Endogenous rhythm shift and adaptation to the tidal environment in the freshwater snail

Organisms have an endogenous timekeeping system to coordinate their biological processes with environmental cycles, allowing adaptation to external rhythmic changes in their environment. Modification of biological clocks could contribute to range expansion in a novel rhythmic environment. We hypothesized that populations of freshwater species near estuaries have acquired a circatidal rhythm to synchronize with the tidal cycle. Here, we compared the locomotion activity and gene expression rhythms of the freshwater snail Semisulcospira reiniana between individuals inhabiting freshwater and brackish-water areas. Individuals inhabiting brackish-water areas exhibited a rhythmic activity pattern coordinated with the tidal cycle under both field and laboratory conditions. Individuals inhabiting upstream freshwater areas showed a nocturnal activity pattern. The proportion of circadian oscillating genes was greater in freshwater than in brackish-water individuals, while that of circatidal oscill..., , ,