Circadian clock involvement in zooplankton diel vertical migration, link to supplementary material

Genetic clocks are a ubiquitous ancient and adaptive mechanism enabling organisms to anticipate environmental rhythms and to regulate behavioral, physiological and behavioral processes accordingly. Whilst terrestrial circadian clocks are well studied and understood, knowledge about the clock systems in marine organisms is still limited. This is particularly true for abundant species displaying large-scale rhythms like diel vertical migration (DVM) that contribute significantly to shaping their respective ecosystems. Here, we describe endogenous and highly rhythmic patterns in the biology of the ecologically important and highly abundant planktic copepod Calanus finmarchicus. This species shows circadian rhythms of DVM, metabolism, and most core circadian clock genes (clock, period1, period2, timeless, cryptochrome2, clockwork orange) in the laboratory. In the field, copepods from shallow water (0-50m) have more robust rhythmic clock gene oscillations than those caught in deeper water (140-50m). Further, peak expressions of clock genes generally occurred at either sunset or sunrise coinciding with peak migration times. Providing one of the first field investigations of clock gene rhythmicity in a marine species this study further couples clock genes measurements with laboratory and field data on DVM. While the mechanistic connection remains elusive, our results imply a high degree of causality between clock gene expression and one of the planet's largest daily migration of biomass. This could increase zooplankton fitness by optimizing the temporal trade-off between feeding and predator avoidance