Fig. 1

a Swim trajectories during baseline illumination and after loss of light. Color scale: time (seconds). Arrowheads, orientation. Scale bar 20 mm. b NTA over 30-s intervals for larvae before/after loss of illumination. Individuals were classified as right- (cyan, N = 25) or left-biased (gray, N = 34) based on the first 30-s interval. Asterisk p < 0.05, d = 3.1, 0.71, 0.66, and 0.61 (first four time points in the dark), t test between groups. c NTA for 30-s light-off trials (black bars). Open circles at time 0 show individual larvae, and were used to classify as right- (cyan, N = 34) or left-biased (black, N = 34). Subsequent points show mean and standard deviation for left/right groups. Asterisk p < 0.05, t test between groups. d As for (c) during constant illumination (right, N = 29; left, N = 35). e Match Index during baseline illumination (yellow, N = 64) and after loss of illumination (gray, N = 68). Asterisk p < 0.05, r = 0.29, Mann–Whitney U test, and ^#p < 0.05, r = 0.76, one-sample permutation test against 0.5. f Visually isolated chamber. Right: time-lapse montage over 10 s following loss of illumination. Color: time (seconds). g, h Percentage of turns executed rightward (mean of four 10-s trials) after loss of illumination (g, gray, N = 89) or during constant illumination (h, yellow, N = 39). Red line: expected distribution, Monte Carlo simulation of unbiased larvae. i Rightward-turn preference over 24 h. Larvae with <33% of rightward turns at 6 dpf were classified as left-biased (black, N = 12), and those >66% as right-biased (cyan, N = 14). At 7 dpf, % rightward-turn use in L/R-classified groups. Repeated measures ANOVA for 7-dpf trials, the effect of 6-dpf first-trial direction F_1,24 = 15.4, p < 0.001, η²_p = 0.39. Asterisk p < 0.05 between groups. j Percentage trials with net rightward bias for larvae tested at 7 or 10 dpf (N = 30, 52), after left/right classification at 6 dpf. Asterisk p < 0.05, r = 0.61 and 0.60 (7 and 10 dpf), Mann–Whitney U test. Error bars: standard error of the mean. Box plots show median and quartiles with whiskers indicating 10–90%. Source data are provided as a Source Data file.