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.

Die stark gefährdete karnivore Wasserfalle (Aldrovanda vesiculosa, Droseraceae) bildet Unterwasser-Schnappfallen zum Fang von aquatischen Kleinstlebewesen aus, jedoch wurde das Beutespektrum bislang noch nicht ausreichend untersucht. Eine genaue Kenntnis der Zusammensetzung der Beutetiere ist jedoch sehr wichtig, um Fragestellungen bzgl. der Ökologie dieser fleischfressenden Pflanze, der Effizienz und Evolution ihrer komplexen Fallen, sowie hinsichtlich Naturschutzmaßnahmen beantworten zu können. Hierfür wurden bei künstlich wieder angesiedelten, etablierten Vorkommen in einem deutschen und in sieben tschechischen Kleinst-Habitaten die Fallengrößen gemessen und Falleninhalte vergleichend analysiert. Die untersuchten Pflanzen entstammen dabei aus mindestens zwei mitteleuropäischen Ursprungspopulationen. Ein tschechischer Standort wurde im Laufe des Jahres 2017 zweimal beprobt und hinsichtlich der Saisonalität ausgewertet. Es wurden sieben Hauptgruppen an Beutetieren erfasst: Cladocera, Copepoda, Ostracoda, Ephemeroptera, Nematocera, Hydrachnidia und Pulmonata. Insgesamt konnten in den 445 untersuchten Fallen, die insgesamt 461 gefangene Tiere enthielten, 43 Beutetaxa bestimmt werden. Die Beute-Spektren korrelierten dabei (mit einer Ausnahme) nicht mit den Standortbedingungen (z.B. Wassertiefe) oder der Größe der Fallen. Unsere Daten zeigen, dass A. vesiculosa keine Beutespezifität aufweist, sondern opportunistisch Beute fängt, unabhängig von Taxonomie, Fortbewegungsmodus (schwimmend oder kriechend) oder der Bewegungsgeschwindigkeit der Beute. Erfolgreiche Fänge durch Einklemmen des Tieres zwischen den Fallenhälften wurden auch bei Beutetieren mit Körpermaßen beobachtet, die die Größe der Falle deutlich überschritten. Durch die hohe Diversität an Beutetaxa erscheint es unwahrscheinlich, dass es spezielle chemische oder Mimikry-basierende Faktoren zur Anlockung gibt. Die Pflanze könnte jedoch als Ruhe- oder Ansitzplatz, sowie als Substrat zum Abweiden (z.B. von Algen) dienen und so indirekt Beuteorganismen in der Umgebung anlocken. Die Beutefangereignisse, wie z.B. die von vergleichsweise großen Notonectiden, könnten jedoch auch rein zufällig sein.

Department of Animal Ecology Evolution and Biodiversity Ruhr University Bochum Universitätsstraße 150 D 44780 Bochum Germany

Freiburg Center for Interactive Materials and Bioinspired Technologies University of Freiburg Georges Koehler Allee 105 D 79110 Freiburg im Breisgau Germany

Freiburg Materials Research Center University of Freiburg Stefan Meier Straße 21 D 79104 Freiburg im Breisgau Germany

Friday Harbor Laboratories University of Washington 620 University Road WA 98250 USA

Institute of Botany of the Czech Academy of Sciences Dukelská 135 CZ 379 82 Třeboň Czech Republic

Plant Biomechanics Group Botanic Garden University of Freiburg Schänzlestraße 1 D 79104 Freiburg im Breisgau Germany

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A Chemometry of Aldrovanda vesiculosa L. (Waterwheel, Droseraceae) Populations