Trichodina is a genus 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 ...

alveolate The alveolates (meaning "pitted like a honeycomb") are a group of protists, considered a major clade and superphylum within Eukarya. They are currently grouped with the stramenopiles and Rhizaria among the protists with tubulocristate mitochon ...

s that is ectocommensal or

parasitic Parasitism is a close relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson ha ...

on aquatic animals, particularly

fish Fish are aquatic, craniate, gill-bearing animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish as well as various extinct related groups. Approximately 95% of ...

. 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 ''Vorticella'' is a genus of bell-shaped ciliates that have stalks to attach themselves to substrates. The stalks have contractile myonemes, allowing them to pull the cell body against substrates. The formation of the stalk happens after the fr ...

'' and ''

Epistylis ''Epistylis'' is a genus of peritrich 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 an ...

'', 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 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 Silver nitrate is an inorganic compound with chemical formula . It is a versatile precursor to many other silver compounds, such as those used in photography. It is far less sensitive to light than the halides. It was once called ''lunar causti ...

staining of

microscope A microscope () is a laboratory instrument used to examine objects that are too small to be seen by the naked eye. Microscopy is the science of investigating small objects and structures using a microscope. Microscopic means being invisi ...

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 copepods and the mantle cavity of

molluscs Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000 extant species of molluscs are recognized. The number of fossil species is estim ...

. 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 Eutrophication is the process by which an entire body of water, or parts of it, becomes progressively enriched with minerals and nutrients, particularly nitrogen and phosphorus. It has also been defined as "nutrient-induced increase in phytopla ...

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