Aldrovanda is a genus of aquatic carnivorous plants in the sundew family Droseraceae (order Saxifragales). It contains a single extant species — the waterwheel plant (Aldrovanda vesiculosa) — along with several extinct taxa known from fossil pollen and seeds. The genus was established by Linnaeus in 1753 and named after the Italian naturalist Ulisse Aldrovandi.
These are small, free-floating, rootless perennial herbs that drift in shallow standing waters. Plants reach 1.5 to 20 cm in length, with leaf whorls 1–2 cm in diameter spaced every 3–4 cm along the stem. Each whorl bears 5–9 leaves up to 11 mm long, united at the base. The leaf blade is articulated into two segments that form a rapid snap trap — a mechanism shared only with the Venus flytrap (Dionaea), its closest relative. Each trap bears up to 6 sensory bristles, 60–80 smaller marginal teeth, and 30–40 trigger hairs, and closes in roughly 0.01–0.02 seconds when stimulated. The petiole is cuneate with 4–8 laciniate bristles, and the stem carries sensitive hairs and numerous digestive glands that release lytic enzymes to break down prey, primarily zooplankton.
Flowers are axillary, solitary, borne on a short scape 0.5–1.5 cm tall that rises just above the water surface for a single day. They have 5 sepals and 5 white or greenish-white oblong petals, 5 stamens 3–4 mm long with subulate filaments, a superior subglobose 5-carpellate ovary, and 5 styles with multifid stigmas. The fruit is a subglobose indehiscent capsule containing 5–8 ovoid-globose seeds. In temperate regions the plant reproduces mostly asexually, producing inviable seeds or no flower at all, and survives winter by forming dormant turions 4–6 mm in diameter that sink to the bottom. In tropical regions it grows year-round without turion formation, favoring temperatures above 25°C and capable of producing up to 200 offspring in a single growing season.
The only extant species, A. vesiculosa, is listed as Endangered on the IUCN Red List, having gone extinct across much of its historical range due to eutrophication of its wetland habitats.
Etymology
The genus Aldrovanda was named by Linnaeus in honor of the Italian naturalist Ulisse Aldrovandi (1522–1605), who founded the Botanical Garden of Bologna (Orto Botanico dell'Università di Bologna) and was one of the pioneering naturalists of the Renaissance.
Distribution
Aldrovanda has been recorded from scattered locations across Europe, central and southern Europe, eastern, northern, and southeastern Asia, Africa (including Madagascar), and Australia. The genus was introduced to the United States and has been reported spreading rapidly in the Catskill Mountains of New York, where it is being evaluated as a potentially problematic invasive species. It is considered extinct across much of its native range.
Ecology
Aldrovanda grows in shallow standing waters rich in humic substances. It is a carnivorous plant that preys primarily on zooplankton, with trigger hairs inside the traps thought to mimic algae to attract small grazing crustaceans. In temperate regions it overwinters as dormant turions that sink to the bottom; in tropical regions it grows year-round. Optimal growth occurs above 25°C, with plants capable of producing up to 200 vegetative offspring per season.
Conservation
The sole extant species Aldrovanda vesiculosa is listed as Endangered on the IUCN Red List (assessed by Cross & Adamec, 2020). The primary threat is eutrophication of its shallow freshwater habitats, which has driven the plant to extinction across much of its historical range in Europe, Asia, Africa, and Australia. Paradoxically, introduced populations in the United States are spreading and being evaluated as potentially invasive.
Taxonomy
Aldrovanda belongs to the family Droseraceae and is one of only two plant genera known to possess snap traps, the other being Dionaea (Venus flytrap). Combined nuclear and chloroplast DNA analyses indicate that Aldrovanda and Dionaea share a most recent common ancestor. However, when chloroplast DNA alone is analyzed, Aldrovanda groups with Drosera (sundews) rather than Dionaea. This discrepancy has been attributed to chloroplast capture. The extinct species of Aldrovanda are known primarily from fossil pollen and seeds, with A. inopinata also known from fossilized leaf laminae. The late Cretaceous form taxon Palaeoaldrovanda splendens, long thought related, was shown in 2010 to represent fossilized insect eggs rather than plant remains.
