3D aquatic plant carnivory by the waterwheel plant and deformed snap-traps

When mentioning carnivorous plants, the Venus flytrap capturing its prey with its clamshell-like leaves is likely to pop into our minds. Whilst researchers have long studied the movement of carnivorous plants, the lesser-known aquatic waterwheel plant (Aldrovanda vesiculosa) is the only known plant species which can change the morphology of its sac-like traps as it digests its prey. 

Dr Anna Westermeier and colleagues from the University of Freiburg developed the first micro-CT method to track the morphology of the delicate snap-traps of the waterwheel plant in 2D and 3D. The researchers found that the trap volume can decrease down to 9.3% of the original size as the trap narrows down upon closure. Dr Anna Westermeier previously studied the trap morphology of carnivorous bladderworts and in 2018, revealed the principles of snap movement in the waterwheel plant.

The waterwheel plant is critically endangered worldwide. It floats under the water surface it consists of 4-9 carnivorous leaves arranged in a whorl. The traps are modified leaf blades which have two lobes, connected at midrib. There are free-side and bristle-side lobes, relative to the bristles emerging from the petioles, which curve differently once the prey is captured. The snap-traps can close in 10-20 ms which makes it ten times faster than the Venus flytrap.
Westermeier and colleagues used waterwheel plants from the Botanic Garden Freiburg which were originally collected from Darwin, Australia. The 3-5 mm long adult traps in the narrowed state were examined with light microscopy, scanning electron microscope (SEM) and micro-CT scanning with two different sample preparation methods. The researchers used the AVIZO image analysis software to segment air bubbles and prey from the traps. The trap volumes were estimated from the midrib length.

The traps were highly sensitive to handling and fixation during the light microscopy and SEM procedures but the trigger hairs, the teeth at the trap margins and the button-shaped glands could be visualised in fresh hand-sections. The micro-CT scans showed no artefacts caused by handling or chemical treatment.

The trap volumes greatly varied between traps which captured different prey. When a mosquito larva was half-digested, the trap volume was 40.5% compared to its theoretical original size. The trap volumes were 15.3% and 9.3% when the prey was a female copepod or a Chironomidae larva respectively. The different prey seemed to have led a change in the whole configuration of the traps and different levels in curvature in the free-side and bristle-side lobes.

“[W]e scanned narrowed A. vesiculosa snap-traps in liquid fixation medium because the exact conservation of turgescent tissues cannot be guaranteed with standard preparation methods (CP-drying or freeze-drying),” wrote Westermeier and colleagues. 

“[T]he traps with different prey animals not only illustrate the opportunistic foraging behaviour of A. vesiculosa, but also show the coupling between midrib bending deformation and the degree of the free-side trap lobe curvature inversion and trap volume decrease. The greater the midrib is bent, the more the trap is narrowed.”

“It remains speculative whether the prey animal was successively and passively moved into the trap, or if its own movements navigated it deeper inside. To answer these questions is an interesting topic for future studies on the narrowing behaviour of A. vesiculosa in respect to predator-prey interactions.”