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Choanocytes (also known as "collar cells") are cells that line the interior of Asconoid,syconoid and leuconoid body type sponges that contain a central flagellum surrounded by a collar of microvilli which are connected by a thin membrane. It is the closest family member to the free-living ancestor called choanoflagellate. The flagellae beat regularly, creating a water flow across the microvilli which can then filter nutrients and other food from the water taken from the collar of the sponge. Food particles are then phagocytosed by the cell.

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 generate a flow of water through the sponges pores, 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 spermatocytes when needed for sexual reproduction, due to the lack of reproductive organs in sponges (amoebocytes become the oocytes).

Evolutionary Significance

They bear more than a passing resemblance to Choanoflagellates, and demonstrate key steps in the evolution of animals. DNA sequencing indicates that multicellular choanoflagellates and poriferans are sister groups, both descended from the same eukaryotic clade. One can see modern choanoflagellates living in small colonies, illustrating the evolution of sponges by analogy. More complex animals, notably the cnidarians, possess cells whose structures are clearly derived from choanocytes, demonstrating their historical ties to phylum porifera.

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