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Heterotrich | |
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Stentor roeselii | |
Scientific classification | |
Domain: | Eukaryota |
Clade: | Diaphoretickes |
Clade: | SAR |
Clade: | Alveolata |
Phylum: | Ciliophora |
Subphylum: | Postciliodesmatophora |
Class: | Heterotrichea Stein 1859 |
Order: | Heterotrichida Stein 1859 |
Families | |
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The heterotrichs are a class of ciliates. They typically have a prominent adoral zone of membranelles circling the mouth, used in locomotion and feeding, and shorter cilia on the rest of the body. Many species are highly contractile, and are typically compressed or conical in form. [1] These include some of the largest protozoa, such as Stentor and Spirostomum , as well as many brightly pigmented forms, such as certain Blepharisma .
The term heterotrich derives from the ancient Greek ἕτερος (héteros), meaning "another, different", and θρίξ , τριχός (thríx, trikhós), meaning 'hair', [2] [3] because of the contrast between the regular somatic ciliation and that of the oral zone. [4]
A number of ultrastructural details characterize the group. The cilia on the body are in dikinetids, in which either the anterior one or both kinetosomes may be ciliated, and which are associated with fibers composed of overlapping postciliary microtubules, called postciliodesmata and found only in this group and the closely related Karyorelictea. A series of oral polykinetids, each containing two or three rows of kinetosomes, support the membranelles. These run from the left to the anterior of the mouth, and often spiral out of the oral cavity. The macronucleus is divided by external microtubules, whereas in the Karyorelictea it forms by differentiation of micronuclei, and in all other ciliates it is divided by internal microtubules. [5] [4]
Many species are highly pigmented, e.g. the blue Stentor coeruleus . In Blepharisma , the red pigment is associated with light sensitivity. Species of blue-pigmented Eufolliculina form extensive mats in the deep sea that have been called "blue mats". [6]
Early classification schemes by Otto Bütschli, Alfred Kahl, Emmanuel Fauré-Fremiet, and John O. Corliss classified the heterotrichs as a subgroup of spirotrichs. They included groups such as Armophorida, Odontostomatida, Licnophorida, Clevelandellida, and Plagiotomida within the Heterotrichea. However, more recent classification systems, which have incorporated information from molecular phylogenetics, place the above groups within the Intramacronucleata, because they are not closely related to the "core" heterotrichs at all. [4] Almost all of the "true" heterotrichs belong to a single order, Heterotrichida.
In the classification scheme of Lynn (2008), the order Heterotrichida contains the following nine families: [4]
Two new families were established in 2014 on the basis of molecular phylogenies: [5]
The spirotrichs are a large and diverse group of ciliate protozoa. They typically have prominent oral cilia in the form of a series of polykinetids, called the adoral zone of membranelles, beginning anterior to the oral cavity and running down to the left side of the mouth. There may also be one or two paroral membranes on its right side. The body cilia are fused to form polykinetids called cirri in some, and are sparse to absent in others.
The plagiopylids are a small order of ciliates, including a few forms common in anaerobic habitats.
The hypotrichs are a group of ciliated protozoa, common in fresh water, salt water, soil and moss. Hypotrichs possess compound ciliary organelles called "cirri," which are made up of thick tufts of cilia, sparsely distributed on the ventral surface of the cell. The multiple fused cilia which form a cirrus function together as a unit, enabling the organism to crawl along solid substrates such as submerged debris or sediments. Hypotrichs typically possess a large oral aperture, bordered on one side by a wreath or collar of membranelles, forming an "adoral zone of membranelles," or AZM.
Blepharisma is a genus of unicellular ciliate protists found in fresh and salt water. The group includes about 40 accepted species, and many sub-varieties and strains. While species vary considerably in size and shape, most are easily identified by their red or pinkish color, which is caused by granules of the pigment blepharismin.
Holotricha is an order of ciliates. The classification has fallen from use as a formal taxon, but the terms "holotrich" and "holotrichous" are still applied descriptively to organisms with cilia of uniform length distributed evenly over the surface of the body.
Stentor, sometimes called trumpet animalcules, are a genus of filter-feeding, heterotrophic ciliates, representative of the heterotrichs. They are usually horn-shaped, and reach lengths of two millimeters; as such, they are among the largest known extant unicellular organisms. They reproduce asexually through binary fission.
Karyorelictea is a class of ciliates in the subphylum Postciliodesmatophora. Most species are members of the microbenthos community, that is, microscopic organisms found in the marine interstitial habitat, though one genus, Loxodes, is found in freshwater.
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 undulating pattern than flagella. Cilia occur in all members of the group and are variously used in swimming, crawling, attachment, feeding, and sensation.
Climacostomum is a genus of unicellular ciliates, belonging to the class Heterotrichea.
Climacostomum virens is a species of unicellular ciliate protists. It is one of just two formally described species in the genus Climacostomum.
Geleiidae is a family of karyorelict ciliates. It is sometimes synonymized with family Aveliidae.
Armophorea is a class of ciliates in the subphylum Intramacronucleata. . It was first resolved in 2004 and comprises three orders: Metopida, Clevelandellida, and Armophorida. Previously members of this class were thought to be heterotrichs because of similarities in morphology, most notably a characteristic dense arrangement of cilia surrounding their oral structures. However, the development of genetic tools and subsequent incorporation of DNA sequence information has led to major revisions in the evolutionary relationships of many protists, including ciliates. Metopids, clevelandellids, and armophorids were grouped into this class based on similarities in their small subunit rRNA sequences, making them one of two so-called "riboclasses" of ciliates, however, recent analyses suggest that Armophorida may not be related to the other two orders.
Kentrophoros is a genus of ciliates in the class Karyorelictea. Ciliates in this genus lack a distinct oral apparatus and depend primarily on symbiotic bacteria for their nutrition.
Licnophora is a genus of ciliates in the family Licnophoridae. They typically have an hourglass-like shape and live as ectocommensals on marine animals.
Bursaria is a genus of ciliates in the class Colpodea. They are relatively large and feed on other protists in freshwater habitats.
Stentor roeselii is a free-living ciliate species of the genus Stentor, in the class Heterotrichea. It is a common and widespread protozoan, found throughout the world in freshwater ponds, lakes, rivers and ditches.
Parablepharismea is a class of free-living marine and brackish anaerobic ciliates that form a major clade of obligate anaerobes within the SAL group, together with the classes Muranotrichea and Armophorea.
Halteria, sometimes referred to as the jumping oligotrich, is a genus of common planktonic ciliates that are found in many freshwater environments. Halteria are easy to locate due to their abundance and distinctive behaviour with observations of Halteria potentially dating back to the 17th century and the discovery of microorganisms. Over time more has been established about their morphology and behavior, which has led to many changes in terms of classification.
Blepharismidae is a family of unicellular ciliate protists found in fresh and salt water. Two genera are recognized: Blepharisma, which contains some model organisms, and Pseudoblepharisma
Pseudoblepharisma is a genus of heterotrich ciliates inhabiting oxygen depleted freshwater habitats. Most sources report that it contains one species, Pseudoblepharisma tenue, but at least four have been seen in literature.