Zooplankton feed in either of three ways: they generate a feeding current, cruise through
the water, or they 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 mm-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. 德國LaVision PIV/PLIF粒子成像測速場儀
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