Rotifer

The rotifers (/ˈroʊtɪfərz/, from Latin rota 'wheel' and -fer 'bearing'), sometimes called wheel animals or wheel animalcules,^[1] make up a phylum (Rotifera /roʊˈtɪfərə/) of microscopic and near-microscopic pseudocoelomate animals.

They were first described by Rev. John Harris in 1696, and other forms were described by Antonie van Leeuwenhoek in 1703.^[2] Most rotifers are around 0.1–0.5 mm (0.0039–0.0197 in) long (although their size can range from 50 μm (0.0020 in) to over 2 mm (0.079 in)),^[1] and are common in freshwater environments throughout the world with a few saltwater species.

Some rotifers are free swimming and truly planktonic, others move by inchworming along a substrate, and some are sessile, living inside tubes or gelatinous holdfasts that are attached to a substrate. About 25 species are colonial (e.g., Sinantherina semibullata), either sessile or planktonic. Rotifers are an important part of the freshwater zooplankton, being a major foodsource and with many species also contributing to the decomposition of soil organic matter.^[3] Genetic evidence indicates that the parasitic acanthocephalans are a highly specialised group of rotifers.^[4]

Most species of the rotifers are cosmopolitan, but there are also some endemic species, like Cephalodella vittata to Lake Baikal.^[5] Recent barcoding evidence, however, suggests that some 'cosmopolitan' species, such as Brachionus plicatilis, B. calyciflorus, Lecane bulla, among others, are actually species complexes.^[6][7] In some recent treatments, rotifers are placed with acanthocephalans in a larger clade called Syndermata.

In June 2021, biologists reported the restoration of bdelloid rotifers after being frozen for 24,000 years in the Siberian permafrost.^[8][9] The earliest record of the rotifer clade is of an acanthocephalan from the Middle Jurassic of China.^[4] Earlier purported fossils of rotifers have been suggested in Devonian^[10] and Permian^[11] fossil beds.