Planktonic predation risk varies with prey life history stage and diurnal phase

Predation risk is believed to shape many aspects of the behavior, morphology and life history of marine organisms. The timing of synchronous larval release, postlarval migrations to adult habitat and diurnal vertical migrations are all considered adaptations to predictable variations in predation risk. However, despite the fact that predation risk is expected to vary predictably over time, this variation remains poorly understood for planktonic animals in the field. In this study, we conducted dock-based predation risk assays using tethered brine shrimp Artemia franciscana. We tested for the combined effects of prey life history stage and diurnal phase by measuring loss of adult and larval tethered individuals. We sampled during day and night, and during large and small amplitude tides on the Pacific coast of Panama. On the Caribbean coast of Panama and the Atlantic coast of Florida (where tidal amplitude is smaller), we sampled during day and night but did not test for an effect of tidal amplitude. Although predation risk differed between sites, the trends were the same at all 3 docks. Predation risk was significantly higher during the day than at night for larvae, whereas adults experienced the opposite trend in risk across the diurnal cycle, although the difference was not significant. Our results demonstrate a temporal gradient in planktonic predation risk across the diurnal cycle that depends on prey life history stage.