Myxosporea is a class of microscopic parasites, belonging to the Myxozoaclade within Cnidaria. They have a complex life cycle which comprises vegetative forms in two hosts, an aquatic invertebrate 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 relatively 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. In 1994 the phylum Myxozoa was redefined to solve the taxonomic and nomenclatural problems arising from the two-host lifecycle of myxozoans. The distinction between the two previously recognised classes Actinosporea and Myxosporea disappeared and the classActinosporea was suppressed, becoming a synonym of the class Myxosporea. The genericnames 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. It has been hypothesized that myxosporeans might have evolved from a transmissible tumor of Polypodium. This hypothesis is called the "Scandal hypothesis", an acronym for speciation by cancer development animals, referencing its radical nature.
Transmission
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.. 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, direct transmission between fish has also been demonstrated, so far in three species of Enteromyxum. Examples of Myxosporean genera are Kudoa, which attacks fish muscle, and Myxobolus, which attacks the hearts of freshwater fishes.
