Choanoflagellate

Choanoflagellates are single-celled or colonial eukaryotes that live freely in water and are the nearest living relatives of animals. Their name comes from the Greek word for "funnel," describing the ring of tiny, finger-like projections called microvilli that surrounds a single whip-like tail, or flagellum. These organisms are found in aquatic habitats around the world and are key to understanding how animals evolved from single cells to multicellular forms.

The flagellum sits at the top of the cell, with microvilli forming a collar around its base. As the flagellum beats, it pulls water through this collar, trapping bacteria and debris against the microvilli, which then engulf the food. This movement also propels the cell forward—unlike most other flagellates, which are pulled—similar to how animal sperm swim. Inside the cell, the arrangement of organelles is consistent: a flagellar basal body sits at the base of the flagellum, with a second, non-flagellar basal body at a right angle. The nucleus is near the top or center, while food vacuoles collect at the bottom. Many choanoflagellates are covered by an extracellular matrix or periplast, which varies in structure and helps taxonomists classify them. Some build intricate basket-like houses called lorica from silica strips. The periplast may aid attachment in sessile species or increase drag in planktonic ones, possibly improving feeding efficiency.

Choanoflagellates can swim freely or attach to surfaces, either directly or via a thin stalk. They are strictly free-living and heterotrophic, though some relatives are parasites. Their life cycles are not well understood. Most species are solitary, but colonial forms have evolved several times, and even colonial species have a solitary stage.

Over 125 species exist, found worldwide in marine, brackish, and freshwater environments, from the Arctic to the tropics, in both open water and bottom sediments. Most sampling occurs in the top 25 meters, but they have been found as deep as 300 meters in open water and 100 meters under Antarctic ice. Some species appear to be cosmopolitan, while others have limited ranges. Even co-occurring species may occupy different microenvironments, but what drives their distribution is still unclear.

Some species, like those in *Proterospongia*, form simple colonies—either flagellated clusters resembling tiny grape bunches or cells on a single stalk. *Barroeca monosierra*, a colonial species from Mono Lake, forms hollow spheres filled with a branched network of extracellular matrix that houses symbiotic bacteria. In 2019, researchers discovered that choanoflagellates can coordinate their response to light. Studies on *Salpingoeca rosetta* show that individual cells use electrical signals—via the same voltage-gated calcium channels found in animal neurons and muscles—to synchronize flagellar beating within a colony.

Choanoflagellates feed on bacteria, linking carbon from otherwise inaccessible sources to higher trophic levels. They remain important in the carbon cycle and microbial food web, and there is evidence they also consume viruses.

Vegetative growth occurs through longitudinal fission, but the full reproductive life cycle is still unknown.