Swept Away: Fine-scale Water Motion Encountered by Microscopic Animals on Surfaces in Turbulent Ambient Flow, Hydrodynamic Forces, and Adhesion

Ambient water flow can sweep away microscopic larvae of benthic animals that settled onto a substratum, and can dislodge zooplanktonic prey from capture surfaces of benthic predators. Measurements of water velocities at the scale of zooplankton on surfaces in different microhabitats within benthic communities showed that the animals experience brief pulses of flow as eddies sweep the substratum. What features of tiny animals determine whether such flow sweeps them off surfaces? We measured adhesive strength of settling larvae on surfaces exposed to realistic flow pulses and found that crawling larvae were more likely to be blown away than stationary larvae, and that adhesive strengths were higher in pulsatile flow than in steady flow for larvae attached by shear-thinning mucus. We measured hydrodynamic forces and moments on dynamically-scaled physical models to determine the effects of body shape and orientation of diverse zooplankters on predator or benthic surfaces exposed to flow, and used Particle Image Velocimetry to determine flow fields around them. Body shapes and orientations that reduced wake size decreased drag. The hydrodynamic moment most affected by body orientation was peeling in the direction of flow for bodies with low aspect ratios, but spinning for bodies with high aspect ratios. Net hydrodynamic force on animals of different shapes was greater if they were attached by a tether than if they were glued flush with a surface. Calculation of shear stress due to hydrodynamic forces and moments on the attachment of a zooplankter to a surface showed that organisms with large attachment areas are pushed off a surface by flowing water, while those on slim tethers are spun off. Thus, the ability of settling larvae to recruit into benthic communities and of benthic predators to retain captured prey depends on the local fine-scale flow in their microhabitat