Fluid Mechanics of Bladderwort
Bladderwort Utricularia gibba is a carnivorous plant that captures zooplankton in underwater bladders. These modified leaf structures are highly specialized for suction feeding. The bladders store elastic energy by pumping out water over a period of hours, during which they become conspicuously flattened. Prey actuate a trap door within the nozzle, and are entrained in a high-speed inward jet as the bladder expands. However, their small size (1-2 mm) and fast action (ca. 1 ms) have obscured the underlying functional kinematics and flows. We have used high-speed digital video and flow visualization (Particle Image Velocimetry) to study U. gibba at frame rates up to 50 000 per second. Shown below are typical bladders as they appears under a dissecting microscope, and in the laser light of a Particle Image Velocimetry (PIV) experiment.
The PIV method is illustrated below: a thin laser sheet is
used to illuminate particles suspended in the water; their
positions, as obtained from the digital video recordings, can be used
to reconstruct the vectorial flow field in the plane of the laser sheet.
We have filmed and analyzed several dozen bladderwort suction events. Just outside of the external "hood", bladderwort generate peak flow speeds of up to 1.2 m/s, corresponding to a Reynolds number of 200. The spatial distribution of flow speed across the gape and along the gape agrees with flow speed distributions observed for adult fish. The time course of the suction event has a distinctively steep onset, followed by a relatively slow decay. Adult fish, by contrast, accelerate water with a symmetric and smoothly varying time profile of longer duration.
While the peak flow speeds generated by suction-feeding fish and
bladderwort are similar, the accelerations observed in bladderwort are
much greater. Within the internal nozzle, the fluid is
accelerated from rest at an extraordinary rate: we have observed
22 000 m/s2
(or 2 400 G) by particle tracking. Peak internal
speeds are approximately 5 m/s. This extreme performance is
necessary in order for such a small organism to feed effectively by
active suction: the short duration of the strike out-paces the
development of a boundary layer, creating a fast and energy-efficient
inward jet.
Capture Success
The most effective predators documented in the literature are
dragonflies (Combes et al., Integrative and Comparative Biology 56 (2013)
787-798) and seahorses (Gemmell, Nature Communications 4 (2013) 2840), which succeed in capturing prey on
approximately 90% of attempts. Our preliminary studies indicate
that bladderwort can be included in this elite group of
super-predators. The number of successful feeding strikes is
easily documented by counting stomach contents. But how to record
the number of attempts? In our recently commisioned audio booth,
we can record the sound produced when bladderwort attack. Below
is a raw sound pressure trace of a 'spontaneously' triggered bladder
(in the absence of prey). The duration of the trace is 0.100
second, and it links to a
corresponding .wav file of 5.0 second duration:
In the course of our observations we have found that the planktonic prey are surprisingly noisy. The following raw audio recording is 15 seconds long, and was obtained while a copepod was apparently resting on the microphone:
The behavior continues for bouts as long as 15 minutes. We have not yet attempted to correlate this sound with video recordings, but it has the makings of an interesting research project.