Myxozoa

Myxozoa (etymology: Greek: μύξα myxa "slime" or "mucus" + thematic vowel o + ζῷον zoon "animal") is a class of aquatic cnidarian animals – all obligate parasites. It contains the smallest animals ever known to have lived. Over 2,180 species have been described and some estimate they are at least as diverse as the free-living cnidarians, with at least 30,000 undiscovered species. Many have a two-host life cycle, involving a fish and an annelid worm or a bryozoan. The average size of a myxosporean spore usually ranges from 10 μm to 20 μm, whereas that of a malacosporean (a subclade of the Myxozoa) spore can be up to 2 mm. Myxozoans can live in both freshwater and marine habitats. Myxozoans are highly derived cnidarians that have undergone dramatic evolution from a free swimming, self-sufficient jellyfish-like creature into their current form of obligate parasites composed of very few cells. As myxozoans evolved into microscopic parasites, they lost many genes responsible for multicellular development, coordination, cell–cell communication, and even, in some cases, aerobic respiration. The genomes of some myxozoans are now among the smallest genomes of any known animal species.