The life cycle from release of myxospore sporoplasm to mature actinospore for the myxozoan parasite Ceratomyxa shasta in the freshwater polychaete host is described. Infection was initiated through exposure to lab-infected waters. Mature myxospores were present in the gut of segment 4 by 2 hr post-exposure (PE). Migration of early developmental stages occurred from the gut epithelium, through the nervous system, and into the epidermis. Once the parasite reached the epidermis, development typically occurred in segments 6–10. Life stages in the epidermis included a proliferative stage (schizogony), development of a pansporocyst (gametogony), sexual reproduction (gametogamy), and spore formation (sporogony). At water temperatures averaging 17.3 C, schizogony began by 3 hr PE, gametogony by 14 days PE, gametogamy by 35 days PE, and sporogony between 35 and 49 days PE. Development of mature actinospores took approximately 49 days PE. Up to 13,650 ± 7,807 actinospores could be released from a single polychaete over a 14-day period. Actinospores appeared to be released directly through the epidermis into the water column.

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