Urceolusspecies are single-celledeukaryotes or protists. Their cells are sack-shaped, narrow at the anterior end and expanded at the posterior end. The cells exhibit flexibility and squirming movements, more vigorous in some species. Like other phagotrophic protists, they present an organelle for ingestion known as a 'feeding apparatus',[8] an arrangement of microtubules beneath a concave portion of the cell membrane used for ingesting prey through phagocytosis.[9] Their cell body is deformable, but can be distinguished from other euglenids by a flared collar[10] or 'neck' in the anterior region, which hosts a canal where the feeding apparatus and the flagellum are located.[2] very small in some species. They have one emergent flagellum, but it is mostly active only at the tip. A rudimentary second flagellum is present in U. cyclostomus.[4]
Urceolus is a genus of phagotrophic flagellates belonging to the Euglenida, a highly diverse group that also contains the phototrophic euglenophyte algae. One trait that has been used to investigate the evolution from heterotrophic euglenids towards their phototrophic counterpart is the number of strips within the feeding canal. In Urceolus cyclostomus, the canal has a number of strips equivalent to the number of strips along the entire exterior of the cell (around 40). In contrast, more 'basal' heterotrophic euglenids such as Dinema have half as many strips in the canal as the cell exterior (around 20). This is known as the 'second strip duplication event', an evolutionary innovation that presumably led to more plastic movement (metaboly) and an increase in cell size for a clade uniting Urceolus, Peranema and the phototrophic euglenophytes,[17] known as Spirocuta.[10][18]
Further morphological traits seen in Urceolus, such as a swelling around the flagellum that resembles a photoreceptor, and a stigma that resembles the eyespot of euglenophyte algae, led to the hypothesis that Urceolus was the sister group of the euglenophytes.[17]Phylogenetic analyses through DNA sequences place all peranemids (e.g., Urceolus, Peranema and others) as a whole as the sister group to euglenophytes, rather than any particular genus. The following cladogram, based on a study published in 2021, represents these findings:[18]
The genusUrceolus was first described by Russian biologist Konstantin Mereschkowsky in 1877,[a] in a memoir on the protozoa of the north of Russia.[2] He described it to accommodate a rare species of flagellate that he discovered that same year in the White Sea, near the Solovetsky Monastery, named U. alenizini. He characterized this new genus by the unique urn or pitcher-shaped cells, and accordingly named it urceolus meaning 'pitcher' in Latin.[19][1] The next year, German zoologist Friderich Stein described a new flagellate by the name of Phialonema cyclostomum, the first species of its genus.[6] Upon comparing the two species, Mereschkowsky concluded that the appearance and description of P. cyclostomum matched his description of Urceolus, and he transferred it to his genus as U. cyclostomus. Consequently, Phialonema became a junior synonym of Urceolus in 1881.[2]
In 1887, American protozoologist Alfred Cheatham Stokes described the genus Urceolopsis to accommodate the species U. sabulosus. This genus is essentially equal in appearance to Urceolus, with the only difference being that the cell surface is covered in adherent sand grains.[7][20] It was later synonimised to Urceolus for that reason.[4][12]
Urceolus and other colorless, non-photosynthetic flexible flagellates such as Heteronema, Peranema and Anisonema were initially lumped together in the family Peranemidae, while similar but phototrophic, green-coloured algae such as Euglena composed the family Euglenidae. Both families belong to the Euglenida,[21] one of the major groups within the phylumEuglenozoa, a basal group of eukaryotes.[10] The taxonomic status of Peranemidae has changed through the years: first classified in the paraphyletic order Heteronematales,[3][4] it later became the sole family of the order Peranemida, regarded as the closest relatives of the photosynthetic euglenids.[18][22] Many colorless euglenids were transferred from Peranemidae to other groups, but Urceolus is one of the few genera that remains in the family.[23]
Several authors note that there is considerable ambiguity in the identification of Urceolus species, because most original descriptions are inadequate and confident identification is not always possible. In addition, detail is often obscured by adhering particles of detritus.[4][14]
^K. S. Mereschkowsky (1877). "Etyudy nad prosteyshimi zhivotnymi severa Rossii" Этюды над простейшими животными севера России [Studies on protozoa of northern Russia]. Trudy S.-Peterburgskago Obshchestva Estestvoispytatelei Труды Санкт-Петербургскаго Общества естествоиспытателей [Proceedings of the St. Petersburg Society of Naturalists] (in Russian). 8: 203–376.
^ abK. I. Prokina (July 2019). "Heterotrophic Flagellates from Sphagnum Bogs and Terrace-Forest and Floodplain Water Bodies of the Central Russian Forest-Steppe". Inland Water Biology. 12 (3): 276–289. doi:10.1134/S199508291903012X. ISSN1995-0829. WikidataQ119981891.
^ abPetr Hašler; Jana Štěpánková; Jana Špačková; et al. (1 September 2008). "Epipelic cyanobacteria and algae: a case study from Czech ponds". Fottea (Praha). 8 (2): 133–146. doi:10.5507/FOT.2008.012. ISSN1802-5439. WikidataQ119649001.
^ abcLax G, Kolisko M, Eglit Y, Lee WJ, Yubuki N, Karnkowska A, Leander BS, Burger G, Keeling PJ, Simpson AGB (2021). "Multigene phylogenetics of euglenids based on single-cell transcriptomics of diverse phagotrophs". Molecular Phylogenetics and Evolution. 159 (107088). doi:10.1016/j.ympev.2021.107088.
^H.W. Conn; C.H. Edmondson (1918). "Chapter IX. Flagellate and Ciliate Protozoa (Mastigophora et Infusoria)". In Henry Baldwin Ward; George Chandler Whipple (eds.). Fresh-water biology. New York: John Wiley & Sons. pp. 238–300. doi:10.5962/bhl.title.160213.
