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an order of the class Cnidosporidia. Many zoologists place the Myxosporidia in a separate subclass. Of the approximately 800 species, 215 are found in the USSR. Myxosporidians parasitize the gall bladder, urinary bladder, ureters, and various tissues of fishes (mostly bony) and, less commonly, of amphibians and turtles.
The vegetative forms are motile multinucleate plasmodia (from 15 μm to 11 mm long) with vegetative nuclei and with generative cells that move actively. In tissue myxosporidians, the plasmodium is fixed and often surrounded by a cyst (about 6 cm in diameter) in the host’s connective tissue.
Asexual reproduction is characterized by division of the nuclei and then the cytoplasm. In sexual reproduction, one or more multicellular spores (5–25 μm in diameter), with valves, polar capsules, and a binucleate amoeboid embryo, arise from the generative cells after several divisions (the last being meiosis). A zygote is formed by the fusion of the embryo’s haploid nuclei.
Spores are released from fish (through the intestine, ureters, tears in tissues) into the water, where they are swallowed by a new animal host. In the intestine of its new host, the amoeboid embryo penetrates the mucous membrane and, along with blood, enters various organs and tissues, where the plasmodium develops and grows. Several myxosporidians cause myxosporidiosis, a dangerous fish disease.
REFERENCEShul’man, S. S. Miksosporidii fauny SSSR. Moscow-Leningrad, 1966.
S. S. SHUL’MAN