| Myxosporea | |
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| Kudoa septempunctata - spores | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Cnidaria |
| Subphylum: | Myxozoa |
| Class: | Myxosporea Buetchli 1881 |
| Orders | |
| Synonyms | |
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Myxosporea is aclass of microscopic animals, all of whom areparasites. They belong to theMyxozoaclade withinCnidaria. They have a complex life cycle that comprises vegetative forms in two hosts—one an aquaticinvertebrate (generally anannelid but sometimes abryozoan) and the other anectothermicvertebrate, usually afish. Each parasitized host releases a different type ofspore. The two forms of spore are so different in appearance that until relatively recently they were treated as belonging to different classes within the Myxozoa.
Thetaxonomy of both actinosporeans and myxosporeans was originally based on spore morphology. In 1994 the phylumMyxozoa was redefined to solve the taxonomic andnomenclatural problems arising from the two-host life cycle of myxozoans.[1][2] The distinction between the two previously recognised classes Actinosporea and Myxosporea disappeared and the classActinosporea was suppressed, becoming asynonym of the classMyxosporea (Bütschli, 1881). Thegeneric names of actinosporeans were retained as collective "type" names, and it was proposed that they be used to characterise different morphological forms of actinosporeans. Those remaining actinosporeans whose myxosporean stage is unattested are being retained asspecies inquirenda until their specific identity is established.
It has been hypothesized that myxosporeans might have evolved from atransmissible tumor ofPolypodium. This hypothesis is called the "SCANDAL hypothesis", an acronym forspeciation by cancer development animals, referencing its radical nature.[3][4]
Until the 1980s, direct transmission of myxosporeans was presumed. In 1984, it was shown experimentally that spores ofMyxobolus cerebralis failed to produce infections when fed tosalmonids.[2] To reproduce successfully, this species requires atubificid worm as an intermediate host, in which the spores develop into a "species" of the "genus"Triactinomyxon. These spores develop inside the oligochaete into forms which are infectious to salmonids.
Such a life cycle—with two different sexual stages, each resulting in two kinds of resistant spores—is unique amongst parasitic organisms, let alone those in thekingdom of animals. This mode of life has been confirmed in several otherMyxobolus species. Thisvector has also been proved in other families.Ceratonova shasta, an economically important parasite of salmonids, has been shown to use apolychaete worm as an alternate host.[5]
Direct transmission between fish has also been demonstrated, so far in three species ofEnteromyxum.
Examples of Myxosporeangenera areKudoa, which attacks fishmuscle,[6] andMyxobolus, which attacks thehearts of freshwater fish.[7]
Authority control databases: National |
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