Paramyxea is a small phylum of marine parasites, using molluscs, crustaceans or polychaetes as hosts. Probably the group is cosmopolitan, but the distribution known so fais restricted to southern Europe, Australia and East Asia. Polychaetes of the genus Nephtys, collected from Danish waters, are infested with a paramyxean parasite in the gut epithelium. The parasite develops intracellularly and is in close contact with the cytoplasm of the host cell. When harbouring mature spores gut cells are lysed and spores are released into the digestive tract.Reproduction is by endogeneous budding, typical for Paramyxea, and the only reproduction observed is sporogony. A stem cell (cell 1) yields two or four sporonts (cell 2), each of which produces four tetracellular spores (cells 3–6). The mature spore is fusiform, about 10 microns long, and surrounded by an individual sac filled with fibrous material. The sacs are united to a spherial body with prominent striated terminal projections. Haplosporosomes, of three different types, are present in cells 1, 4 and 5. The species, which is new to science, is the first find of Paramyxea from Scandinavia.
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