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((Refimprove|date=December 2009))
((Annotated image/Porifera cell types))
'''Choanocytes''' (also known as "collar cells") are [[cell (biology)|cell]]s that line the interior of [[asconoid]], syconoid and leuconoid body types of [[sea sponge|sponges]] that contain a central [[flagellum]], or ''cilium,'' surrounded by a collar of [[microvilli]] which are connected by a thin membrane.   
[[File:Choanocyte.jpg|thumb|Choanocyte]]
They make up the [[choanoderm]], a type of cell layer found in [[sponges]]. The cell has the closest resemblance to the [[choanoflagellates]] which are the closest related single celled protists to the animal kingdom (metazoans).  The flagellae beat regularly, creating a water flow across the microvilli which can then [[filter feeder|filter]] nutrients from the water taken from the collar of the sponge. Food particles are then [[phagocytosed]] by the cell. <ref>Anderson, D. (2001) ''Invertebrate Zoology'' Oxford University Press</ref>

==Location==
Choanocytes are found dotting the surface of the spongocoel in [[asconoid]] sponges and the radial canals in [[syconoid sponges]], but they comprise entirely the chambers in [[leuconoid]] sponges.

==Function==
By cooperatively moving their flagella, choanocytes filter particles out of the water and into the [[spongocoel]], and out through the [[osculum]]. This improves both respiratory and digestive functions for the sponge, pulling in oxygen and nutrients and allowing a rapid expulsion of carbon dioxide and other waste products. Although all cells in a sponge are capable of living on their own, choanocytes carry out most of the sponge's ingestion, passing digested materials to the [[amoebocytes]] for delivery to other cells.

Choanocytes can also turn into [[spermatocyte]]s when needed for sexual reproduction, due to the lack of reproductive organs in sponges (amoebocytes become the [[oocyte]]s).

==Evolutionary Significance==
Choanocytes bear a superficial resemblance to [[Choanoflagellates]]. Molecular phylogenies indicate that choanoflagellates and [[metazoans]] are sister groups. One can see some modern choanoflagellates living in small colonies. The evolutionary relationship between the two cell types is debated.<ref name=mah2014> Jasmine L. Mah, Karen K. Christensen‐Dalsgaard, Sally P. Leys "Choanoflagellate and choanocyte collar‐flagellar systems and the assumption of homology", 2014, https://doi.org/10.1111/ede.12060</ref>

==See also==
* [[Choanoderm]]

==References==
((reflist))

[[Category:Histology]]
[[Category:Animal cells]]
[[Category:Sponge anatomy]]

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