Danger of zooplankton feeding: the fluid signal generated by ambush-feeding copepods

Abstract

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.

Flow disturbances generated by feeding and swimming zooplankton

“…All of the jumping and swimming copepods in fact produce two such vortex rings ( Fig. 3 and Movies S2-S4) consistent with previous observations in different species (4,17,24), and the observed far field spatial attenuation of the flow (u ∼ r -4 ) is consistent with that predicted from the idealized impulsive stresslet model (Table 1). Thus, the rapid power strokes may be considered an adaptation to minimize the production of fluid signals.…”

“…The frequency of reposition jumps in copepods and ciliates is between 1.0 s −1 and 0.01 s −1 (28,29) (reviewed in ref. 4) with each jump lasting only a few milliseconds. The males of the copepod Oithona spp.…”

“…These highly idealized models yield very different predictions of the spatial attenuation of the fluid disturbance and, thus, of how far away the feeding and swimming animal can be detected. A few studies have compared observed flow patterns with those predicted from these simple models and in some cases found fair comparisons (4,(14)(15)(16)(17). However, numerical simulations as well as observations of self-propelled microplankton have demonstrated that…”

“…However, moving comes at a predation risk: Swimming increases the predator encounter velocity (encounter rate increases with prey velocity to a power ≤1), and feeding and swimming generate fluid disturbances that may be perceived by rheotactic predators, thus increasing the predator's detection distance (encounter rate increases with detection distance squared) (1)(2)(3)(4)(5). So, the advantages of moving and feeding must be traded off against the associated risks, and organisms should aim at moving and foraging in ways that reduce the predation risk and optimize the trade-off (6,7).…”

Strike mechanics of an ambush predator: the spearing mantis shrimp

“…Some ambush predators, including snakes, fish and insects (Daniels, 1982;Cooper et al, 1985;Bailey, 1986;Formanowicz and Brodie, 1988;Alfaro, 2002;Bilcke et al, 2006;Hulbert et al, 2006;Ostrand et al, 2004;Sano and Kurokura, 2011) orient their bodies toward the prey so that they can strike quickly and accurately, while also minimizing disturbance to the water around them. Alternatively, some ambush predators, such as copepods that sit motionlessly in the water column to prevent detection by the prey (Kiørboe et al, 2010), are known to locate prey using hydrodynamic cues, which they then exploit to precisely time attacks (Jiang and Paffenhöfer, 2008). Upon striking, aquatic ambush predators must effectively manipulate their strikes so as not to push water, and therefore the prey, out of the range of attack.…”