Air-breathing behavior in Heterotis niloticus fingerlings: Response to changes in oxygen, temperature, and exercise...

Air-breathing in fish is believed to have arisen as an adaptation to aquatic hypoxia. Although air-breathing has been widely studied in numerous fish species, little is known about the obligate air-breathing African bonytongue, Heterotis niloticus. We evaluated if abiotic factors and physical activity affect air-breathing patterns in fingerlings. The air-breathing frequency (f_AB ) and behavioral responses of H. niloticus fingerlings were assessed in response to environmental oxygen, temperature, and exhaustion and activity in a series of experiments. The air-breathing behavior of H. niloticus fingerlings under optimum water conditions was characterized by swift excursions lasting less than 1 s to the air-water interface to gulp air. The intervals between air-breaths were highly variable, ranging from 3 to 259 s. Body size only slightly affected f_AB , while hypoxia, hyperthermia, and exercise stress significantly increased f_AB . Progressive hypoxia from 17.69 to 2.17 kPa caused a ~2.5-fold increase in f_AB . Increasing temperatures to 27 and 32°C, from a baseline temperature of 22°C, significantly increased f_AB from 0.4 ± 0.2 to 1.3 ± 0.5 and 1.6 ± 0.4 breaths min^-1 , respectively. Lastly, following exhaustive exercise, f_AB increased up to 3-fold. These observations suggest that H. niloticus fingerlings are very reliant on aerial oxygen, and their air-breathing behavior is sensitive to environmental changes and activity levels.