Close Encounters

Daphnia are bite-size for the carnivorous waterwheel plant

The Venus Fly Trap is not the only carnivorous plant with a snap-trap. Aldrovanda vesiculosa, the waterwheel plant, has one too but it's not always been clear how it works.

The Venus Fly Trap is not the only carnivorous plant with a snap-trap. Aldrovanda vesiculosa, the waterwheel plant, has one too but it’s not always been clear how it works. Simon Poppinga and colleagues have taken a closer look, and published their findings in Scientific Reports. “Our study is the first to show in detail how the carnivorous waterwheel plant (A. vesiculosa) captures its daphniid prey,” they write.

Aldrovanda vesiculosa
Aldrovanda vesiculosa Jan Wieneke / Wikipedia

A. vesiculosa is plant that lives in the water, eating small prey as it passes. The A. vesiculosa trap looks similar to the Venus Fly Trap trap, with two lobes that snap shut around the prey. The challenge for studying A. vesiculosa is the speed. A. vesiculosa is fast, with the traps operating in as short a time as tenth thousands of a second, about the same time as the blink of an eye.

To tackle the traps, Poppinga’s team used high-speed photography, similar to the setup they had already used for examining the slightly faster bladderwort plants. Keeping a focus on the traps, the scientists could review how the prey triggered the trap and, if it was lucky, how it escaped.

Source: Poppinga et al. 2019

The team captured fourteen prey capture attempts (PCAs) on film, with nine of them being successful, a strike rate of around 64%. Quite a few of the escapes were due not to the flea triggering the trip directly, but causing the trap to trigger as they swam past.

The use of live prey is a key element of the research, write Poppinga and colleagues. “In most studies on the functional principles of carnivorous plant traps, the respective experiments and kinematical analyses were performed with artificially triggered traps. There are only few reports available, which (in some cases only superficially) deal with the interrelation of prey and carnivorous plant trap movements.”

While the plant is similar to the Venus Fly Trap, there are some big differences in the trapping mechanism. As A. vesiculosa has to operate in water it has to avoid triggering its traps accidentally from passing water currents. The authors propose that A. vesiculosa‘s triggers are deeper in the trap to protect them from currents and, possibly, to ensure the prey is deeper into the trap before it snaps. However, triggering the A. vesiculosa trap is a different matter compared to the Venus Fly Trap (Dionaea muscipula).

“Whereas in D. muscipula there are always at least two consecutive stimuli on one or on different trigger hairs necessary within a certain period to entail trap closure, the traps of A. vesiculosa are more variable in this respect,” write Poppinga and colleagues. “Here, some traps close after reception of one stimulus, some close after two stimuli, some require even more mechanical perturbations, and some do not respond at all. The reasons for this variability in trap irritability are still unknown.”

“Future experiments could complementarily analyse PCAs with different natural prey animals, e.g. members of Cladocera, Copepoda, Ostracoda, Ephemeroptera, Nematocera, Hydrachnidia, and Pulmonata. Such analyses would be helpful for further evaluating the capture efficiency of the A. vesiculosa traps, which feed on a very high diversity of prey animals regarding taxonomy, size, and movement behaviour. Therefore, a broad spectrum of A. vesiculosa trap (motion) characteristics could possibly be a selective advantage.”

Determining how the traps work could lead to a better understanding of a plant that is both fussy in its habitats and also invasive in places, as well as both endangered and found across many continents outside its home of Australia.

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