“Early birds” take it easy: diurnal timing is correlated with overall level in activity of zebrafish larvae

in Behaviour
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Individual animals show consistent differences in behavioural responses when coping with environmental challenges. Consistency over time and across context in a behavioural trait is an indication for animal personality. Chronotypes refer to consistent inter-individual differences in diurnal rhythmicity driven by underlying variation in circadian clock processes. Personality traits and chronotype may relate to a single behavioural syndrome, but few studies have investigated such a link explicitly. Here, we explored zebrafish larvae for the presence of consistency in activity levels and timing, and their correlation with and without external cues (Zeitgeber: light–dark cycle versus constant light). We found individual consistency in activity level and timing, and their correlation independent of the presence of Zeitgeber: early-active individuals were less active overall than late-active individuals. Our study suggests a link between personality and chronotype and provides new insights into the early development of individual variation in behavioural tendencies of zebrafish.



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  • A schematic overview of the experimental procedure. Eggs were collected on the day of spawning (0 dpf) and transported into a 48-well plate one day later (1 dpf). On the third day post-fertilization, the well plate was placed inside the setup and the larval activity was recorded. Furthermore, there were two groups: A diurnal rhythm group that kept receiving a light–dark cycle throughout the treatment, and a free running circadian rhythm group that received constant light without dark periods from the 3rd day post-fertilization and onward. These light–dark regimes are also shown in this figure.

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  • Typical actogram for a larva of the diurnal rhythm group (a), under the influence of a light dark cycle, indicated by white and grey bars, and for a larva of the circadian rhythm group (b), in constant light, during two consecutive days, i.e., 5 and 6 dpf. Swimming velocity (V, mm/s) as a proxy for activity, is plotted against time (hZT). The time of peak activity (Tpeak), i.e., the time point of maximum swimming velocity (Vmax), is marked by an arrow above the graph. Each dot represents a 30-min average after three-point smoothing.

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  • Relationship between activity levels, at 5 and 6 dpf, in the same individual zebrafish larvae, as mean swimming velocity (Vmean, mm/s) for the diurnal (a) and the circadian rhythm group (b). Vmean was averaged over the light period of the diurnal rhythm group (i.e., 0–14 hZT) for both, diurnal and circadian rhythm group. Each dot represents one individual zebrafish larvae. Linear regression analysis for the diurnal rhythm group revealed a relationship of 5 dpf to 6 dpf Vmean, described by the equation Vmean6 dpf=0.64±0.16×Vmean5 dpf+0.15±0.08. Similarly, for the circadian rhythm group, linear regression was described by the equation Vmean6 dpf=0.31±0.63×Vmean5 dpf+0.14±0.03 (mean ± SD).

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  • Relationship between timing of activity, at 5 and 6 dpf, in the same individual zebrafish larvae, as time of peak activity (Tpeak, hZT) for the diurnal (a) and the circadian rhythm group (b). Tpeak was determined as the time point with maximum activity as swimming speed (V, mm/s) in 30-min intervals, after application of a running three-point average. Each dot represents an individual zebrafish larva. Linear regression analysis for the diurnal rhythm group revealed a relationship of 5 dpf to 6 dpf Tpeak, described by the equation Tpeak at 6 dpf=0.24±0.10×Tpeak at 5 dpf+0.35±0.76. Similarly, for the circadian rhythm group, linear regression was described by the equation Tpeak at 6 dpf=0.36±0.15×Tpeak at 5 dpf+4.55±1.13 (mean ± SD).

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  • Relationship between timing of activity as time of peak activity (Tpeak, hZT), and level of activity, as mean swimming velocity (Vmean, mm/s), at 6 dpf, for the diurnal (a) and the circadian rhythm group (b). Each dot represents an individual zebrafish larva. Linear regression analysis for the diurnal rhythm group revealed a relationship of Tpeak to Vmean, described by the equation Vmean=6.95±1.53×Tpeak1.00±0.73. Similarly, for the circadian rhythm group, linear regression was described by the equation Vmean=12.50±4.09×Tpeak+3.80±1.15 (mean ± SD).

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