Feeding status modulates diel vertical migration ofzooplankton via effects on circadian rhythms

Diel vertical migration (DVM) of aquatic animals is arguably the largest migration on Earth, and occurs each day in most marine and freshwater ecosystems. DVM is influenced directly by factors such as light, food, and predator abundance, but is also regulated by internal circadian clocks. Untangling the mechanistic controls of DVM, and the relative importance of external versus endogenous cues, has been hampered by a lack of experimental systems. Here, we leverage advances in animal tracking software to develop an imaging system allowing us to quantify the positions of dozens of individual copepods (Acartia tonsa) at sub-second resolution over multiple days. We use this approach to characterize group-level diel behavioral patterns at much higher resolution than has previously been possible. We find that behavioral rhythms entrain to light cycles and regular feeding and persist during constant conditions (darkness, no feeding), indicating true circadian regulation and establishing A. tonsa as an experimental system for DVM. We test the hypothesis that food availability impacts DVM by acting as an entraining cue for circadian rhythms. Daytime-restricted feeding weakens circadian behavioral rhythms compared to nighttime-restricted feeding, illustrating that food availability can impact DVM indirectly via effects on internal clocks in zooplankton. Our results provide a detailed view of zooplankton populations over diel cycles, establish a mechanism by which temporal variation in food supply can affect zooplankton migrations, and lay the groundwork for new experimental studies of circadian behavior.