The Sleepy Shrimp Is Caught by the Tide: Parhyale hawaiensis As a New Model Organism for Circatidal Rhythms

Abstract

Biological rhythms allow organisms to time their physiologies to match environmental cycles. Circatidal rhythms are synchronized by tidal cycles along coastlines, and have a period of 12.4 or 24.8 hours (h). Little is known about their underlying molecular mechanisms, despite how essential they are for many marine species. We chose to utilize the crustacean Parhyale hawaiensis as they are amenable to genetic manipulation. I demonstrate that Parhyale exhibits robust circatidal rhythms of activity entrained by our artificial tidal regimen. These circatidal rhythms differ from those entrained by a 12h:12h light:dark cycle, in pattern and periodicity. To investigate if the circadian molecular clock is required for circatidal rhythms, we used CRISPR-Cas9 and generated a line of Parhyale carrying a null allele of the core circadian clock gene Bmal1. I show individuals lacking Bmal1 exhibit defects both in circadian behavior, as expected, but also in circatidal behavior, labeling Bmal1 the first circatidal clock gene. I then address how circatidal rhythms adapt to the three types of tidal cycle occurring on Earth, characterized by differing periodicities. I show that Parhyale can entrain to artificial regimens mimicking all types of tidal cycle, but as regimens got further from 12.4h in period, animals exhibited behavioral flexibility. Under these challenging tidal cycles, some animals exhibited 12.4h circatidal-like rhythms of behavior, while other had more 24h circadian-like patterns of activity, demonstrating that the interaction between circadian and circatidal cycles is quite complex. In summary, this work introduces Parhyale hawaiensis as a robust model organism to study circatidal rhythms.