thumb|right|Germinating Aldrovanda seeds
Aldrovanda vesiculosa, commonly known as the waterwheel plant, is the sole extant species in the flowering plant genus Aldrovanda of the family Droseraceae. The plant captures small aquatic invertebrates using traps similar to those of the Venus flytrap. The traps are arranged in whorls around a central, free-floating stem, giving rise to the common name. This is one of the few plant species capable of rapid movement.
While the genus Aldrovanda is now monotypic, up to 19 extinct species are known in the fossil record. While the species displays a degree of morphological plasticity between populations, A. vesiculosa possesses a very low genetic diversity across its entire range. It is kept by hobbyists.
Morphology
Aldrovanda vesiculosa is a rootless aquatic plant. Seedlings develop a short protoroot; however, this fails to develop further and senesces. The plant consists of floating stems reaching a length of . The trap leaves grow in whorls of between 5 and 9 in close succession along the plant's central stem. The actual traps are held by petioles which have air sacs that aid in flotation. One end of the stem continually grows while the other end dies off. Growth is quite rapid ( per day in Japanese populations), so that in optimal conditions a new whorl is produced once or more each day.
Trap
The actual traps consist of two lobes which fold together to form a snap-trap similar to that of the Venus flytrap, except that it is smaller and located underwater. These traps, which are twisted so that the trap openings point outward, are lined on the inside by a fine coating of trigger hairs, snapping shut in response to contact with aquatic invertebrates and trapping them. The closing of this trap takes 10–20 milliseconds, making it one of the fastest examples of plant movement in the kingdom. This trapping is only possible in warm conditions of at least . Each trap is surrounded by between four and six long bristles that prevent triggering of traps by debris in the water.
Nutrient acquisition
A. vesiculosa is able to grow in nutrient-poor habitats not only due to its carnivory, but also due to its ability to re-utilize nutrients from senesced shoots, and its high affinity for mineral nutrients in water.
Reproduction
Flowers
The small, solitary white flowers of A. vesiculosa are supported above the water by short peduncles which arise from whorl axes. The flower only opens for a few hours, after which the structure is brought back beneath the water for seed production. The seeds are cryptocotylar: the cotyledons remain hidden within the seed coat and serve as an energy store for the seedling. Flowering, however, is rare in temperate regions and poorly successful in terms of fruit and seed development. resulting in new plants as the tips continue to grow and the old ends die off and separate. Due to the rapid growth rate of this species, countless new plants can be produced in a short period of time in this fashion.
thumb|Trap of Aldrovanda
Turions
Winter-hardy Aldrovanda form turions as a frost survival strategy. At the onset of winter, the growth tip starts producing highly reduced non-carnivorous leaves on a severely shortened stem. This results in a tight bud of protective leaves which, being heavier and having released flotational gases, breaks off the mother plant and sinks to the water bottom, where temperatures are stable and warmer. Here it can withstand temperatures as low as . In the wild, Aldrovanda turions have been observed to have a relatively low rate of successful sinking.
The Waterwheel (Aldrovanda vesiculosa) thrives in a range of aquatic habitats, including small fens, peat-bog pools, billabongs, lakes, lagoons, and river deltas. It prefers oligo-mesotrophic and dystrophic systems with low nutrient levels. These plants are commonly found in shallow backwaters or the littoral zones of larger lakes, where they face less competition from other aquatic species and where water levels remain relatively stable throughout the growing season. The Waterwheel is highly intolerant of habitat degradation, and even minor changes in water chemistry can lead to local extinction. When Carl Linnaeus published his Species Plantarum in 1753, the "i" was dropped from the name (an apparent copying error) to form the modern binomial.