The choanoflagellates are a group of free-living unicellular and colonial flagellate eukaryotes considered to be the closest living relatives of the animals. Choanoflagellates are collared flagellates, having a funnel shaped collar of interconnected microvilli at the base of a flagellum. Choanoflagellates are capable of both asexual and sexual reproduction. They have a distinctive cell morphology characterized by an ovoid or spherical cell body 3–10 μm in diameter with a single apical flagellum surrounded by a collar of 30–40 microvilli. Movement of the flagellum creates water currents that can propel free-swimming choanoflagellates through the water column and trap bacteria and detritus against the collar of microvilli, where these foodstuffs are engulfed. This feeding provides a critical link within the global carbon cycle, linking trophic levels. In addition to their critical ecological roles, choanoflagellates are of particular interest to evolutionary biologists studying the origins of multicellularity in animals. As the closest living relatives of animals, choanoflagellates serve as a useful model for reconstructions of the last unicellular ancestor of animals. According to a 2021 study, crown group craspedids appeared 422.78 million years ago, Although a previous study from 2017 recovered the divergence of the crown group choanoflagellates (craspedids) at 786.62 million years.
Amorphea is a taxonomic supergroup that includes the basal Amoebozoa and Obazoa. That latter contains the Opisthokonta, which includes the Fungi, Animals and the Choanomonada, or Choanoflagellates. The taxonomic affinities of the members of this clade were originally described and proposed by Thomas Cavalier-Smith in 2002.
The Ichthyosporea are a small group of Opisthokonta in Eukaryota, mostly parasites of fish and other animals.
Cafeteria is a genus of marine bicosoecid described in 1988 by Tom Fenchel and D. J. Patterson. It was created after the discovery of a new species, Cafeteria roenbergensis, a tiny (5–10 μm) eukaryotic organism that is eaten by protozoa and small invertebrates. The name is meant to indicate the importance of the genus in the food web.
Micronuclearia is a genus of free-living protozoa containing the single species Micronuclearia podoventralis. While originally thought to be a nucleariid, as reflected in the name, it is now inferred to be a member of the taxon Rigifilida, and to belong to the 'CRuMs' assemblage.
Loukozoa is a proposed taxon used in some classifications of eukaryotes, consisting of the Metamonada and Malawimonadea. Ancyromonads are closely related to this group, as sister of the entire group, or as sister of the Metamonada. Amorphea may have emerged in this grouping, specifically as sister of the Malawimonads.
Thaumatomonadida is an order of flagellates.
Bigyra is a phylum of microscopic eukaryotes that are found at the base of the Stramenopiles clade. It includes three well-known heterotrophic groups Bicosoecida, Opalinata and Labyrinthulomycetes, as well as several small clades initially discovered through environmental DNA samples: Nanomonadea, Placididea, Opalomonadea and Eogyrea. The classification of Bigyra has changed several times since its origin, and its monophyly remains unresolved.
Holozoa is a clade of organisms that includes animals and their closest single-celled relatives, but excludes fungi and all other organisms. Together they amount to more than 1.5 million species of purely heterotrophic organisms, including around 300 unicellular species. It consists of various subgroups, namely Metazoa and the protists Choanoflagellata, Filasterea, Pluriformea and Ichthyosporea. Along with fungi and some other groups, Holozoa is part of the Opisthokonta, a supergroup of eukaryotes. Choanofila was previously used as the name for a group similar in composition to Holozoa, but its usage is discouraged now because it excludes animals and is therefore paraphyletic.
Varisulca was a proposed basal Podiate taxon. It encompassed several lineages of heterotrophic protists, most notably the ancyromonads (planomonads), collodictyonids (diphylleids), rigifilids and mantamonadids. Recent evidence suggests that the latter three are closely related to each other, forming a clade called CRuMs, but that this is unlikely to be specifically related to ancyromonads.
The Flabellinia are a subclass of Amoebozoa. During locomotion the cells are flattened and have a clear layer called hyaloplasm along the front margin. Some form slender subpseudopodia projecting outward from the hyaloplasm, but the cell mass does not flow into these as in true pseudopodia, and advances without a definite central axis as in the Tubulinea. They also lack distinctive features like shells and flagella, and are united mainly by evidence from molecular trees.
Codonosiga elegans is a species of choanoflagellate in the family Codonosigidae. It may be a species complex containing multiple cryptic species – four distinct genotypes are known.
Salpingoeca rosetta is a species of Choanoflagellates in the family Salpingoecidae. It is a rare marine eukaryote consisting of a number of cells embedded in a jelly-like matrix. This organism demonstrates a very primitive level of cell differentiation and specialization. This is seen with flagellated cells and their collar structures that move the cell colony through the water.
Similar low level cellular differentiation and specification can also be seen in sponges. They also have collar cells and amoeboid cells arranged in a gelatinous matrix.
Unlike S. rosetta, sponges also have other cell-types that can perform different functions. Also, the collar cells of sponges beat within canals in the sponge body, whereas Salpingoeca rosetta's collar cells reside on the inside and it lacks internal canals. Despite these minor differences, there is strong evidence that Proterospongia and Metazoa are highly related.
The neokaryotes are a proposed eukaryote clade consisting of the unikonts and the bikonts as sister of for instance the Jakobea. It arises because the Euglenozoa, Percolozoa, Tsukubea, and Jakobea are seen in this view as more basal eukaryotes. These four groups, are traditionally grouped together in the Discoba. However, the Discoba may well be paraphyletic as the neokaryotes may have emerged in them.
The Scotokaryotes (Cavalier-Smith) is a proposed basal Neokaryote clade as sister of the Diaphoretickes. Basal Scotokaryote groupings are the Metamonads, the Malawimonas and the Podiata. In this phylogeny the Discoba are sometimes seen as paraphyletic and basal Eukaryotes.
The paracercomonads are a group of cercozoan protists. Taxonomically, they comprise the family Paracercomonadidae, order Paracercomonadida and subclass Paracercomonada. Due to their morphological similarities to the cercomonads, members of this family were grouped with Cercomonas and similar taxa from the beginning. However, their similarities are due to convergent evolution.
Massisteriidae is a family of granofilosean protists within the phylum Cercozoa.
Crinolina isefiordensis is a species of choanoflagellate in the family Acanthoecidae. It is the type species of the genus Crinolina and is named for the first location of its collection, the Ise Fjord in Denmark.
An amoeboflagellate is any eukaryotic organism capable of behaving as an amoeba and as a flagellate at some point during their life cycle. Amoeboflagellates present both pseudopodia and at least one flagellum, often simultaneously.
Parviluciferaceae is a family of perkinsozoans, a group of endoparasitic protists present in aquatic environments.
