Therefore, we concluded that the structural variations of T. lalius in the acidic group mainly resulted from cell proliferation. The cell proliferation of gill tissue in the hypoxic stress treatment was also found in the aquatic air-breathing fish, T. leeri (Lee et al., 2008). A decrease in cell apoptosis in the gills might also be involved in the observed morphological modifications (Sollid et al., 2003; Nilsson, 2007; Matey et al., 2008).