Myxosporea are microscopic parasites that belong to the phylum Myxozoa. They have a "double life". The complex life cycle comprises vegetative forms in two hosts, an aquatic invertebrate (generally an annelid) and an aquatic vertebrate (generally a fish). Each host releases a different type of spores. The two types of spores are so different that until recently they were being treated as different species.
The myxosporean species are identified generally by the size and shape of the spores released by vertebrate hosts. For instance, the genus Ceratomyxa is a very frequent parasite of the gallbladder of many fish species; they have "boomerang-like" spores with two polar capsules resembling eyes in the middle of the spore. The size of these spores generally ranges between 15 and 100 micrometers in most Ceratomyxa species. Other genuses have round or oval spores with only one polar capsules or with more than two. The second type of spores, which is generally released by an annelid host, resembles three or four hooks united by the base, and can also be used for identification.
Examples of Myxosporean genuses are Kudoa , destroying the muscle of fishes, Myxobolum from the hearts of freshwater fishes, Myxidium , Sphaerospora or Henneguya . Some myxosporeans cause important diseases in cultured fishes, for instance the kidney disease named PDK in salmonids .
