Rhythm in the vertical distribution of Danio rerio under photic and magnetic zeitgebers

Circadian rhythms in the vertical distribution of adult zebrafish and their potential sensitivity to geomagnetic fluctuations remain poorly characterized. In this study, we investigated Danio rerio maintained under different light-dark regimes and exposed to natural diurnal geomagnetic variation (~30 nT, with a 24-h period) or slow magnetic oscillations (~100-150 nT, with periods of 26.8, 33.76, or 36 h). Behavioral endpoints included the proportion of time spent in the upper versus lower part of the water column and the frequency of midline crossings. Under standard and modified light-dark cycles, zebrafish exhibited a robust rhythm, occupying upper layers during darkness and shifting downward during the light phase. This pattern differs from that described for zebrafish in natural environments and probably reflects long-term adaptation to aquaculture. Under constant illumination, free-running rhythms with periods close to 24 h persisted across groups. Analysis of midline crossing frequency revealed a significant ~33.8 h rhythm in fish pre-exposed to the atypical 24:12 h light-dark cycle, consistent with the periodicity of the applied magnetic oscillation. These results suggest that geomagnetic fluctuations may function as an extremely weak zeitgeber compared to the dominant influence of the photoperiod. Prior exposure to an atypical light-dark regime, which may have reset the biochemical processes sustaining the endogenous oscillators, appears to be required for the manifestation of magnetically driven rhythm entrainment.