A myxosporean producing actinospores of the tetractinomyxon type in Hydroides norvegicus Gunnerus (Serpulidae) in Denmark was identified as a member of the family Parvicapsulidae based on small-subunit ribosomal DNA (SSU rDNA) sequences. Myxosporean samples from various Danish and Norwegian marine fishes were examined with primers that detect the novel myxosporean. Sprattus sprattus (Linnaeus) and Clupea harengus Linnaeus (Teleostei, Clupeidae) were found to be infected. The sequences of this parvicapsulid from these hosts were consistently slightly different (0.8% divergence), but both these genotypes were found in H. norvegicus. Disporic trophozoites and minute spores of a novel myxosporean type were observed in the renal tubules of some of the hosts found infected through PCR. The spores appear most similar to those of species of Gadimyxa Køie, Karlsbakk et Nylund, 2007, but are much smaller. The actinospores of the tetractinomyxon type from H. norvegicus have been described previously. In GenBank, the SSU rDNA sequences of Parvicapsulidae gen. sp. show highest identity (82%) with Parvicapsula minibicornis Kent, Whitaker et Dawe, 1997 infecting salmonids (Oncorhynchus spp.) in fresh water in the western North America. A phylogenetic analysis places P. minibicornis and Parvicapsulidae gen. sp. in a sister clade to the other parvicapsulids (Parvicapsula spp. and Gadimyxa spp.).
Sequencing of SSU rDNA showed that actinospores of the tetractinomyxon type, which develop in Chone infundibuliformis Krøyer (Annelida, Polychaeta, Sabellidae) from the northern Øresund, Denmark, are identical with Ceratomyxa auerbachi Kabata, 1962 (Myxozoa, Ceratomyxidae). This myxosporean was found in the gallbladder of the Atlantic herring Clupea harengus L. from the northern Øresund, Denmark, and from the Bergen area, western Norway. The pansporocysts and actinospores of C. auerbachi are described. This is the third elucidated two-host life cycle of a marine myxozoan, and the first involving a marine ceratomyxid.