Crosstalk between the circatidal and circadian clocks mediates behavioral adaptation to tidal patterns.

The ability to anticipate tides is critical for a wide range of marine organisms, but this task is complicated by the diversity of tidal patterns on Earth. Previous findings suggest that organisms whose geographic range spans multiple types of tidal cycles can produce distinct patterns of rhythmic behavior that correspond to the tidal cycles they experience. How this behavioral plasticity is achieved, however, is unclear. Here, we show that Parhyale hawaiensis adapts its rhythmic behavior to various naturally occurring tidal regimens through the plastic contribution of its circatidal and circadian clocks. After entrainment to a tidal cycle that deviated only mildly from a regular 12.4 h tidal cycle, animals exhibited strong circatidal rhythms. By contrast, following entrainment to more irregularly spaced tides or to tides that occurred every 24.8 h, a significant fraction of animals instead synchronized to the light/dark (LD) cycle and exhibited circadian behavior, while others showed rhythmic behavior with both circatidal and circadian traits. We also show that the circatidal clock, while able to entrain to various naturally occurring tidal patterns, does not entrain to an unnatural one. We propose that Parhyale hawaiensis's ecological success around the world relies in part on the plastic interactions between the circatidal and circadian clocks, which shape its rhythmic behavior appropriately according to tidal patterns.