Aldrovanda

The waterwheel is a small, free floating and rootless aquatic plant, with a length of about 1.5 to 20 cm (9⁄16 to 7+7⁄8 in), and whorls of about 1 to 2 cm (3⁄8 to 13⁄16 in) in diameter.[6] At every 3 to 4 cm the plant branches, sometimes forming offshoots. An average of 12 to 19 whorls spans the length of the plant, each with about 5 to 9 leaves, each up to 11 mm long. The growth is faster than terrestrial carnivorous plants, sometimes growing about 4 to 9 mm a day.

In temperate regions the plant goes dormant in the winter, forming turions of about 4–6 mm (3⁄16–1⁄4 in) and sinking to the bottom. In tropical regions, the plant grows all year long without forming turions. The plant flourishes and produces seeds only in these warmer regions, with temperatures higher than 25 °C (77 °F), and produces only one flower, white or light pink, that arises above the surface. In temperate regions, the plant reproduces mostly via asexual means, producing inviable seeds or no flower at all.

The leaf structure is very similar to the Dionaea (commonly known as the Venus flytrap), the main difference being air chambers present in the "stem". The traps, at the end of the leaf, contains up to 6 bristles, analogous to Dionaea teeth, that prevent debris from activating the trap. Each trap contains an additional 60–80 smaller "teeth", and circa 30–40 trigger hairs inside. The speed of closing is about 0.01 to 0.02 seconds.

The extinct species are known only from fossil pollen and seeds,[8] with the exception of A. inopinata, which is also known from fossilised laminae.[9] Aldrovanda was for a long time thought to be related to the Late Cretaceous form taxon Palaeoaldrovanda splendens, but research published in 2010 suggests that remains attributed to Palaeoaldrovanda actually represent fossilised insect eggs.[10][11]

The organ of carnivory in Aldrovanda is the snap trap. Snap traps are only found in one other carnivorous plant genus, Dionaea. The two genera have been shown to share a most recent common ancestor by analysis of combined nuclear and chloroplast DNA sequences.[12][13] When sequences of chloroplast DNA of Aldrovanda, Dionaea, and Drosera were analyzed alone, however, Aldrovanda and Drosera (sundews) were shown to share a most recent common ancestor.[14] It has been proposed that this discrepancy between gene trees based on nuclear and cytoplasmic DNA analyses can be explained by chloroplast capture,[14] as similar inconsistencies have been explained by this phenomenon.[15][16]

Aldrovanda vesiculosa is listed as “Endangered” on the IUCN Red List.[17] It is native to Europe, Asia, Africa, and Australia, but is considered extinct in much of its native range, primarily due to eutrophication.[18] Despite being endangered in its native range, it was introduced in the U.S. and has been reported to be rapidly spreading in the waters of the Catskill Mountains of New York, USA, where it is being evaluated as a potentially problematic invasive species.[19]

Several undescribed species are also known.

The distinctions between the various named species have been criticised,[20] although SEM analysis of the seed structures seems to confirm the existence of different species.[21]