Armophorea is a class of
ciliate
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 differen ...
s in the subphylum
Intramacronucleata
Intramacronucleata is a subphylum of ciliates. The group is characterized by the manner in which division of the macronucleus is accomplished during binary fission of the cell. In ciliates of this subphylum, division of the macronucleus is achiev ...
.
.
[ ] It was first resolved in 2004 and comprises three orders: Metopida, Clevelandellida, and Armophorida. Previously members of this class were thought to be
heterotrich
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 ...
s 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
protist
A protist () is any eukaryotic organism (that is, an organism whose cells contain a cell nucleus) that is not an animal, plant, or fungus. While it is likely that protists share a common ancestor (the last eukaryotic common ancestor), the exc ...
s, including ciliates. Metopids, clevelandellids, and armophorids were grouped into this class based on similarities in their
small subunit rRNA
Small subunit ribosomal ribonucleic acid (SSU rRNA) is the smaller of the two major RNA components of the ribosome.
Associated with a number of ribosomal proteins, the SSU rRNA forms the small subunit of the ribosome. It is encoded by SSU- rDNA.
...
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.
Etymology
The name Armophorea is thought to be derived from the Latin word ''arma,'' meaning weapons, or ''armus'', meaning shoulder. This name refers to the caenomorphid members of this class, which have a characteristic military helmet-like morphology, and also a twisted appearance that looks like a shoulder.
Habitat and ecology
Free-living armophoreans live in
anoxic
The term anoxia means a total depletion in the level of oxygen, an extreme form of hypoxia or "low oxygen". The terms anoxia and hypoxia are used in various contexts:
* Anoxic waters, sea water, fresh water or groundwater that are depleted of diss ...
or microaerobic habitats, in the sediment or water column where there is reduced or absent oxygen. Thus their distribution is quite limited, although they are found globally in both marine and freshwater habitats, as well as in terrestrial sediment. Clevelandellids live as
commensal symbionts inside of the digestive tracts of terrestrial and aquatic animals.
Armophoreans can survive by
encystment
A microbial cyst is a resting or dormant stage of a microorganism, usually a bacterium or a protist or rarely an invertebrate animal, that helps the organism to survive in unfavorable environmental conditions. It can be thought of as a state of ...
when in unfavorable environmental conditions. This is quite important for clevelandellids because it facilitates their transmission between hosts.
Like most anaerobic ciliates, armophoreans have mitochondria-derived organelles called
hydrogenosome
A hydrogenosome is a membrane-enclosed organelle found in some anaerobic ciliates, flagellates, and fungi. Hydrogenosomes are highly variable organelles that have presumably evolved from protomitochondria to produce molecular hydrogen and ATP i ...
s. These specialized organelles produce energy for the cell in absence of oxygen by the
fermentation of pyruvate into acetate and hydrogen. Armophoreans harbor
methanogenic
endosymbiotic
An ''endosymbiont'' or ''endobiont'' is any organism that lives within the body or cells of another organism most often, though not always, in a mutualistic relationship.
(The term endosymbiosis is from the Greek: ἔνδον ''endon'' "within ...
archaea that have been located in the cytoplasm adjacent to their hydrogenosomes. Host cells can contain up to 10,000 methanogens, and they are thought to play roles in host growth and metabolism.
It is hypothesized that these symbionts consume the hydrogen that is produced as an end-product of fermentation, making it a more favorable reaction and increasing its energetic yield. Marine armophoreans also harbor ectosymbiotic bacteria that are
sulfate reducers, which are thought to play a similar role as the endosymbiotic methanogens in that they also consume hydrogen as a metabolite of host fermentation.
by these encosymbiotic archaea can contribute substantially to the production of methane in sulphate-rich, anoxic detrital sediments and the anoxic water column, but the contribution is modest (>2%) in sandy sediments where the ciliates are lower in number.
This is in contrast to clevelandellids: over 80% of the methane produced by the American cockroach can be attributed to these ciliates via their methanogenic endosymbionts.
Taxonomy
Order Metopida
Order Clevelandellida (de Puytorac & Grain, 1976)
Family Clevelandellidae (Kidder, 1938)
*Genus ''
Anteclevelandella'' gen. nov.
*Genus ''
Clevelandella''
Kidder, 1938
*Genus ''
Paraclevelandia''
Kidder, 1937
*Genus ''
Rhynchoclevelandella'' gen. nov.
Family Nyctotheridae (Amara, 1972)
*Genus ''
Nyctotherus''
Leidy, 1849
Order Armophorida
References
External links
*
{{Taxonbar, from=Q23753112
Intramacronucleata
Ciliate classes