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

Abstract

Copepods are small marine invertebrates that are one of the most abundant multicellular organisms on Earth. They serve as an important link in the marine food chain between small oceanic plant life, called phytoplankton, and larger organisms such as fish. As with all organisms, they must adapt to the surrounding fluid environment. Since copepods are small, they inhabit an aquatic flow regime that provides a balance of inertial and fluid viscous forces on the organism. The flow created by copepods controls, to a large degree, the interaction with prey, predators, and other individuals of the same species. Hence, examination of the flow disturbances created during cruise and escape behaviors provides insight into the method and consequences of propulsion in this unique flow environment. The genus Euchaeta includes species that inhabit ocean waters at different latitudes extending from the subtropics to the poles. The body shapes of these species are very similar, with the primary difference being much larger body size in colder waters. Thus, Euchaeta species provide a natural experiment to study adaptation to fluid property variation between habitats. Here, variation in body size, swimming and escape speeds, or viscosity has direct consequences on the hydrodynamic disturbance created by organism motion. 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 copepod species. The results reveal an intriguing interplay between body size and the fluid environment that alters planktonic interactions, including the ability to prey on food items and the ability to escape from predators, in a complex manner. The complex interaction for the genus Euchaeta partially explains species adaptations to the local environmental conditions. 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.