Daily rhythms in the behavioural stress response of the zebrafish Danio rerio

In nature, animals are exposed to stressors that occur with different likelihood throughout the day, such as risk of
predation and human disturbance. Hence, the stress response is expected to vary plastically to adaptively match
these challenges. Several studies have supported this hypothesis in a wide range of vertebrate species, including
some teleost fish, mostly through evidence of circadian variation in physiology. However, in teleost fish,
circadian variation in behavioural stress responses is less understood. Here, we investigated the daily rhythm of
stress response at the behavioural level in the zebrafish Danio rerio. We exposed individuals and shoals to an open
field test every 4 h over a 24 h cycle, recording three behavioural indicators of stress and anxiety levels in novel
environments (thigmotaxis, activity and freezing). Thigmotaxis and activity significantly varied throughout the
day with a similar pattern, in line with a stronger stress response in the night phase. The same was suggested by
analysis of freezing in shoals, but not in individual fish, in which variation appeared mostly driven by a single
peak in the light phase. In a control experiment, we observed a set of subjects after familiarisation with the open-
field apparatus. This experiment indicated that activity and freezing might present a daily rhythmicity that is
unrelated to environmental novelty, and thus to stress responses. However, the thigmotaxis was constant through
the day in the control condition, suggesting that the daily variation of this indicator is mostly attributable to the
stress response. Overall, this research indicates that behavioural stress response of zebrafish does follow a daily
rhythm, although this may be masked using behavioural indicators other than thigmotaxis. This rhythmicity can
be relevant to improve welfare in aquaculture and reliability of behavioural research in fish models