Diversity
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.
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.
- Biogeographic Regions
- nearctic
- palearctic
- oriental
- ethiopian
- neotropical
- australian
- antarctica
- oceanic islands
- arctic ocean
- indian ocean
- atlantic ocean
- pacific ocean
- mediterranean sea
- Other Geographic Terms
- holarctic
- cosmopolitan
Habitat
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.
- Habitat Regions
- temperate
- tropical
- polar
- saltwater or marine
- freshwater
- Aquatic Biomes
- pelagic
- benthic
- reef
- lakes and ponds
- rivers and streams
- temporary pools
- coastal
- brackish water
- Other Habitat Features
- riparian
- estuarine
- intertidal or littoral
Systematic and Taxonomic History
The name
Choanoflagellata
was first applied to this group by Kent in 1880. The name refers to the funnel-like
collar structure surrounding the single flagellum. For the next hundred years, these
organisms were grouped in phylum
Zoomastigophora
(zooflagellates). However, choanoflagellate species do not conform to many characteristics
of that group, and have since been placed into their own phylum. Several authors have
proposed alternative names for the group containing different choanoflagellate families
and species, although
Choanoflagellata
remains the most widely used.
Due to their morphological similarities to sponge choanocytes, these two phyla have
long been thought to be related, with choanoflagellates being the evolutionary ancestors
of metazoans (multicellular organisms). Other researchers, however, have suggested
that choanoflagellates may actually represent extremely reduced sponges. Recent molecular
phylogenetic studies have had mixed success in evaluating these hypotheses, although
the majority support choanoflagellates as ancestors or the closest relatives of metazoans.
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.
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.
- Other Physical Features
- heterothermic
- radial symmetry
- Sexual Dimorphism
- sexes alike
Development
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.
Reproduction
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.
The reproductive systems of choanoflagellate species remains to be determined, although
several species are known to undergo longitudinal fission.
- Key Reproductive Features
- year-round breeding
- asexual
Choanoflagellate species do not exhibit any sort of parental investment.
- Parental Investment
- no parental involvement
Lifespan/Longevity
There is currently no published information available regarding the lifespan of any choanoflagellate species.
Behavior
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.
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.
- Key Behaviors
- natatorial
- diurnal
- nocturnal
- crepuscular
- sessile
- motile
- solitary
- colonial
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.
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.
- Primary Diet
- planktivore
- detritivore
Predation
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.
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.
- 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.
Additional Links
Contributors
Jeremy Wright (author), University of Michigan-Ann Arbor, Leila Siciliano Martina (editor), Texas State University.
- Nearctic
-
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.
- native range
-
the area in which the animal is naturally found, the region in which it is endemic.
- Palearctic
-
living in the northern part of the Old World. In otherwords, Europe and Asia and northern Africa.
- native range
-
the area in which the animal is naturally found, the region in which it is endemic.
- oriental
-
found in the oriental region of the world. In other words, India and southeast Asia.
- native range
-
the area in which the animal is naturally found, the region in which it is endemic.
- Ethiopian
-
living in sub-Saharan Africa (south of 30 degrees north) and Madagascar.
- native range
-
the area in which the animal is naturally found, the region in which it is endemic.
- Neotropical
-
living in the southern part of the New World. In other words, Central and South America.
- native range
-
the area in which the animal is naturally found, the region in which it is endemic.
- Australian
-
Living in Australia, New Zealand, Tasmania, New Guinea and associated islands.
- native range
-
the area in which the animal is naturally found, the region in which it is endemic.
- Antarctica
-
lives on Antarctica, the southernmost continent which sits astride the southern pole.
- native range
-
the area in which the animal is naturally found, the region in which it is endemic.
- 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.
- native range
-
the area in which the animal is naturally found, the region in which it is endemic.
- Arctic Ocean
-
the body of water between Europe, Asia, and North America which occurs mostly north of the Arctic circle.
- native range
-
the area in which the animal is naturally found, the region in which it is endemic.
- native range
-
the area in which the animal is naturally found, the region in which it is endemic.
- 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.
- native range
-
the area in which the animal is naturally found, the region in which it is endemic.
- 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.
- native range
-
the area in which the animal is naturally found, the region in which it is endemic.
- native range
-
the area in which the animal is naturally found, the region in which it is endemic.
- holarctic
-
a distribution that more or less circles the Arctic, so occurring in both the Nearctic and Palearctic biogeographic regions.
Found in northern North America and northern Europe or Asia.
- cosmopolitan
-
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.
- temperate
-
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).
- tropical
-
the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.
- polar
-
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.
- saltwater or marine
-
mainly lives in oceans, seas, or other bodies of salt water.
- freshwater
-
mainly lives in water that is not salty.
- pelagic
-
An aquatic biome consisting of the open ocean, far from land, does not include sea bottom (benthic zone).
- benthic
-
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.
- reef
-
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.
- coastal
-
the nearshore aquatic habitats near a coast, or shoreline.
- brackish water
-
areas with salty water, usually in coastal marshes and estuaries.
- marsh
-
marshes are wetland areas often dominated by grasses and reeds.
- swamp
-
a wetland area that may be permanently or intermittently covered in water, often dominated by woody vegetation.
- bog
-
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.
- riparian
-
Referring to something living or located adjacent to a waterbody (usually, but not always, a river or stream).
- estuarine
-
an area where a freshwater river meets the ocean and tidal influences result in fluctuations in salinity.
- intertidal or littoral
-
the area of shoreline influenced mainly by the tides, between the highest and lowest reaches of the tide. An aquatic habitat.
- heterothermic
-
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.
- 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).
- year-round breeding
-
breeding takes place throughout the year
- asexual
-
reproduction that is not sexual; that is, reproduction that does not include recombining the genotypes of two parents
- natatorial
-
specialized for swimming
- diurnal
-
- active during the day, 2. lasting for one day.
- nocturnal
-
active during the night
- crepuscular
-
active at dawn and dusk
- sessile
-
non-motile; permanently attached at the base.
Attached to substratum and moving little or not at all. Synapomorphy of the Anthozoa
- motile
-
having the capacity to move from one place to another.
- solitary
-
lives alone
- colonial
-
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.
- planktivore
-
an animal that mainly eats plankton
- detritivore
-
an animal that mainly eats decomposed plants and/or animals
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