The effect of fluid viscosity, habitat temperature, and body size on the flow disturbance of Euchaeta

The spatial extent and temporal decay of copepod-generated hydrodynamic disturbances during cruise and escape behavior were examined using the particle image velocimetry technique combined with theoretical models. Our study compared results for two species in the genus Euchaeta: the larger E. elongata living in colder water of higher viscosity versus the smaller E. rimana living in warmer water of lower viscosity. We expected that body size and viscosity would work in opposite directions in shaping the spatial and temporal properties of the hydrodynamic disturbances generated by these two copepod species. We found that the spatial extent of the copepod-induced hydrodynamic signal in front of the copepods during cruising was equivalent, with the peak strength of the signal to preferred prey showing no significant difference. In contrast, the spatial extent and strength of the hydrodynamic disturbance during escape were larger for E. elongata, although the decay time of the flow disturbance to a threshold value was equivalent between the species. Importantly, the observation of vortex rings during escape for Euchaeta strongly supports the appropriateness of the impulsive stresslet model over the impulsive Stokeslet model. Moreover, our empirical data discount the validity of using a sphere in creeping flow to model copepod–fluid interactions. Rather, these results suggest a complicated interaction of fluid viscosity, body size, and swimming speed for the genus Euchaeta that partially explains the adaptations to the local environmental conditions.