Myxozoa

Myxozoa are a group of microscopic parasitic cnidarians that were previously classified as protozoa. These aquatic invertebrates are characterized by their complex life cycles, which often require multiple hosts including fish and annelid worms. Myxozoans are extremely small, with some species measuring only a few micrometers in diameter. They possess polar capsules, structures unique to this group, which are used for attachment to host tissues.

The life cycle of myxozoans typically involves stages in both vertebrate and invertebrate hosts, with spore formation occurring in the final host. They are known to cause various diseases in fish, including whirling disease and proliferative kidney disease, which can have significant economic impacts on aquaculture industries.

Recent molecular studies have revealed that myxozoans are highly reduced cnidarians, having lost many typical cnidarian characteristics during their evolutionary adaptation to parasitism. This discovery has provided valuable insights into the extremes of morphological simplification in animal evolution.