Colponemids are free-living

alveolate The alveolates (meaning "pitted like a honeycomb") are a group of protists, considered a major unranked clade or superphylum within Eukaryota. They are currently grouped with the Stramenopiles and Rhizaria among the protists with tubulocristate ...

s, unicellular

flagellate A flagellate is a cell or organism with one or more whip-like appendages called flagella. The word ''flagellate'' also describes a particular construction (or level of organization) characteristic of many prokaryotes and eukaryotes and the ...

s related to

dinoflagellate The Dinoflagellates (), also called Dinophytes, are a monophyletic group of single-celled eukaryotes constituting the phylum Dinoflagellata and are usually considered protists. Dinoflagellates are mostly marine plankton, but they are also commo ...

apicomplexa The Apicomplexa (also called Apicomplexia; single: apicomplexan) are organisms of a large phylum of mainly parasitic alveolates. Most possess a unique form of organelle structure that comprises a type of non-photosynthetic plastid called an ap ...

ns and

ciliate The ciliates are a group of alveolates characterized by the presence of hair-like organelles called cilia, which are identical in structure to flagellum, eukaryotic flagella, but are in general shorter and present in much larger numbers, with a ...

s. They are predators of other small eukaryotes, found in freshwater, marine and soil environments. They do not form a solid clade, but a sparse group of deep-branching alveolate lineages.

Biology

Colponemids are

d unicellular

protist A protist ( ) or protoctist is any eukaryotic organism that is not an animal, land plant, or fungus. Protists do not form a natural group, or clade, but are a paraphyletic grouping of all descendants of the last eukaryotic common ancest ...

s, 8 to 30 μm long, with rounded cell bodies that bear two heterodynamic

flagella A flagellum (; : flagella) (Latin for 'whip' or 'scourge') is a hair-like appendage that protrudes from certain plant and animal sperm cells, from fungal spores ( zoospores), and from a wide range of microorganisms to provide motility. Many pr ...

for rapid swimming. They are only present as swimming cells during their life cycle.

Reproduction Reproduction (or procreation or breeding) is the biological process by which new individual organisms – "offspring" – are produced from their "parent" or parents. There are two forms of reproduction: Asexual reproduction, asexual and Sexual ...

or resting

cyst A cyst is a closed sac, having a distinct envelope and division compared with the nearby tissue. Hence, it is a cluster of cells that have grouped together to form a sac (like the manner in which water molecules group together to form a bubb ...

s are unknown.

Cell structure

As other alveolates, all colponemids exhibit

cortical alveoli The cortical alveolum () is a cellular organelle consisting of a vesicle located under the cytoplasmic membrane, to which they give support. The term " corticate" comes from an evolutionary hypothesis about the common origin of kingdoms Plantae ...

and tubular mitochondrial cristae. They present a three-membrane alveolar pellicle without fibrils, theca or micropores. Within the cell is a vesicular

nucleus Nucleus (: nuclei) is a Latin word for the seed inside a fruit. It most often refers to: *Atomic nucleus, the very dense central region of an atom *Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA Nucleu ...

with a central

nucleolus The nucleolus (; : nucleoli ) is the largest structure in the cell nucleus, nucleus of eukaryote, eukaryotic cell (biology), cells. It is best known as the site of ribosome biogenesis. The nucleolus also participates in the formation of signa ...

, as well as a very large digestive vacuole (or

food vacuole The food vacuole, or digestive vacuole, is an organelle found in simple eukaryotes such as protists. This organelle is essentially a lysosome. During the stage of the symbiont parasites' lifecycle where it resides within a human (or other mammalia ...

) located at the posterior end of the cell, and a

contractile vacuole A contractile vacuole (CV) is a sub-cellular structure (organelle) involved in osmoregulation. It is found predominantly in protists, including unicellular algae. It was previously known as pulsatile or pulsating vacuole. Overview The contrac ...

at the anterior end. In addition, the

cytoplasm The cytoplasm describes all the material within a eukaryotic or prokaryotic cell, enclosed by the cell membrane, including the organelles and excluding the nucleus in eukaryotic cells. The material inside the nucleus of a eukaryotic cell a ...

contains flask-shaped

extrusome Extrusomes are membrane-bound organelles found in eukaryotic cells that are capable of discharging material contained within to the exterior of the cell. Due to the diversity in structure and function, it is unlikely that different types of extruso ...

s. The anterior

flagellum A flagellum (; : flagella) (Latin for 'whip' or 'scourge') is a hair-like appendage that protrudes from certain plant and animal sperm cells, from fungal spores ( zoospores), and from a wide range of microorganisms to provide motility. Many pr ...

has thin non-hollow

mastigoneme Mastigonemes are lateral "hairs" that attach to protistan flagella. Flimsy hairs attach to the flagella of euglenid flagellates, while stiff hairs occur in stramenopile and cryptophyte protists.Hoek, C. van den, Mann, D. G. and Jahns, H. M. ( ...

s at its base (or proximal end), unlike the hollow tripartite mastigonemes found in

stramenopiles The stramenopiles, also called heterokonts, are Protist, protists distinguished by the presence of stiff tripartite external hairs. In most species, the hairs are attached to flagella, in some they are attached to other areas of the cellular sur ...

