The aquatic bladderwort Utricularia gibba captures zooplankton in mechanically triggered underwater traps. With characteristic dimensions <1 mm, the trapping structures are among the smallest known that work by suction-a mechanism that would not be effective in the creeping-flow regime. To understand the adaptations that make suction feeding possible on this small scale, we have measured internal flow speeds during artificially triggered feeding strikes in the absence of prey. These data are compared with complementary analytical models of the suction event: an inviscid model of the jet development in time and a steady-state model incorporating friction. The initial dynamics are well described by a time-dependent Bernoulli equation in which the action of the trap door is represented by a step increase in driving pressure. According to this model, the observed maximum flow speed (5.2 m/s) depends only on the pressure difference, whereas the initial acceleration (3 × 10 4 m/s 2 ) is determined by pressure difference and channel length. Because the terminal speed is achieved quickly (~0.2 ms) and the channel is short, the remainder of the suction event (~2.0 ms) is effectively an undeveloped viscous steady state. The steady-state model predicts that only 17% of power is lost to friction. The energy efficiency and steady-state fluid speed decrease rapidly with decreasing channel diameter, setting a lower limit on practical bladderwort size. K E Y W O R D S bladderwort, carnivorous plants, suction feeding, unsteady Bernoulli equation, Utricularia

The carnivorous plant bladderwort exemplifies the use of accumulated elastic energy to power motion: respiration-driven pumps slowly load the walls of its suction traps with elastic energy (∼1 h). During a feeding strike, this energy is released suddenly to accelerate water (∼1 ms). However, due to the traps’ small size and concomitant low Reynolds number, a significant fraction of the stored energy may be dissipated as viscous friction. Such losses and the mechanical reversibility of Stokes flow are thought to degrade the feeding success of other suction feeders in this size range, such as larval fish. In contrast, triggered bladderwort traps are generally successful. By mapping the energy budget of a bladderwort feeding strike, we illustrate how this smallest of suction feeders can perform like an adult fish.

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