Phylum Choanoflagellata, formerly placed in phylum Sarcomastigophora, is a group of unicellular protozoans containing three families (Codosigidae, Salpingoecidae, Acanthoecidae) and over 125 species. Family-level divisions are mainly based on differences in the composition and structure of the periplast (the outer cell covering). Codosigids have no periplast, salpingoecids have a firm theca made of cellulose or other polysaccharides enclosing the cell, and acanthoecids have loricae, complex cell coverings made of silica strips arranged in a basket-weave pattern. Depending on the species, these organisms may be sessile or free swimming and exist singly or in colonies. They are an important part of the microbial food web and carbon cycle. (Brusca and Brusca, 2003; Fairclough and King, 2006; Karpov and Leadbeater, 1998; Leadbeater and Thomsen, 2000)

Geographic Range

Members of this phylum are found globally in marine, brackish and freshwater environments, from the Arctic to the tropics. Some species are considered to be cosmopolitan. (Fairclough and King, 2006; King, et al., 2008)


Choanoflagellates are found in marine, brackish and freshwater environments, in pelagic and benthic zones; from the Arctic and Antarctic (even under Antarctic ice sheets at depths of 100 m) to the tropics; and from depths of 0 to 300 m. They may be sessile or free swimming and exist singly or in colonies. (Fairclough and King, 2006; King, et al., 2008)

Physical Description

Choanoflagellates are unicellular protozoa, and are very similar in appearance to the choanocytes of sponges. They are non-pigmented, ovoid or spherical cells, 3 to 10 µm in diameter, and are covered in a periplast. Each choanoflagellate has a single apical flagellum surrounded by a transparent collar of 30 to 40 actin-filled microvilli, acting as a food catching net. All choanoflagellates have a flagellar basal body at the base of the apical flagellum and another, non-flagellar basal body, at a right angle. The cell’s nucleus is apically-centrally located and there are food vacuoles in the basal area of the cytoplasm. (Brusca and Brusca, 2003; Fairclough and King, 2006; Leadbeater and Kelly, 2001; Leadbetter, 2008)

The make-up of the periplast is diagnostic at the family level. Codosigidae species have no periplasts, Salpingoecidae species have a firm theca made of cellulose or other polysaccharides enclosing the cell, and Acanthoecidea species have loricae, which are complex cell coverings made of silica strips arranged in a basket weave-like pattern. Generally, thecae and loricae not only protect the cell but also aid in substrate attachment. All choanoflagellate cells are coated in a glycocalyx, a layer of fibrils on the outer surface of the cell membrane. (Fairclough and King, 2006; Leadbeater and Kelly, 2001; Leadbetter, 2008)

  • Sexual Dimorphism
  • sexes alike


Little is known of choanoflagellate development, but it is believed that they reproduce through longitudinal fission. Codonosigidae species divide laterally, while Salpingoecidae emerge from the limited space of the theca and become amoeboid in order to divide. When a thecate cell divides, the resulting cell may be motile, dispersing and undergoing division; if it does not separate, it will create a colony. Some Acanthoecidae species divide and produce a flagellated cell that swims away from the parent, settles, and produces its own lorica. Others produce costal strips and store them in the collar; the new cell is pushed out of the parent, taking the strips with it and using them to create its own lorica. (Fairclough and King, 2006; Karpov and Leadbeater, 1998; Leadbetter, 2008)


Only asexual reproduction has been observed in choanoflagellates; however, recent research suggests that they may have sexual periods during their lives, due to the presence of LTR retrotransposons and genes associated with meiosis in the genomes of some species. (Carr, et al., 2010; Karpov and Leadbeater, 1998; Leadbetter, 2008)

The reproductive systems of choanoflagellate species remains to be determined, although several species are known to undergo longitudinal fission. (Karpov and Leadbeater, 1998)

Choanoflagellate species do not exhibit any sort of parental investment. (Leadbetter, 2008)

  • Parental Investment
  • no parental involvement


There is currently no published information available regarding the lifespan of any choanoflagellate species.


The flagellum beats to create a forwardly directed current of water, moving from base to tip, creating a feeding current. When free swimming, the flagellum is used in locomotion, pushing the cell along rather than pulling it, as in many flagellates. In some colonial species, flagellar movements of individuals may be cancelled out by each other as they beat in different directions; this may enable the colony to remain suspended in the water column. (Brusca and Brusca, 2003; Leadbeater and Kelly, 2001; Leadbetter, 2008)

At least one choanoflagellate species is capable of undergoing encystment under unfavorable conditions, retracting the flagellum and collar in a fibrous, electron-dense wall. When placed in a new medium, they emerge from this encysted form. (Fairclough and King, 2006)

Communication and Perception

Choanoflagellates do not have nervous systems. They do, however, express ion-channel genes similar to those associated with sodium channels in sensory neurons. (Liebeskind, et al., 2011)

Food Habits

The water flow created by the beating of a flagellum directs particles into the collar where they are taken into the cell by pseudopodia. In some colonial species, the amalgamation of periplasts may create drag, which counteracts the forces of the flagella and increases feeding efficiency. Food items typically include bacteria and detritus; choanoflagellates are an important part of the microbial food web and carbon cycle, providing a link between their bacterial prey and higher trophic levels. (Carr, et al., 2008; Fairclough and King, 2006; Leadbeater and Kelly, 2001)


As unicellular organisms, choanoflagellates are likely prey to many larger organisms, although no confirmed predators of these species are known.

