Relatedness of hypoxia and hyperthermia tolerances in the Nile tilapia (Oreochromis niloticus) and their relationships with cardiac and gill traits.

In fish, thermal and hypoxia tolerances may be functionally related, as suggested by the oxygen- and capacity-limited thermal tolerance (OCLTT) concept, which explains performance failure at high temperatures due to limitations in oxygen delivery. In this study the interrelatedness of hyperthermia and hypoxia tolerances in the Nile tilapia (Oreochromis niloticus), and their links to cardiorespiratory traits were examined. Different groups of O. niloticus (n = 51) were subjected to hypoxia and hyperthermia challenges and the O2 tension for aquatic surface respiration (ASR pO2) and critical thermal maximum (CTmax) were assessed as measurement endpoints. Gill filament length, total filament number, ventricle mass, length and width were also measured. Tolerance to hypoxia, as evidenced by ASR pO2 thresholds of the individual fish, was highly variable and varied between 0.26 and 3.39 kPa. ASR events increased more profoundly as O2 tensions decreased below 2 kPa. The CTmax values recorded for the O. niloticus individuals ranged from 43.1 to 44.8 °C (Mean: 44.2 ± 0.4 °C). Remarkably, there was a highly significant correlation between ASR pO2 and CTmax in O. niloticus (r = -0.76, p < 0.0001) with ASR pO2 increasing linearly with decreasing CTmax. There were, however, no discernible relationships between the measured cardiorespiratory properties and hypoxia or hyperthermia tolerances. The strong relationship between hypoxia and hyperthermia tolerances in this study may be related to the ability of the cardiorespiratory system to provide oxygen to respiring tissues under thermal stress, and thus provides some support for the OCLTT concept in this species, at least at the level of the entire organism.

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

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 (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.