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The “collared flagellates” (choanoflagellates) are either unicells or colonies, each cell having one apical flagellum surrounded by a unique cone-shaped collar of microvilli (a.k.a. tentacles) made of contractile actin for capturing and ingesting bacteria. The cone may appear to be a solid surface except in high-resolution microscopy (see 19th Century sketches). Cells are 3 - 10 um diameter. The flagellum pushes the cell backward, similar to animal sperm, in contrast to most flagellates that are pulled apically. Cell propulsion draws water and potential food particles inward through the microvilli and outward, pushing the cell backward if it's not attached to a substrate. Many of the ~150 species are unicells while others form colonies. Choanoflagellates are considered the closest predecessor of animals (metazoans), as they are almost identical to sponge choanocytes. Conversely they could be seen as highly reduced sponges. For an excellent concise summary of choanoflatellates see Fairclough and King (2006). Earlier classified as Craspedomonadophycidees within the Chrysophyceae (Bourrelly 1968), their anatomical structure was described as so different that they had no place in either 'algae' or the plant kingdom (Hibberd 1975).
Computer-generated models of progressive assembly of silica costal strips into loricas (Leadbeater et al. 2009) online.
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| Biosilicification: | ||||
Loricate choanoflagellates, similar to diatoms, chrysophytes and other groups of organisms, produce silica (silicon dioxide) structures. Germanium interferes with the process, and causes deformities with increasing Ge/Si ratios (Aiken et al. 2016). While sponges produce silica spicules, the genes related to silicification in choanoflagellates are missing, suggesting a different mechanism (Marron et al. 2013).
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Habitat of choanoflagellates: |
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Occupy aquatic habitats globally from pole to pole, freshwater, brackish and marine locations, planktonic and benthic. Planktonic forms are propelled by the single anterial flagellum that pushes the cell backward, while simultaneously collecting bacteria as food filtered through the microvilli.
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| References: | ||||
Aitken, Z.H., A. Luo, S.N. Reynolds, C. Thaulow, and J.R. Greer 2016. Microstructure provides insights into evolutionary design and resilience of Coscinodiscus sp. frustule. Proceedings of the National Academy of . Science USA 113(8):2017-2012. Bourrelly, P. 1968. Les Algues d'eau douce: Initiation a'la Systematique. Tome II, p Les Algues jaunes et brunes. Chrysophycees, Pheophycees, Xanthopycees et Diatomees. N. Boubee & Cie, Paris. Fairclough, Stephen and Nicole King. 2006. Choanoflagellates, collared-flagellates. Version 14 August 2006. online in The Tree of Life Web Project, http://tolweb.org/ Hibberd, D.J. 1975. Observations on the ultrastructure of the choanoflagellate Codosiga botrytis (Ehr.) Saville-Kent with special reference to the flagellar apparatus. Journal of Cell Science 17(1):191-219. Leadbeater, B.S.C., Q. Yu, J. Kent, and D.J. Stekel 2009. Three-dimensional images of choanoflagellate loricae. Proc. R. Soc. B 276(1654): 3-11. online. Marron, A.O., M.J. Alston, D. Heavens, M. Akam, M Caccamo, P.W.H. Holland and G. Walker 2013. A family of diatom-like silicon transporters in the siliceous loricate choanoflagellates. Proceedings of the Royal Society B Biological Sciences 280(1756): 20122543
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