Gill remodeling responses of the clown knifefish (Chitala ornata) to temperature and hypoxic stress

Gill remodeling ability of clown knifefish exposed to different temperature levels (27°C and 33°C) and dissolved oxygen levels (hypoxia - 25% and 30% air saturation (at 27°C and 33°C), and normoxia - 95% air saturation) were performed applying vertical section to estimate lamellar surface area (SA), gill filament volume, lamellar volume, harmonic mean water blood thickness and calculate anatomic diffusion factor (ADF). The initial lamellar SA and harmonic mean water blood thickness of Chitala (33.12 ± 1.09 g) were 51.43 ± 3.10 mm2g-1 and 3.59 ± 0.15 µm in normoxia, respectively. The lamellae SA increased strongly after one month of exposure to elevated temperature and hypoxia. A significant reduction in the harmonic mean water–blood barrier thickness was observed following one month of hypoxia, whereas temperature effects were observed significantly after two months. The value of lamellar SA in the hypoxic group at 33°C (47.02 ± 2.44 mm2g-1) was twice as high as that of the normoxic group at 27°C (22.38 ± 1.06 mm2g-1) while the ADF in the hypoxic group at 33°C was nearly 4-fold higher than the normoxic group at 27°C. Findings in Chitala suggest that gill remodeling represents an ancient adaptation that has existed for over 300 million years.