Choanocytes (also known as "collar cells") are cells that line the interior of

asconoid Sponges or sea sponges are primarily marine invertebrates of the animal phylum Porifera (; meaning 'pore bearer'), a basal clade and a sister taxon of the diploblasts. They are sessile filter feeders that are bound to the seabed, and are on ...

, syconoid and leuconoid body types of

sponges Sponges or sea sponges are primarily marine invertebrates of the animal phylum Porifera (; meaning 'pore bearer'), a basal clade and a sister taxon of the diploblasts. They are sessile filter feeders that are bound to the seabed, and ar ...

that contain a central

flagellum A flagellum (; : flagella) (Latin for 'whip' or 'scourge') is a hair-like appendage that protrudes from certain plant and animal sperm cells, from fungal spores ( zoospores), and from a wide range of microorganisms to provide motility. Many pr ...

, or ''cilium,'' surrounded by a collar of

microvilli Microvilli (: microvillus) are microscopic cellular membrane protrusions that increase the surface area for diffusion and minimize any increase in volume, and are involved in a wide variety of functions, including absorption, secretion, cellula ...

which are connected by a thin membrane. They make up the

choanoderm The choanoderm is a type of cell layer composed of flagellated collar cells, or choanocytes, found in sponges. The sponge body is mostly a connective tissue; the mesohyl, over which are applied epithelioid monolayers of cells, the outer pinaco ...

, a type of cell layer found in

. The cell has the closest resemblance to the

choanoflagellates Choanoflagellates are a group of free-living unicellular and colonial flagellate eukaryotes considered to be the closest living relatives of animals. The name refers to the characteristic funnel-shaped "collar" of interconnected microvilli and t ...

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 nutrients from the water taken from the collar of the sponge. Food particles are then phagocytosed by the cell.Anderson, D. (2001) ''Invertebrate Zoology'' Oxford University Press Choanocytes are found dotting the surface of the spongocoel in

sponges and the radial canals in syconoid sponges, but they comprise entirely the chambers in

leuconoid Sponges or sea sponges are primarily marine invertebrates of the animal phylum Porifera (; meaning 'pore bearer'), a Basal (phylogenetics) , basal clade and a sister taxon of the Eumetazoa , diploblasts. They are sessility (motility) , sessile ...

sponges.

Morphology

Function

By cooperatively moving their flagella, choanocytes filter particles out of the water and into the

spongocoel A spongocoel (), also called paragaster (or paragastric cavity), is the large, central cavity of sponges. Water enters the spongocoel through hundreds of tiny pores ( ostia) and exits through the larger opening ( osculum). Depending on the body pl ...

, and out through the

osculum The osculum (: oscula) is an excretory structure in the living sponge, a large opening to the outside through which the current of water exits after passing through the spongocoel. Wastes diffuse into the water and the water is pumped through ...

. 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 Spermatocytes are a type of male gametocyte in animals. They derive from immature germ cells called spermatogonia. They are found in the testis, in a structure known as the seminiferous tubules. There are two types of spermatocytes, primary and s ...

s when needed for sexual reproduction, due to the lack of reproductive organs in sponges (amoebocytes become the

oocyte An oocyte (, oöcyte, or ovocyte) is a female gametocyte or germ cell involved in reproduction. In other words, it is an immature ovum, or egg cell. An oocyte is produced in a female fetus in the ovary during female gametogenesis. The female ger ...

s).

Evolutionary significance

Choanocytes bear a superficial resemblance to

Choanoflagellates Choanoflagellates are a group of free-living unicellular and colonial flagellate eukaryotes considered to be the closest living relatives of animals. The name refers to the characteristic funnel-shaped "collar" of interconnected microvilli and t ...

. 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. 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

References

{{reflist Histology Animal cells Sponge anatomy

Morphology

Function

Evolutionary significance

See also

References