Locomotor kinematics and behaviour in the escape response of European sea bass, Dicentrarchus labrax L., exposed to hypoxia

To assess the effect of oxygen reduction on the escape response of sea bass, Dicentrarchus labrax, an integrative experimental approach was employed. The effect of hypoxia was tested on locomotor variables, i.e. cumulative distance (D), maximum swimming speed (V _max) and maximum acceleration (A _max). Behavioural variables, such as responsiveness (i.e. the proportion of individuals responding out of the total number of fish tested), response latency (i.e. the time interval between stimulus onset and the first detectable movement leading to the escape of the animal) and directionality (i.e. the proportion of escape responses in which the first detectable movement of the head was oriented away or towards the stimulus at its onset) were also considered. Four levels of oxygen were used: >85% (i.e. normoxia, the control treatment), 50, 20 and 10% of air saturation. Sea bass responsiveness decreased significantly at 10% of air saturation, while hypoxia did not have any effect on the response latency. At the onset of the escape response, the proportion of away/towards responses was random when oxygen was ≤50% of air saturation, suggesting an impairment of the left–right discrimination. Whatever the level of hypoxia, none of the locomotor variables (i.e. D, V _max and A _max) was significantly different from normoxia. Our study suggests that hypoxia may reduce sea bass elusiveness facing a predator by directly affecting its escape behaviour, possibly related to an impairment of the mechano-sensory performance and/or in the Mauthner cells involved in triggering the escape response.