Hunters come in all shapes and sizes. In freshwater ponds, one particularly formidable hunter is actually an aquatic plant, the bladderwort. Bladderworts (in particular, Utricularia spp.) hunt small invertebrates. Along their branches, the plants have hollow pods (bladders) under negative pressure. Each pod is about 1 mm in diameter and has a trapdoor on a hair trigger. When a small critter brushes by, the door opens and sucks the prey into the pod (where it is eventually digested). It turns out that the biomechanics of how bladderworts do this is not well known. However, Amit Singh, Sunil Prabhakar and Sanjay Sane at the National Institute for Biological Sciences in Bangalore recently used high-speed videography to describe the mechanics of bladderwort trapdoor movements at sub-millisecond resolution and they published their work in Biology Letters.
First, the team set out to determine the pressure difference across bladder walls. To do this, they used a glass micropipette filled with water and a bubble as a pressure probe. After estimating the pressure within the bubble, they inserted the micropipette into individual bladders. The bubble then expanded towards the pod (pulled by the negative pressure); by measuring this expansion, they were able to estimate the internal pressures within individual bladders. As expected, internal pressures within pods were much lower than the pressure of the surrounding water. This differential disappeared immediately after trapdoor opening, then reset over the course of 20–30 min. Interestingly, trapdoor opening could not be triggered while the pressure differential was resetting. This suggests that each bladder possesses an internal sensing mechanism that keeps the door firmly shut until a pressure threshold is reached.
Next, Singh and colleagues filmed individual pods opening and closing with a very high-speed camera. The kinematics of trapdoor opening turned out to be incredibly fast. In response to mechanical stimulation, trapdoors opened within 300–700 μs, stayed open for 1–3 ms, then closed in 1–2 ms. These swings are an order of magnitude faster than previous estimates and are the fastest recorded movements in any carnivorous plant. By filming neutrally buoyant beads being sucked into bladders, the group was also able to measure the speed at which water moves through the trapdoors. The speeds involved were similar to theoretical estimates based solely on the pressure differentials across the bladder; in addition, the suction flows that developed were dominated by inertial forces. This helps explain why bladderworts are so effective at hunting; the speed of trapdoor movements and the large inertial forces easily outpace sensorimotor responses in prey animals. Small animals just have no chance to react.
The work of Singh, Prabhakar and Sane is important because it shows that carnivorous plants are capable of moving much faster than previously thought. It is also significant because it is a descriptive study. This work represents a traditional way of doing biology that has fallen out of favour. Indeed, for many young biologists, ‘purely descriptive’ work is something to be avoided at all costs. But the work of Singh and colleagues bucks this trend and sends an important message. It shows that new discoveries can still be made by those who simply take the time to carefully describe what they see in nature.
