Thecofilosea | |
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Two preserved specimens of Capsellina sp. (Tectofilosida: Chlamydophryidae), from the Eugène Penard collection | |
Scientific classification | |
Domain: | Eukaryota |
Clade: | Diaphoretickes |
Clade: | SAR |
Phylum: | Cercozoa |
Superclass: | Ventrifilosa |
Class: | Thecofilosea Cavalier-Smith, 2003 emend. 2011 |
Subgroups [1] | |
Thecofilosea is a class of unicellular testate amoebae belonging to the phylum Cercozoa. They are amoeboflagellates, organisms with flagella and pseudopodia, distinguished from other cercozoa by their scale-lacking test composed of organic material. They are closely related to the Imbricatea, a group of testate amoebae with tests composed of inorganic silica scales.
Thecofilosean organisms evolved from an ancestor with a robust extracellular theca made of organic material, unlike most other Cercozoa, which are usually naked or have inorganic silica scales. They present thin pseudopodia (filopodia) that emerge from a ventral groove. They also have two flagella which have been secondarily lost in Rhizaspidae and the tectofilosid amoebae, and are restricted to zoospores within the phaeodarian amoebae. They ancestrally glide on their posterior flagellum only and have a benthic distribution, but many lineages have evolved as planktonic swimmers, like the Ebriacea which have lost their pseudopodia. [1] [2]
The thecofilosean theca or test has perforations for flagella and pseudopodia. In the phaeodarian amoebae, the test has three perforations. Although they lack silica scales, unlike many Imbricatea, they present a hollow silica endoskeleton in all ebriids and most phaeodarians. [1] [2]
Thecofilosea is a clade or monophyletic group. It belongs to the subphylum Monadofilosa, a group of cercozoan classes that evolved after the early divergence of Reticulofilosa. Thecofilosea is the only group of testate cercozoan amoebae with ancestrally organic scale-lacking shells, while all the Imbricatea (to which the scale-bearing Kraken belongs) have non-organic silica scaled tests. The Thecofilosea appear to have evolved from the Imbricatea, a group previously considered monophyletic but based on new analyses seems paraphyletic to Thecofilosea. Their common ancestor carried scales in its theca and no organic cement, a trait lost in Thecofilosea in favour of an organic theca without scales. [3] [4] [5]
Cercozoa (=Filosa) |
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An earlier analysis that recovers the Kraken within Sarcomonadea, a group of naked amoebae, proposes the unlikely possibility that not only the common ancestor of Imbricatea+Thecofilosea, but also the common ancestor of all Filosa, already bore inorganic silica scales in its test. In this scenario, both Thecofilosea and Sarcomonadea would have lost their scales independently. [6]
The class Thecofilosea was created in 2003 by protozoologist Cavalier-Smith to accommodate the testate orders Cryomonadida and Tectofilosida. They were first described as uninucleate cells surrounded by an organic flexible tectum (in the case of Tectofilosida) or a rigid test (in Cryomonadida) with one or two apertures for filopodia, either two flagella or none, and tubular mitochondrial cristae. [7] It was later emended in 2011 by the same author, to include Ventricleftida, Ebriida and Phaeodaria. [2] Finally, the elusive Mataza was added to the class in its own order Matazida. [8]
Two different classification schemes are in place, despite containing the same orders.
Cavalier-Smith (2018) [3] | Adl et al. (2019) [1] |
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Heliozoa, commonly known as sun-animalcules, are microbial eukaryotes (protists) with stiff arms (axopodia) radiating from their spherical bodies, which are responsible for their common name. The axopodia are microtubule-supported projections from the amoeboid cell body, and are variously used for capturing food, sensation, movement, and attachment. They are similar to Radiolaria, but they are distinguished from them by lacking central capsules and other complex skeletal elements, although some produce simple scales and spines. They may be found in both freshwater and marine environments.
The Heliomonadida are a small group of heliozoan amoeboids that are unusual in possessing flagella throughout their life cycle.
Cercozoa is a phylum of diverse single-celled eukaryotes. They lack shared morphological characteristics at the microscopic level, and are instead united by molecular phylogenies of rRNA and actin or polyubiquitin. They were the first major eukaryotic group to be recognized mainly through molecular phylogenies. They are the natural predators of many species of bacteria. They are closely related to the phylum Retaria, comprising amoeboids that usually have complex shells, and together form a supergroup called Rhizaria.
The Rhizaria are a diverse and species-rich supergroup of mostly unicellular eukaryotes. Except for the Chlorarachniophytes and three species in the genus Paulinella in the phylum Cercozoa, they are all non-photosynthethic, but many foraminifera and radiolaria have a symbiotic relationship with unicellular algae. A multicellular form, Guttulinopsis vulgaris, a cellular slime mold, has been described. This group was used by Cavalier-Smith in 2002, although the term "Rhizaria" had been long used for clades within the currently recognized taxon. Being described mainly from rDNA sequences, they vary considerably in form, having no clear morphological distinctive characters (synapomorphies), but for the most part they are amoeboids with filose, reticulose, or microtubule-supported pseudopods. In the absence of an apomorphy, the group is ill-defined, and its composition has been very fluid. Some Rhizaria possess mineral exoskeletons, which are in different clades within Rhizaria made out of opal, celestite, or calcite. Certain species can attain sizes of more than a centimeter with some species being able to form cylindrical colonies approximately 1 cm in diameter and greater than 1 m in length. They feed by capturing and engulfing prey with the extensions of their pseudopodia; forms that are symbiotic with unicellular algae contribute significantly to the total primary production of the ocean.
