Choanoflagellate

Choanoflagellates are single-celled or colonial eukaryotes that swim freely and are the closest living relatives of animals. Their name combines the Greek word for "funnel," referring to their collar shape, with the Latin word for "whip," describing their flagellum. Found in aquatic habitats worldwide, they interest evolutionary biologists because they help reveal how animals became multicellular.

Each choanoflagellate has one flagellum, surrounded at its base by a ring of finger-like microvilli that form a collar. The flagellum’s motion pulls water through this collar, trapping bacteria and debris against the microvilli for ingestion. This current also pushes the cell forward—unlike most other flagellates, which are pulled—similar to how animal sperm move. Inside the cell, 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 in the upper to middle part of the cell, while food vacuoles gather near the bottom. Many species also have an outer covering, or periplast, which can be a simple coat or a complex basket-like house called a lorica, built from silica strips. The periplast’s function is unclear, but it may help sessile forms attach or, in planktonic ones, increase drag to improve feeding efficiency.

Choanoflagellates can be free-swimming or attached to surfaces, either directly or via a stalk or periplast. They are strictly free-living and heterotrophic, though some close relatives are parasitic. Their life cycles are poorly understood. Many species live alone, but colonial forms have evolved several times, and even colonial species have a solitary stage. Over 125 known species exist worldwide, from the Arctic to the tropics, in marine, brackish, and freshwater environments, from surface waters down to 300 meters deep or under Antarctic ice. Some species appear to be cosmopolitan, while others have limited ranges. Factors controlling their distribution remain unclear.

Some species form simple colonies, like clusters of flagellated cells or cells on a single stalk. One colonial species, *Barroeca monosierra*, creates hollow spheres filled with a branched matrix housing 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 their flagellar beating and movement within a colony.

Choanoflagellates feed on bacteria, linking otherwise inaccessible carbon to higher trophic levels, and play a role in the carbon cycle and microbial food web. Some evidence suggests they also consume viruses. They reproduce vegetatively through longitudinal fission, but the full reproductive cycle remains unknown. Morphologically, they resemble the choanocyte cells of sponges, making them a key model for reconstructing the last unicellular ancestor of animals. A 2021 study estimated that crown group craspedids (and possibly all crown choanoflagellates) appeared 422.78 million years ago, while a 2017 study placed the divergence of crown choanoflagellates at 786.62 million years.