浮游动物进食的危险:桡足类潜伏进食者产生的流体信号
Zooplankton feed in either of three ways: they generate a feeding current, cruise throughthe water, or they are ambush feeders. Each mode generates different hydrodynamic disturbancesand hence exposes the grazers differently to mechanosensory predators. Ambush feeders sinkslowly and therefore perform occasional upward repositioning jumps. We quantified the fluiddisturbance generated by repositioning jumps in a mm-sized copepod (Re ~ 40). The kick of theswimming legs generates a viscous vortex ring in the wake another ring of similar intensity butopposite rotation is formed around the decelerating copepod. A simple analytical model, that of animpulsive point force, properly describes the observed flow field as a function of the momentum ofthe copepod, including the translation of the vortex and its spatial extension and temporal decay.We show that the time-averaged fluid signal and the consequent predation risk is much less for anambush feeding than a cruising or hovering copepod for small individuals, while the reverse is truefor individuals larger than about 1 mm. This makes inefficient ambush feeding feasible in smallcopepods and is consistent with the observation that ambush feeding copepods in the ocean are allsmall, while larger species invariably use hovering or cruising feeding strategies.