Anisonema

Anisonema is a genus of phagotrophic, single-celled flagellates found in fresh, marine and brackish waters.^[2] Each cell has two unequal, heterodynamic flagella: a shorter, thinner anterior flagellum that flails freely in front of the cell; and a longer (usually two to three times longer than the cell), often thicker, hook-shaped posterior flagellum that trails behind the cell when moving. Both flagella emerge from the anterior region of a slit-like groove located on the ventral side of the cell, where the posterior one is thickest.^[3][4][5] When moving, Anisonema cells glide on their posterior flagellum,^[6] and can reverse the direction of their movement quickly^[7] and periodically.^[4] Unlike other flagellates, the flagellar apparatus of Anisonema replicates in the late stages of mitosis, as opposed to the early stages.^[2]

The cells themselves are generally ovoid, more or less flattened and asymmetrical, usually measuring 20–70 μm in length and 12–19 μm in width.^[3] They contain a nucleus at the posterior end, contractile vacuole at the anterior end, and a long, wedge-shaped cytopharynx or feeding organelle that can be difficult to observe.^[7][8] They also contain a reservoir on the left side.^[3] The cell surface maintains a constant rigid shape most of the time, but is capable of some deformation.^[5] It is supported underneath by a non-thickened pellicle composed of more than 18 protein strips, as in other spirocute euglenids.^[3][9] Anisonema is similar in appearance to Dinema, but lacks its flexibility and thickened pellicle.^[4]

The genus Anisonema was first named by French biologist Félix Dujardin in his 1841 work Histoire naturelle des zoophytes. He distinguished it from other genera of biflagellates with unequal flagella due to its more resistant, non-contractile surface that is also often perfectly transparent. He erected two species, A. acinus and A. sulcata.^[10] The latter was transferred to the genus Entosiphon,^[3] while A. acinus is considered the type species.^[9][2]

Since its publication, around 20-36^[4] additional species of Anisonema have been described, but it is unclear how many of these actually belong to Anisonema, as most of them are only known from morphology and lack any genetic data. They are therefore considered morphospecies.^[9] Assignment to this genus is often problematic. For example, some authors have defended that Anisonema species are distinguished by the lack of a visible specialized ingestion organelle, but other authors have observed this organelle in several specimens.^{[7]: 75 [9]} Moreover, the description of the type species itself is considered insufficient to be accurately identified.^[3]

The only molecular data available up to date has been obtained from A. acinus-like cells, which, according to phylogenetic analyses, belong to the clade Spirocuta within euglenids.^[9] In particular, Anisonema belongs to the monophyletic family Anisonemidae which also contains the genus Dinema; both genera glide only on their posterior flagellum and are moderately flexible. Anisonemidae is, in turn, part of the clade Anisonemia, which also contains the osmotrophic Aphagea and the genus Neometanema.^[11] Additional genetic sequences have been isolated from species that were previously assigned to Anisonema, but these are phylogenetically more related to ploeotiids and have been transferred to the new genera Hemiolia and Liburna.^[9]

Listed below are the currently accepted species of Anisonema:

Previously recognized species include A. glaciale,^[3] transferred to Liburna, and A. trepidum,^[15] transferred to Hemiolia.^[9]