Locomotor activity rhythms in cavefish Astyanax mexicanus under slow magnetic fluctuations.

The Earth's magnetic field undergoes slow diurnal variations, yet their potential role as environmental cues influencing biological rhythms remains poorly understood. This study examines whether such geomagnetic fluctuations can entrain locomotor activity rhythms in the blind Mexican cavefish (Astyanax mexicanus), a species with reduced circadian regulation due to life in perpetual darkness. Fish were exposed to artificially altered geomagnetic variations with extended (27 h) or shortened (21 h) periods, while control groups experienced natural diurnal fluctuations. Locomotor activity was continuously recorded and analyzed using periodograms. The results indicate that slow geomagnetic fluctuations did not entrain locomotor rhythms in A. mexicanus. Instead, significant individual variability was observed, with some fish displaying rhythmicity, while others did not. Notably, rhythmic individuals exhibited lower average swimming speeds than arrhythmic ones. Arrhythmic individuals were more common among larger fish. These findings suggest that the circadian regulation reduction in A. mexicanus prevents possible synchronization with weak magnetic cues. Furthermore, the observed correlation between locomotor rhythmicity, body size, and swimming speed highlights potential physiological or metabolic differences underlying inter-individual variability.