Hydrodynamics of the bladderwort feeding strike

Abstract

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

Suction Flows Generated by the Carnivorous Bladderwort Utricularia—Comparing Experiments with Mechanical and Mathematical Models

“…In order to increase the spatial and temporal resolution of experimentally measured flow fields, we designed a dynamically scaled model for study by Particle Image Velocimetry. The start-up time for a bladderwort feeding strike is less than 0.1 ms [53]; this study focuses on events after 0.1 ms, when the external flow is no longer time-dependent. We therefore use Reynolds number rather than Womersley number to scale the model.…”

“…Linear dimensions in the model were scaled up by a factor of 320 (32 mm vs 0.10 mm) based on the fluid-flow bottleneck observed in a typical specimen of U. gibba [53]. Although a proportional decrease in flow speed would preserve Reynolds number, it would lead to impractically long event durations.…”

“…To simulate the feeding strike generated by a bladderwort trap (high velocity/strong pressure) and larval fish (low velocity/weak pressure), the bulk flow speed in the test section was set to 0.54 m s −1 (Reynolds number Re = 250) and 0.002 m s −1 (Re = 1), respectively. These Reynolds numbers are based on experimental observations of bladderworts and larval fish, following conventions for experimental studies on suction feeding [21] that use channel diameter as the characteristic length and peak flow speed near the mouth or bulk flow speed inside the channel [46,47,53].…”

“…The result is a higher density of useful vectors, suppression of peak-locking artifacts, and less random error in the transects. In all cases, the transects represent a time shortly after the flow-speed maximum, at which time the flow is in steady state [53]. The various events span a sevenfold range of absolute peak speeds.…”

“…In this study we compare the suction flow profile of bladderwort traps with theoretical predictions for inviscid and creeping flow into an orifice or through a contraction [24,52,53] as well as with predictions of the dynamically scaled model [54]. We chose bladderwort as our model organism for a small suction feeder because its suction mechanism is not complicated by supplementary strategies such as ram feeding or jaw protrusion, making bladderwort a suitable organism to compare its suction flows with theoretical and mechanical-model predictions.…”