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|Kudoa septempunctata - spores|
The Myxosporea are a class of microscopic parasites, belonging to the Myxozoa clade within Cnidaria. They have a complex life cycle which comprises vegetative forms in two hosts, an aquatic invertebrate (generally an annelid) and an ectothermic vertebrate, usually a fish. Each host releases a different type of spore. The two forms of spore are so different that until recently they were treated as belonging to different classes within the Myxozoa.
The taxonomy of both actinosporeans and myxosporeans was originally based on spore morphology. Recently Kent et al. (1994) redefined the phylum Myxozoa to solve the taxonomic and nomenclatural problems arising from the two-host life cycle of myxozoans as first described by Wolf and Markiw (1984). The distinction between the two previously recognised classes Actinosporea and Myxosporea disappeared and the class Actinosporea was suppressed, becoming a synonym of the class Myxosporea Bütschli, 1881. The generic 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 as species inquerandae until their specific identity is established.
Until the 1980s, direct transmission of myxosporeans was presumed. In 1984, it was shown experimentally that spores of Myxobolus cerebralis failed to produce infections when fed to salmonids (Wolf & Markiw, 1984). To reproduce successfully, this species requires a tubificid 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 infective to salmonids. Such a life cycle, with two different sexual stages, resulting in two kinds of resistant spores, is unique amongst the parasitic organisms. This mode of life cycle has been confirmed in several other Myxobolus species.
This mode of infection has also been proved in other families. Ceratomyxa shasta, an economically important parasite of salmonids, has been shown to use a polychaete worm as an alternate host. Surprisingly, however, direct transmission between fish has also been demonstrated, so far in three species of Enteromyxum.
- Bartholomew, J. L., M. J. Whipple, D. G. Stevens and J. L. Fryer. (1997). The life cycle of Ceratomyxa shasta, a myxosporean parasite of salmonids, requires a freshwater polychaete as an alternate host. American Journal of Parasitology. 83:859-868.
- Wolf, K. & Markiw, M.E. (1984). Biology contravenes taxonomy in the Myxozoa: new discoveries show alternation of invertebrate and vertebrate hosts. Science 225:1449-1452
- Kent, M. L., Margolis. L., & Corliss, J. 0. (1994). The demise of a class of protists: Taxonomic and nomenclatural revisions proposed for the protist phylum Myxozoa Grasse, 1970. Canadian Journal of Zoology 72:932-937.