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Myxozoa
Triactinomyxon stage of Myxobolus cerebralis.
Scientific classification
Kingdom: Animalia (unranked) Bilateria Phylum: Myxozoa Grassé, 1970
Classes
Malacosporea Myxosporea

The Myxozoa are a group of microscopic, single-celled, parasitic animals. Originally taxonomists classed them as protozoa, and included them with other non-motile forms in the group Sporozoa. However, as their distinct nature became clear they gained their own phylum. Many evolutionary theorists now generally consider them to have developed from multicellular animals and classify them accordingly. Over 1300 species of Myxozoa have been described[2].

The Greek roots of the name myxozoa express the ideas of "slime" or "mucus" (myx-) and "animal" (zo-).

Many Myxozoa have a two-host lifecycle, involving a fish and an annelid worm or bryozoan. Infection occurs through valved spores. These contain one or two sporoblast cells and one or more polar capsules that contain filaments which anchor the spore to its host. The sporoblasts are then released as a motile form, called an amoebula, which penetrates the host tissues and develops into one or more multinucleate plasmodia. Certain nuclei later pair up, one engulfing another, to form new spores.

In structure and appearance the polar capsules closely resemble the stinging cells of Cnidaria. On account of this, biologists have generally regarded the Myxozoa as extremely reduced cnidarians, and in particular as close relatives of Polypodium, with some genetic support. More recent studies of Hox genes, however, point to an origin among the Bilateria. Strong support for this comes from the discovery that Buddenbrockia, a worm-like parasite of bryozoans up to 2 mm in length, belongs among the Myxozoa^[1]. Genetically difficult to distinguish from the other forms, it has Myxozoan-like spore capsules, but it retains a bilateral body form with longitudinal muscles. This serves as a missing link between the Myxozoa and their multicellular ancestors.

Taxonomists now recognize the outdated subgroup Actinosporea as a life-cycle phase of Myxosporea ^[2].

Some species of myxozoa include:

• Class Malacosporea
  • Buddenbrockia plumatellae
  • Tetracapsuloides bryosalmonae - an important salmon parasite
• Class Myxosporea
  • Myxobolus cerebralis - an important parasite of salmon and trout

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Myxozoa

1. ^ Monteiro, A. S., Okamura, B., and P. W. H. Holland. "Orphan Worm Finds a Home: Buddenbrockia is a Myxozoan." Molecular Biology and Evolution 19:968-971 (2002)[1]
2. ^ Kent, M. L., Margolis, L. & Corliss, J.O. (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(5):932-937.