Kudoa sp.

There are many species of Kudoa - which are common muscle parasites of marine fish. Most are apparantly harmless to their hosts and go unrecognized if fish are caught and consumed. Several species (most notable Kudoa thyrcites) cause a condition called myoliquefaction ("jelly muscle" or "soft flesh") after the fish has been killed and not immediately frozen or cooked. Although harmless to humans, this effect lowers the appeal and value of the fillet, and is a major economic concern to aquaculture.
A different species Kudoa septempunctata, is a parasite of olive flounder in South Korea and Japan, and is the only myxozoan parasite known to cause human illness. Consumption of raw infected olive flounder can cause food poisoning.