Myxozoa

Myxozoa are microscopic, spore-forming parasites in the phylum Cnidaria. Most infect fish, while some infect other aquatic animals, often using complex life cycles involving multiple hosts.

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Cnidaria: An animal phylum that includes jellyfish, corals, sea anemones, and Myxozoa. Molecular and developmental evidence places Myxozoa within this phylum despite their radically reduced bodies.
Myxobolus cerebralis: A myxozoan parasite that causes whirling disease in salmonid fish. Its two-host cycle with a freshwater worm illustrates myxozoan transmission.
Joseph Leidy: An American naturalist and anatomist who studied microscopic organisms and fossils. Leidy described Myxobolus muelleri in 1843, among the earliest named myxozoans.
Whirling disease: A disease of salmonid fish caused by the parasite Myxobolus cerebralis. It is a well-studied example of myxozoan pathology and environmental transmission.
Microsporidia: A group of spore-forming, intracellular parasites related to fungi. Their microscopic spores resemble myxozoan spores, but they belong to a different lineage.
Parasite: An organism that lives on or in a host and gains resources at the host’s expense. Myxozoan development and transmission depend on exploiting one or more animal hosts.
Tubifex tubifex: A freshwater oligochaete worm found in sediments and polluted waters. It serves as the invertebrate host for Myxobolus cerebralis.
Small-subunit ribosomal RNA: A ribosomal RNA molecule whose sequence is widely used to infer evolutionary relationships. Sequence comparisons helped reveal that myxozoans are highly reduced cnidarians.
Proliferative kidney disease: A disease of salmonids caused by the myxozoan Tetracapsuloides bryosalmonae. It can cause severe losses in farmed and wild fish populations.
Ciliophora: A diverse group of single-celled eukaryotes characterized by hairlike cilia. Myxozoans were once grouped with protists, including forms mistaken for ciliates.