Ecosystem Roles

Choanoflagellates serve an important ecosystem role as part of the microbial food web and carbon cycle. Although it has not been recorded among choanoflagellates, some related eukaryotes (classes Mesomycetozoea and Ichthyosporea) are known to be parasitic. Choanoflagellates have no known parasites. (Fairclough and King, 2006; Mendoza, et al., 2002)

Economic Importance for Humans: Positive

Beyond the role they play in the food web and carbon cycle and their use in scientific research, there are no known positive effects of choanoflagellates on humans. (Fairclough and King, 2006)

  • Positive Impacts
  • research and education

Economic Importance for Humans: Negative

There are no known adverse effects of choanoflagellates on humans.

Conservation Status

Choanoflagellates are not considered threatened or in danger of extinction.

  • IUCN Red List [Link]
    Not Evaluated


Jeremy Wright (author), University of Michigan-Ann Arbor, Leila Siciliano Martina (editor), Animal Diversity Web Staff.



lives on Antarctica, the southernmost continent which sits astride the southern pole.

Arctic Ocean

the body of water between Europe, Asia, and North America which occurs mostly north of the Arctic circle.

Atlantic Ocean

the body of water between Africa, Europe, the southern ocean (above 60 degrees south latitude), and the western hemisphere. It is the second largest ocean in the world after the Pacific Ocean.

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Living in Australia, New Zealand, Tasmania, New Guinea and associated islands.

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living in sub-Saharan Africa (south of 30 degrees north) and Madagascar.

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living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.

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living in the southern part of the New World. In other words, Central and South America.

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Pacific Ocean

body of water between the southern ocean (above 60 degrees south latitude), Australia, Asia, and the western hemisphere. This is the world's largest ocean, covering about 28% of the world's surface.

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living in the northern part of the Old World. In otherwords, Europe and Asia and northern Africa.

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reproduction that is not sexual; that is, reproduction that does not include recombining the genotypes of two parents


Referring to an animal that lives on or near the bottom of a body of water. Also an aquatic biome consisting of the ocean bottom below the pelagic and coastal zones. Bottom habitats in the very deepest oceans (below 9000 m) are sometimes referred to as the abyssal zone. see also oceanic vent.


a wetland area rich in accumulated plant material and with acidic soils surrounding a body of open water. Bogs have a flora dominated by sedges, heaths, and sphagnum.

brackish water

areas with salty water, usually in coastal marshes and estuaries.


the nearshore aquatic habitats near a coast, or shoreline.


used loosely to describe any group of organisms living together or in close proximity to each other - for example nesting shorebirds that live in large colonies. More specifically refers to a group of organisms in which members act as specialized subunits (a continuous, modular society) - as in clonal organisms.


having a worldwide distribution. Found on all continents (except maybe Antarctica) and in all biogeographic provinces; or in all the major oceans (Atlantic, Indian, and Pacific.


active at dawn and dusk


an animal that mainly eats decomposed plants and/or animals

  1. active during the day, 2. lasting for one day.

an area where a freshwater river meets the ocean and tidal influences result in fluctuations in salinity.


mainly lives in water that is not salty.


having a body temperature that fluctuates with that of the immediate environment; having no mechanism or a poorly developed mechanism for regulating internal body temperature.


a distribution that more or less circles the Arctic, so occurring in both the Nearctic and Palearctic biogeographic regions.

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Found in northern North America and northern Europe or Asia.

intertidal or littoral

the area of shoreline influenced mainly by the tides, between the highest and lowest reaches of the tide. An aquatic habitat.


marshes are wetland areas often dominated by grasses and reeds.


having the capacity to move from one place to another.


specialized for swimming

native range

the area in which the animal is naturally found, the region in which it is endemic.


active during the night

oceanic islands

islands that are not part of continental shelf areas, they are not, and have never been, connected to a continental land mass, most typically these are volcanic islands.


found in the oriental region of the world. In other words, India and southeast Asia.

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An aquatic biome consisting of the open ocean, far from land, does not include sea bottom (benthic zone).


an animal that mainly eats plankton


the regions of the earth that surround the north and south poles, from the north pole to 60 degrees north and from the south pole to 60 degrees south.

radial symmetry

a form of body symmetry in which the parts of an animal are arranged concentrically around a central oral/aboral axis and more than one imaginary plane through this axis results in halves that are mirror-images of each other. Examples are cnidarians (Phylum Cnidaria, jellyfish, anemones, and corals).


structure produced by the calcium carbonate skeletons of coral polyps (Class Anthozoa). Coral reefs are found in warm, shallow oceans with low nutrient availability. They form the basis for rich communities of other invertebrates, plants, fish, and protists. The polyps live only on the reef surface. Because they depend on symbiotic photosynthetic algae, zooxanthellae, they cannot live where light does not penetrate.


Referring to something living or located adjacent to a waterbody (usually, but not always, a river or stream).

saltwater or marine

mainly lives in oceans, seas, or other bodies of salt water.


non-motile; permanently attached at the base.

Attached to substratum and moving little or not at all. Synapomorphy of the Anthozoa


lives alone


a wetland area that may be permanently or intermittently covered in water, often dominated by woody vegetation.


that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).


the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.

year-round breeding

breeding takes place throughout the year


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