Comparative Prey Spectra Analyses on the Endangered Aquatic Carnivorous Waterwheel Plant (Aldrovanda vesiculosa, Droseraceae) at Several Naturalized Microsites in the Czech Republic and Germany

Abstract

Synopsis The critically endangered carnivorous waterwheel plant (Aldrovanda vesiculosa, Droseraceae) possesses underwater snap traps for capturing small aquatic animals, but knowledge on the exact prey species is limited. Such information would be essential for continuing ecological research, drawing conclusions regarding trapping efficiency and trap evolution, and eventually, for conservation. Therefore, we performed comparative trap size measurements and snapshot prey analyses at seven Czech and one German naturalized microsites on plants originating from at least two different populations. One Czech site was sampled twice during 2017. We recorded seven main prey taxonomic groups, that is, Cladocera, Copepoda, Ostracoda, Ephemeroptera, Nematocera, Hydrachnidia, and Pulmonata. In total, we recorded 43 different prey taxa in 445 prey-filled traps, containing in sum 461 prey items. With one exception, prey spectra did not correlate with site conditions (e.g. water depth) or trap size. Our data indicate that A. vesiculosa shows no prey specificity but catches opportunistically, independent of prey species, prey mobility mode (swimming or substrate-bound), and speed of movement. Even in cases where the prey size exceeded trap size, successful capture was accomplished by clamping the animal between the traps’ lobes. As we found a wide prey range that was attracted, it appears unlikely that the capture is enhanced by specialized chemical- or mimicry-based attraction mechanisms. However, for animals seeking shelter, a place to rest, or a substrate to graze on, A. vesiculosa may indirectly attract prey organisms in the vicinity, whereas other prey capture events (like that of comparably large notonectids) may also be purely coincidental.

Prey capture analyses in the carnivorous aquatic waterwheel plant (Aldrovanda vesiculosa L., Droseraceae)

“…11 ) were discussed to mimic filamentous algae in order to lure grazing crustaceans into the snap-trap 28 , but clarifying studies are missing so far. Furthermore, since Middle European A. vesiculosa shows no prey specificity but catches opportunistically, independent of prey species, size, mobility mode, and speed of movement, it is unlikely that the capture is enhanced by specialized chemical- or mimicry-based attraction mechanisms 7 . From the erratic swimming behaviour by D. longicephala we could not discriminate and observe any behaviour indicating attraction towards the trap.…”

“…How A. vesiculosa is able to capture and retain animals larger than the actual traps, as e.g. chironomid pupae or notonectid nymphs 6,7 , remains to be investigated. Probably, the teeth on the infolded rim (Fig.…”

“…Future experiments could complementarily analyse PCAs with different natural prey animals, e.g. members of Cladocera, Copepoda, Ostracoda, Ephemeroptera, Nematocera, Hydrachnidia, and Pulmonata 7 . 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.…”

“…by their color 5 . However, prey spectra have only been analysed in plants from Middle-European natural 6 and naturalized 7 sites. The results suggest that the waterwheel plant non-selectively captures a wide range of different taxonomic animal groups (including different water flea taxa) of different sizes and with different movement behaviour (substrate-bound, grazer, slow and fast swimmers).…”

“…Even faster are aquatic bladderworts ( Utricularia spp., Lentibulariaceae) with suction traps, which are comparable in size and prey spectra to A. vesiculosa snap-traps. The Southern bladderwort ( U. australis ), which occasionally co-occurs with A. vesiculosa 6,7 , can engulf its prey within 5.2 ms at fastest (9 ms on average) 12 .

Figure 1The snap-trap of Aldrovanda vesiculosa and the prey daphniid Daphnia longicephala . ( a ) A trap in the open condition, prior to triggering.

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