Abstract
Circadian rhythms regulate physiological and behavioural processes, with the light-dark cycle acting as the principal environmental cue synchronising the biological clock; disruption of this cue can affect brain function and behaviour. Using zebrafish (Danio rerio) as a translational model, we examined how chronic photoperiod alteration, applied from the larval stage to adulthood, affects exploratory activity, anxiety-like behaviour, aggression, and social preference, and whether these changes are paralleled by shifts in gut microbiota composition and brain transcriptomic profile. Zebrafish were reared under three photoperiod regimes: 14L/10D (control), 20L/4D (extended light), and 4L/20D (extended dark), followed by behavioural, gut metagenomic, and brain transcriptomic analyses. Extended darkness (4L/20D) reduced anxiety-like behaviour, whereas extended light (20L/4D) increased aggression both extended photoperiods altered social behaviour, but with distinct behavioural profiles. Gut microbiota in the 20L/4D group showed phylum-level co-dominance of Actinomycetota and Pseudomonadota, including a greater presence of potentially pathogenic taxa, while the 4L/20D group was dominated by Bacillota. Brain transcriptome profiling, based on a single pooled sample per condition, identified transcripts showing the largest expression differences in each group: in 20L/4D, transcripts linked to phototransduction (gnat1, saga, pde6ga, exorh), ribosomal function (rpl13a, rpl9, knop1), melatonin synthesis (asmt), calcium signalling (plcb4a), and glycolysis (eno1a) were elevated; in 4L/20D, transcripts linked to neuroprotection (nr4a1), DNA repair and chromatin remodelling (fance, uimc1, histh1l), synaptic plasticity (serpina10a), neurodevelopment (six6a), and cell-cycle regulation (btg2) were elevated. These findings indicate that photoperiod shapes zebrafish behaviour, gut microbial composition, and brain gene expression, providing a hypothesis-generating basis for future studies incorporating biological replication.