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

Organisms have an endogenous time-keeping system to coordinate their biological processes with environmental cycles and thereby adapt to external rhythmic environments. The modification of biological clocks could contribute to range expansion to the novel rhythmic environment. In freshwater species, we hypothesized that populations near the estuary have acquired the circatidal rhythm to synchronize with the tidal cycle. We analysed gene expression rhythms of the freshwater snail, Semisulcospira reiniana, between individuals inhabiting the freshwater and brackish water area. The proportion of circadian oscillating genes was greater in the freshwater than brackish water individuals, while that of circatidal oscillating genes was greater in the brackish water than freshwater individuals. Interestingly, we found 16 genes that exhibited circadian and circatidal rhythm in their expression in the freshwater and brackish water individuals, respectively. Additionally, transcriptome-wide population genetic analyses supported our assumption that these two populations are genetically almost identical. These findings suggest that the slight divergence contributed to the shift in the time-keeping system and endogenous rhythms could differ in their periods between the freshwater and brackish water populations. Our results could be evidence of the evolution of endogenous rhythm via range expansion to the novel rhythmic environment in a single species.