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

Krizma Singh,Otto Berg,Roberto C Reyes,Ulrike K Müller,Gabriel Campa,Matthew D Brown,Fatima Hidalgo

doi:10.3390/fluids5010033

Abstract

Suction feeding is a well-understood feeding mode among macroscopic aquatic organisms. The little we know about small suction feeders from larval fish suggests that small suction feeders are not effective. Yet bladderworts, an aquatic carnivorous plant with microscopic underwater traps, have strong suction performances despite having the same mouth size as that of fish larvae. Previous experimental studies of bladderwort suction feeding have focused on the solid mechanics of the trap door’s opening mechanism rather than the mechanics of fluid flow. As flows are difficult to study in small suction feeders due to their small size and brief event durations, we combine flow visualization on bladderwort traps with measurements on a mechanical, dynamically scaled model of a suction feeder. We find that bladderwort traps generate flows that are more similar to the inertia-dominated flows of adult fish than the viscosity-dominated flows of larval fish. Our data further suggest that axial flow transects through suction flow fields, often used in biological studies to characterize suction flows, are less diagnostic of the relative contribution of inertia versus viscosity than transverse transects.

Highlights

Suction is a force exerted on a fluid or solid suspended in a fluid in the presence of a sub-ambient pressure
We aim to characterize the suction flows of bladderwort traps in two stages: we begin with experimental observations on actual traps, which yield the parameters used to tune the dynamically scaled model; subsequent measurements on the model yield flow fields of greater sensitivity, resolution, and experimental control
The flow fields obtained from the mechanical model (Figure 4) are significantly more uniform than those from the live plant (Figure 3)

Summary

Introduction

Suction is a force exerted on a fluid or solid suspended in a fluid in the presence of a sub-ambient pressure. Suction feeding has been studied most extensively in fish [1,2,3,4,5,6,7,8], and in tadpoles [9], aquatic salamanders [10], and seals [11]. The suction flows generated by large suction feeders have been described extensively [6,15,16]: adult fish generate strong sub-ambient pressures by opening their mouth and expanding their buccal cavity [17,18,19]; their suction pressures range from 1.5 to 55.6 kPa and produce strong flows of 1.4 to 6.8 m·s−1 , measured typically at half gape from the plane of the mouth [6,17,18,19,20,21,22]

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