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

Abstract

AbstractAir‐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 (fAB) 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 fAB, while hypoxia, hyperthermia, and exercise stress significantly increased fAB. Progressive hypoxia from 17.69 to 2.17 kPa caused a ~2.5‐fold increase in fAB. Increasing temperatures to 27 and 32°C, from a baseline temperature of 22°C, significantly increased fAB from 0.4 ± 0.2 to 1.3 ± 0.5 and 1.6 ± 0.4 breaths min−1, respectively. Lastly, following exhaustive exercise, fAB 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.