The effect of hydrostatic pressure on grazing in three calanoid copepods

The vertical distribution of copepods, and the factors determining it, has been studied extensively. Some copepods appear to maintain their vertical position in the water column with high precision, even against downwelling and upwelling currents. The adaptive benefits of this behavior are unknown. We hypothesized that changes in hydrostatic pressure alter the buoyancy of copepods and, in turn, affect their grazing rate. We tested this hypothesis in laboratory grazing experiments with three calanoid species, Calanus helgolandicus, Pleuromamma indica and Rhincalanus nasutus, grazing on cultured phytoplankton. The study was based on a comparison of gut pigment content between individuals that fed in chambers at two different pressure levels: 1 bar and 4 bars. A significant effect of pressure was found for C. helgolandicus (lower gut content at 4 bars) but not for P. indica and R. nasutus. If the effect observed in C. helgolandicus is common, it would add a new dimension to our understanding of copepod ecology in the oceanic realm, where plankton is often exposed to vertical currents and internal waves.