Utricularia sucks: Aquatic carnivorous plants that evolved vacuum traps

"Hi." Trap of Utricularia inflata, clearly showing
the door, trigger hairs, and concave walls.
Scale bar = 500 μm
Source: Vincent et al., 2011.

Utricularia, commonly known as the bladderworts, is a genus of approximately 230 species of carnivorous plants that have evolved an amazing suction trap to supplement their nutrient requirements by trapping and digesting convenient little arthropoid or crustacean packets of nitrogen, phosphorous, and other essential chemicals. Not all species are aquatic, as this cosmopolitan genus has also evolved species with lithophytic (growing in or on rocks), epiphytic, and terrestrial habits.

The rootless aquatic species are most notable for their tiny underwater bladder-shaped traps dotting the web-like system of stolons like aquatic chandeliers. Each trap is only a few millimeters long or less and possess a trap door surrounded by sensitive hairs that trigger the trap door mechanism to open, quickly sweeping the water - and any tasty prey contained therein - adjacent to the trap into the bladder. Keep in mind that each trap is only two cell layers thick when considering the pressure differentials and forces involved in prey capture.


Gazing upon this wondrously evolved botanical curiosity, naturalists in the 19th century thought that it was a passive system as comically illustrated in F. E. Lloyd's 1942 book on carnivorous plants (see below). Charles Darwin and others thought prey was simply enticed into entering the trap, much like a mouse entering a passive mousetrap. Since that time, and thanks to Lloyd's research in the early 20th century, we now know that the bladder traps of Utricularia are much more complex, involving the active setting of a trap and a rapid response once triggered, as illustrated in Lloyd's figure (below), which can only be described as the potential inspiration for the elaborate and beguiling board game Mouse Trap. Rube Goldberg would be proud!

Source: F.E. Lloyd. 1942. The Carnivorous Plants. Waltham, Mass.: Chronica Botanica Co.
The description is too long to reproduce here, but the following amused me: "...which allows the lever l to swing
downwards when the door is actuated again by, it is confidently hoped, a second mouse. In the meantime, the mouse
first caught can employ his time admiring the interior effect, and possibly suggest improvements." (pg. 267)

So by the mid-20th century, we had a pretty good idea of how these traps worked. Water is pumped out of the trap, producing the familiar "set" concave wall appearance. An unlucky crustacean, perhaps a Daphnia, swims too close to the trigger hairs, which relays that signal to the trap door, which swings open so quickly, no one had been able to quantify it before now. And here's where the exciting new research comes in. Physicists decided to record prey capture using high-speed cameras and measure the morphology of the door as it opens. The best thing about this, I believe, is that they put all of their supplemental material on YouTube.






The above video from the new article shows a copepod from the genus Cyclops being trapped by a Utricularia inflata bladder. The whole process occurs in less than one millisecond and is thus one of the fastest plant movements known. The poor little copepod seems utterly stunned. And no wonder! Olivier Vincent at the Laboratoire Interdisciplinaire de Physique, University of Grenoble and colleagues estimated that fluid velocities entering the trap can reach 1.5 meters per second (approximately 3.4 miles per hour) with maximum fluid accelerations of 600g. (Most humans lose consciousness at 4-6g.) Furthermore, in the video above you'll notice the copepod swirls down and around in the trap. The authors propose an interesting idea, that the trap morphology propels prey forward, then down into a swirling motion, preventing the immediate escape before the trap door closes again.

More impressive is the work they did investigating the door morphology as it opens. I can only imagine how precise this microscope, camera, and laser setup had to be in order to capture the exact moment when the door buckles and lets water flow in:






The also produced a dynamic simulation of the door opening:






So there we have it. Amazing new research adds to our understanding of one of the most unique carnivorous plant capture mechanisms. We've come a long way from Darwin's day and I certainly hope there's more to uncover. I'll leave us with just one more video, produced directly by the authors and posted on YouTube:

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References:

Vincent O, Weißkopf C, Poppinga S, Masselter T, Speck T, Joyeux M, Quilliet C, & Marmottant P (2011). Ultra-fast underwater suction traps. Proceedings. Biological sciences / The Royal Society PMID: 21325323