. The posterior flagellum has a fold or 'vane'. As many basal eukaryotes, they present a ventral groove for feeding, tiny in '' Loeffela'', substantial in '' Palustrimonas'' and absent in '' Acavomonas''. Supporting this longitudinal groove are two

microtubule Microtubules are polymers of tubulin that form part of the cytoskeleton and provide structure and shape to eukaryotic cells. Microtubules can be as long as 50 micrometres, as wide as 23 to 27 nanometer, nm and have an inner diameter bet ...

bands.

Feeding

All colponemids share a

raptorial In biology (specifically the anatomy of arthropods), the term ''raptorial'' implies much the same as ''predatory'' but most often refers to modifications of an arthropod leg, arthropod's foreleg that make it function for the grasping of prey whi ...

predatory lifestyle, where they move actively to encounter and capture their prey. Prey cells are ingested through the anterior region of the ventral groove, on the right side of the cell (i.e. to the right of the posterior

). During the early stages of

phagocytosis Phagocytosis () is the process by which a cell (biology), cell uses its plasma membrane to engulf a large particle (≥ 0.5 μm), giving rise to an internal compartment called the phagosome. It is one type of endocytosis. A cell that performs ph ...

, flask-shaped extrusomes within the colponemid cell seem to play a role in subduing the much larger prey cells.

Ecology

Colponemids are found in freshwater, marine and soil environments. They are obligate eukaryovores or eukaryotrophs, free-living predators that feed on other microscopic

eukaryote The eukaryotes ( ) constitute the Domain (biology), domain of Eukaryota or Eukarya, organisms whose Cell (biology), cells have a membrane-bound cell nucleus, nucleus. All animals, plants, Fungus, fungi, seaweeds, and many unicellular organisms ...

s by capturing and ingesting entire cells.

Evolution

Colponemids are not a

clade In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...

, i.e. do not form an independent evolutionary lineage within

s. Instead, they are a series of deep-branching lineages spread within the larger Alveolata group. Phylogenetic analyses recover some colponemids at the base of

Myzozoa Myzozoa is a grouping of specific phyla within Alveolata, that either feed through myzocytosis, or were ancestrally capable of feeding through myzocytosis. Many protist orders are included within Myzozoa. It is sometimes described as a phylum ...

(the clade uniting

ns and

s), such as '' Acavomonas'', while others appear at the base of all alveolates. The method of feeding in colponemids perhaps represents the ancestral alveolate state, preceding

myzocytosis Myzocytosis (from Greek: myzein, (') meaning "to suck" and kytos (') meaning "container", hence referring to "cell") is a method of feeding found in some heterotrophic organisms. It is also called "cellular vampirism" as the predatory cell pierce ...

in their

myzozoa Myzozoa is a grouping of specific phyla within Alveolata, that either feed through myzocytosis, or were ancestrally capable of feeding through myzocytosis. Many protist orders are included within Myzozoa. It is sometimes described as a phylum ...

n relatives. In addition, they possibly constitute the evolutionary link to more basal protists with ventral grooves used in feeding, i.e. ' excavates' (such as

metamonad The metamonads are a large group of flagellate amitochondriate microscopic eukaryotes. They include the retortamonads, diplomonads, parabasalids, oxymonads, and a range of more poorly studied taxa, most of which are free-living flagellates. ...

jakobid Jakobida is an order (sole order in the class Jakobea) of free-living, heterotrophic, flagellar eukaryotes in the clade Discoba. They are small (less than 15 μm), and can be found in aerobic and anaerobic environments. The order Jakobida, b ...

s and malawimonads). These basal organisms feed on suspended particles driving them into their ventral groove by the beating of their posterior flagellum, an arrangement comparable to that seen among colponemids during feeding. However, there are large differences between their feeding processes. Namely, colponemids are not suspension feeders, and phagocytosis occurs in their anterior end, not the posterior end as in excavates.

Classification

Due to the non-

monophyletic In biological cladistics for the classification of organisms, monophyly is the condition of a taxonomic grouping being a clade – that is, a grouping of organisms which meets these criteria: # the grouping contains its own most recent co ...

nature of this group of protists, a new method of classification was proposed in 2014 which divided the Alveolata

in four

phyla Phyla, the plural of ''phylum'', may refer to: * Phylum, a biological taxon between Kingdom and Class * by analogy, in linguistics, a large division of possibly related languages, or a major language family which is not subordinate to another Phy ...

, Acavomonidia (with the newly described genus '' Acavomonas''), Colponemidia (with the genus ''

Colponema ''Colponema'' is a genus of single-celled flagellates that feed on eukaryotes in aquatic environments and soil. The genus contains 6 known species and has not been thoroughly studied. ''Colponema'' has two flagella which originate just below the ...

'') and

Ciliophora The ciliates are a group of alveolates characterized by the presence of hair-like organelles called cilia, which are identical in structure to eukaryotic flagella, but are in general shorter and present in much larger numbers, with a different ...

. This change was proposed to strive away from the paraphyletic taxon "Colponemida" and instead group each colponemid lineage in its own separate phylum. In 2023, three more genera were added to Colponemida: newly described '' Loeffela'' and '' Neocolponema'', together with the previously unsequenced genus '' Palustrimonas'', without their inclusion in new phyla. Currently, five genera of colponemids are recognised: * '' Acavomonas'' – 1 sp. * ''

'' – at least 5 spp. * '' Loeffela'' – 1 sp. * '' Neocolponema'' – 1 sp. * '' Palustrimonas'' – 1 sp.

References

{{taxonbar, from=Q21447133 Alveolata taxa