Trichodina is a genus of
ciliate alveolates that is
ectocommensal or
parasitic on aquatic animals, particularly
fish. They are characterised by the presence of a ring of interlocking cytoskeletal denticles, which provide support for the cell and allow for adhesion to surfaces including fish tissue.
Taxonomy
Trichodinids are members of the peritrichous ciliates, a paraphyletic group within the
Oligohymenophorea
The Oligohymenophorea are a large class of ciliates. There is typically a ventral groove containing the mouth and distinct oral cilia, separate from those of the body. These include a paroral membrane to the right of the mouth and membranelles, ...
. Specifically, they are mobiline peritrichs because they are capable of locomotion, as opposed to sessiline peritrichs such as ''
Vorticella'' and ''
Epistylis
''Epistylis'' is a genus of peritrich ciliates with a short oral disc
The word oral may refer to:
Relating to the mouth
* Relating to the mouth, the first portion of the alimentary canal that primarily receives food and liquid
**Oral administr ...
'', which adhere to the substrate via a stalk or lorica. There are over 150 species in the genus ''Trichodina''. ''
Trichodinella'', ''
Tripartiella'', ''
Hemitrichodina'', ''
Paratrichodina'' and ''
Vauchomia'' are similar genera.
Morphology
Trichodinids are round ciliates that may be disc-shaped or hemispherical. The cytostome (cell mouth) is on the surface that faces away from the host; this is termed the oral surface. The other side, or aboral surface, attaches to the skin of the host or other substrate. There is a spiral of
cilia
The cilium, plural cilia (), is a membrane-bound organelle found on most types of eukaryotic cell, and certain microorganisms known as ciliates. Cilia are absent in bacteria and archaea. The cilium has the shape of a slender threadlike projecti ...
leading towards the cytostome and several rings of cilia at the periphery of the cell, responsible for creating adhesive suction and locomotory power. In the taxonomy of trichodinids, the exact number, shape and arrangement of the cytoskeletal denticles is critical for determining taxonomic relationships. These characters are usually revealed by
silver nitrate staining of
microscope slides, which stains the cell cytoplasm black and leaves the denticles white.
Life history
Trichodinids have a simple direct life cycle. That is, they have a single host and do not use alternation of generations or mass asexual replication off the host. They reproduce by binary fission, literally cell-splitting. This produces daughter cells with half the number of denticles of the parent cell. The full complement of denticles is restored by synthesis of new denticles from the outer edge of the cell, working inwards.
Trichodinids are typically found on the gills, skin and fins of fishes, though some species parasitise the urogenital system. A range of invertebrates is also host to trichodinid infections, including the surfaces of
copepod
Copepods (; meaning "oar-feet") are a group of small crustaceans found in nearly every freshwater and saltwater habitat (ecology), habitat. Some species are planktonic (inhabiting sea waters), some are benthos, benthic (living on the ocean floor) ...
s and the mantle cavity of
molluscs. Transmission occurs by direct contact of infected and uninfected hosts, and also by active swimming of trichodinids from one host to another. ''Trichodina'' cells swim with the aboral surface facing forwards. On surfaces, they move laterally, with the aboral surface facing the substrate.
Pathogenesis
Most trichodinids are ectocommensal in that they use the fish only as a substrate for attachment, while they feed on suspended bacteria. Some species are certainly primary pathogens, however, since they occur in sterile sites (e.g. urinary system), or provoke pronounced responses on the part of the host (e.g. ''Tripartiella'' on gills).
When trichodinids become a problem in aquaculture, it usually indicates
eutrophication or poor water quality. High bacterial loads provide abundant food for trichodininds, which subsequently proliferate on hosts and then cause attachment-related pathologies.
References
*
{{Taxonbar, from=Q2114597
Oligohymenophorea
Ciliate genera
Parasitic alveolates
Parasites of fish