SEM) in the scaleless carp exposed to hypoxia (0.3 mg O₂ l^–1) for 4 h (A), 8 h (B), 12 h (C) and 24 h (D). In A and B, note the `wavy-convex' mitochondria-rich cell (MRCs) with short and slightly branched microvilli (arrows) and numerous mucous cells releasing mucous granules; in C, note the small openings of MRCs (arrows), abundant mucous cells, and mucous deposition on the epithelial surface; in D, note `shallow-basin' MRCs (arrows) with a thick film of mucous covering epithelial surface. Scale bars, 10 µm.