When thinking of predators roaming in and around pools and ponds, a large fish or a proud heron might be the first things that come to mind. But some animals instead encounter unusual predators, such as the waterwheel plant (Aldrovanda vesiculosa). This carnivorous plant bears small snap-traps arranged in leaf whorls along its stem. Each trap is equipped with two lobes that snap together when capturing prey. The prey is then digested and provides nutrients for the critically endangered plant that lives free-floating in nutrient poor waters. But who exactly ends up as prey in these deadly traps? And does the waterwheel lure the prey into its traps in some way (like its close relative the Venus fly trap does on land ) or does the prey get trapped by chance?
To find out more about the waterwheel’s “taste” in prey, a team of researchers opened 445 “fully-fed” snap-traps (with a size between about two to four and a half millimeters), which they collected from different sites in the Czech Republic and in Germany . They found that the traps contained mostly crustaceans: Water fleas (Cladocera) and two other groups of tiny crustaceans, ostracods and copepods, to be more precise. But a variety of prey from other taxonomic groups was also found in the traps, including insect larvae, snails, and water mites. The waterwheel plant thus seems to be a not very picky eater. Its diet contains a wide range of prey groups instead of concentrating on one particular group. However, the proportions of different prey groups found in its traps do vary, as well as the more exact composition within the groups. This shows that its meal plan is likely to be simply determined by what is available around. As the set of prey available at different locations and times will vary, the sample plants collected at different locations and times can reveal quite different prey compositions in their traps.
Researchers also looked into the prey’s behavior in an attempt to analyze the waterwheel’s preference: Some can swim rather fast (e.g. copepods can swim up to 350 mm/s ), while some swim more slowly (e.g. planktonic insect larvae); some can swim but are mostly bound to the substrate (e.g. ostracods) while some cannot swim at all (e.g. snails). But it turned out that the waterwheel plant didn’t mind these differences either and caught them all. It seems like it’s just bad luck for the prey: Snails using the waterwheel plant as a substrate to graze at might end up in the trap just like by-swimmers might get in between the trap’s lobes.
Overall, the waterwheel plant can apparently make use of a large variety of food sources, making it a generalist predator. Its snap-traps capture a broad range of different organisms with different mobility modes and of a broad range of sizes: The prey ranged from small water mites (0.67 mm) to the longest prey item, a chironomid (a family of flies) pupa of 4.32 mm, which could just barely fit in the traps. It seems that as long as they fit inside, they will be consumed. And the researchers also found a few traps that even contained more than one piece of prey. Well, enjoy your meal!
Frederike Klimm holds a Bachelor’s degree in Biology from University of Freiburg and is pursuing her studies there. Her field of work is plant biomechanics, particularly the attachment systems of marine algae and climbing plants and she loves sharing her fascination with others. Twitter: @Kelp_Action
 review by Horner J.D., Płachno B.J., Bauer U. &, Di Giusto B. (2018): Attraction of prey. In: Ellison A.M., Adamec L. (eds) Carnivorous plants: physiology, ecology, and evolution.
Oxford: Oxford University Press, pp. 157-66.
 Horstmann M., Heier L., Kruppert S., Weiss L.C., Tollrian R., Adamec L., Westermeier A., Speck T. & Poppinga S. (2019): Comparative Prey Spectra Analyses on the Endangered Aquatic Carnivorous Waterwheel Plant (Aldrovanda vesiculosa, Droseracea) at Several Naturalized Microsites in the Czech Republic and Germany.
 Strickler J.R. (1975): Swimming of Planktonic Cyclops Species (Copepoda, Crustacea): Pattern, Movements and Their Control. In: Wu T.Y.T., Brokaw C.J., Brennen C. (eds) Swimming and Flying in Nature. Boston, MA: Springer, pp. 599-613.