Phaeodarea or Phaeodaria is a group of amoeboid cercozoan organisms. They are traditionally considered radiolarians, but in molecular trees do not appear to be close relatives of the other groups, and are instead placed among the Cercozoa. They are distinguished by the structure of their central capsule and by the presence of a phaeodium, an aggregate of waste particles within the cell.
Lobosa is a taxonomic group of amoebae in the phylum Amoebozoa. Most lobosans possess broad, bluntly rounded pseudopods, although one genus in the group, the recently discovered Sapocribrum, has slender and threadlike (filose) pseudopodia. In current classification schemes, Lobosa is a subphylum, composed mainly of amoebae that have lobose pseudopods but lack cilia or flagella.
Cercomonads are small amoeboflagellates, widespread in aqueous habitats and common in soils.
Monadofilosa is a grouping of Cercozoa. These organisms are single-celled amoeboid protists.
Imbricatea is a class of Rhizaria characterised by silica scales. It is sometimes described as "Imbricatea/Silicofilosea", due to the similarity of those two groupings. Imbricatea is divided into the orders Euglyphida and Thaumatomonadida.
Cryomonadida is a group of heterotrophic Rhizaria, that belong to the Cercozoa.
The sarcomonads or class Sarcomonadea are a group of amoeboid biciliate protists in the phylum Cercozoa. They are characterized by a propensity to move through gliding on their posterior cilium or through filopodia, a lack of scales or external theca, a soft cell surface without obvious cortical filamentous or membranous skeleton, two cilia without scales or hairs, tubular mitochondrial cristae, near-spherical extrusomes, and a microbody attached to the nucleus.
Katabia is a genus of soil-dwelling heterotrophic flagellate cercozoans containing the single species Katabia gromovi, and the only member of family Katabiidae.
The vampyrellids, colloquially known as vampire amoebae, are a group of free-living predatory amoebae classified as part of the lineage Endomyxa. They are distinguished from other groups of amoebae by their irregular cell shape with propensity to fuse and split like plasmodial organisms, and their life cycle with a digestive cyst stage that digests the gathered food. They appear worldwide in marine, brackish, freshwater and soil habitats. They are important predators of an enormous variety of microscopic organisms, from algae to fungi and animals. They are also known as aconchulinid amoebae.
Kraken is a genus of amoebae within the Cercozoa, containing the sole species Kraken carinae. These amoebae are characterized by a small round cell body and a network of thin and very long filopodia that can reach up to a mm in diameter. Kraken amoebae feed on bacteria and live in freshwater and soil systems.
Ventrifilosa is a highly diverse group of phagotrophic protists that glide through their flagella and emit filose pseudopods from their ventral side for feeding. Because of their mixture of amoeba and flagellate characteristics, they are amoeboflagellates. Members of this group are the Imbricatea, Sarcomonadea and Thecofilosea.
Cryptofilida is an order of small heterotrophic protists in the phylum Cercozoa. They are filose amoebae that lack cilia and gliding, and are instead characterized by movement through branching or unbranched granular filopodia that are appressed to the substrate during their feeding.
Pediglissa is a subclass of phagotrophic protists that inhabit soil or freshwater habitats. They were defined in 2018 according to phylogenetic analyses that showed a clade containing the orders Cercomonadida and Glissomonadida. They're the sister group of Paracercomonadida.
Rhogostoma minus is a species of thecate amoeba that belongs to the phylum Cercozoa. It was first described by Belar in 1921. These amoebae are heterotrophic, which means that they consume other organisms for nutrition. The cells of Rhogostoma minus are typically between 8 and 12 µm in diameter and are surrounded by a protective organic covering called a theca. The theca has a unique cleft-like opening that allows the amoebae to extend and retract thread-like projections called filose pseudopodia. The amoebae move along surfaces by pulling themselves forward using their pseudopodia.
Rhogostomidae is a family of thecate amoebae with a ventral cleft-like aperture. Their theca is thin and flexible and adheres to the cell. The cleft-like aperture allows them to extend and retract their filose pseudopodia, which they use to move and feed. They are primarily feeding on bacteria, but they are also known to consume yeasts and algae. The family contains three genera: Capsellina, Sacciforma, and Rhogostoma.
Limnofila is a genus of heterotrophic protists that live in freshwater habitats and feed on bacteria. They are also present in the soil ecosystem, where they play an important role as predators of bacteria. They are classified as a single family Limnofilidae and order Limnofilida.