National Institute of Aquatic Resources, Technical University of Denmark, Kavalergården 6, 2920 Charlottenlund, Denmark.
Proc Biol Sci. 2010 Nov 7;277(1698):3229-37. doi: 10.1098/rspb.2010.0629. Epub 2010 Jun 10.
Zooplankton feed in any of three ways: they generate a feeding current while hovering, cruise through the water or are ambush feeders. Each mode generates different hydrodynamic disturbances and hence exposes the grazers differently to mechanosensory predators. Ambush feeders sink slowly and therefore perform occasional upward repositioning jumps. We quantified the fluid disturbance generated by repositioning jumps in a millimetre-sized copepod (Re ∼ 40). The kick of the swimming legs generates a viscous vortex ring in the wake; another ring of similar intensity but opposite rotation is formed around the decelerating copepod. A simple analytical model, that of an impulsive point force, properly describes the observed flow field as a function of the momentum of the 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 an ambush-feeding than a cruising or hovering copepod for small individuals, while the reverse is true for individuals larger than about 1 mm. This makes inefficient ambush feeding feasible in small copepods, and is consistent with the observation that ambush-feeding copepods in the ocean are all small, while larger species invariably use hovering or cruising feeding strategies.
悬停时产生摄食流，巡游或伏击式捕食。每种方式都会产生不同的水动力干扰，从而使掠食者以不同的方式暴露于机械感觉掠食者面前。伏击式捕食者会缓慢下沉，因此会偶尔进行向上的复位跳跃。我们量化了毫米大小的桡足类（Re∼40）复位跳跃产生的流体干扰。游泳腿的踢腿在尾迹中产生粘性涡环；在减速桡足类周围形成另一个强度相似但旋转方向相反的涡环。一个简单的分析模型，即脉冲点力模型，能够很好地描述观测到的流场，其函数形式与桡足类的动量有关，包括涡的平移及其空间扩展和时间衰减。我们表明，对于小型个体，伏击式捕食的平均流体信号和由此产生的捕食风险比巡游或悬停的桡足类要小得多，而对于大于约 1 毫米的个体则相反。这使得小型桡足类的低效伏击式捕食成为可能，这与海洋中伏击式捕食的桡足类都是小型的观察结果一致，而较大的物种总是采用悬停或巡游的摄食策略。
