Myxosporea

Myxosporea is a subclass of microscopic animals, all of whom are parasites. They belong to the Myxozoa clade within Cnidaria. They have a complex life cycle that comprises vegetative forms in two hosts—one an aquatic invertebrate (generally an annelid but sometimes a bryozoan) and the other an ectothermic vertebrate, usually a fish. Each parasitized host releases a different type of spore. The two forms of spore are so different in appearance that until relatively recently they were treated as belonging to different classes within the Myxozoa.

Bivalvulida is an order of myxosporean parasites which contains a number of species which cause economically significant losses to aquaculture and fisheries, such as Myxobolus cerebralis and Ceratomyxa shasta. The Myxosporean stages of members of the bivalvulida are characterised by their two spore valves (hence the name), which meet in a "suture line" which encircles the spore. They usually contain two polar capsules, but species have been reported which contain either one or four.

Multivalvulida is an order of myxozoan.