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Choanocytes (also known as "collar cells") arecells that line the interior ofasconoid, syconoid and leuconoid body types ofsponges that contain a centralflagellum, orcilium, surrounded by a collar ofmicrovilli which are connected by a thin membrane.
They make up thechoanoderm, a type of cell layer found insponges. The cell has the closest resemblance to thechoanoflagellates 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 thenfilter nutrients from the water taken from the collar of the sponge. Food particles are thenphagocytosed by the cell.[2]
Choanocytes are found dotting the surface of the spongocoel inasconoid sponges and the radial canals insyconoid sponges, but they comprise entirely the chambers inleuconoid sponges.
By cooperatively moving their flagella, choanocytes filter particles out of the water and into thespongocoel, and out through theosculum. 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 theamoebocytes for delivery to other cells.
Choanocytes can also turn intospermatocytes when needed for sexual reproduction, due to the lack of reproductive organs in sponges (amoebocytes become theoocytes).
Choanocytes bear a superficial resemblance toChoanoflagellates. Molecular phylogenies indicate that choanoflagellates andmetazoans are sister groups. One can see some modern choanoflagellates living in small colonies. The evolutionary relationship between the two cell types is debated.[3]
