The choanoflagellates are a group of flagellateprotozoa. They are considered to be the closest relatives of the animals, and in particular may be the direct ancestors of sponges.
Each choanoflagellate has a single flagellum, surrounded by a ring of hairlike protrusions called microvilli, forming a cylindrical or conical collar (choanos in Greek). The flagellum pulls water through the collar, and small food particles are captured by the microvilli and ingested. It also pushes free-swimming cells along, as in animal sperm, whereas most other flagellates are pulled by their flagella.
Most choanoflagellates are sessile, with a stalk opposite the flagellum. A number of species are colonial, usually taking the form of a cluster of cells on a single stalk. Of special note is Proterospongia, which takes the form of a glob of cells, of which the external cells are typical flagellates with collars, but the internal cells are non-motile.
The choanocytes of sponges have the same basic structure as choanoflagellates. Collared cells are occasionally found in a few other animal groups, such as flatworms.
Yet choanoflagellates must have existed on the Earth since the Late Precambrian, because they are the closest living protist relatives of the sponges, the most primitive metazoans.
Choanoflagellates are almost identical in shape and function with the choanocytes, or collar cells, of sponges; these cells generate a current that draws water and food particles through the body of a sponge, and they filter out food particles with their microvilli.
A few living choanoflagellates, such as Proterospongia, are colonial for part of their life cycle, and show a limited degree of cell differentiation and integration into a unit; these colonial choanoflagellates are the best living examples of what the ancestor of all metazoans may have looked like.
Choanoflagellates are found globally in marine, brackish and freshwater environments from the Arctic to the tropics, occupying both pelagic and benthic zones.
Irrespective of their distribution, choanoflagellates are in high abundance relative to other nanoplankton members in their communities, and there is a positive correlation between primary producers and choanoflagellate densities, supporting a model in which choanoflagellates play a pivotal role in the microbial food web and carbon cycling (Buck and Garrison, 1988).
Choanoflagellates are either free-swimming in the water column or sessile, adhering to the substrate directly or through either the periplast or a thin pedicel (Leadbeater, 1983).
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