Overlooked Parasite Diversity in Staurozoa: Two Species of Lepocreadiidae (Trematoda: Digenea) Parasitic in Haliclystus tenuis

Three non-encysted digenean metacercariae were found parasitic in the mesoglea of two of 13 individuals of the staurozoan cnidarian <i>Haliclystus tenuis</i> Kishinouye, 1910 from Yoichi, Hokkaido, Japan. The metacercariae comprised two morphospecies (one oval, the other elongate), for which the gross morphology is described. Partial sequences of the 18S and 28S rRNA genes and the internal transcribed spacer 2 region were determined for both species, and a partial sequence of the cytochrome <i>c</i> oxidase subunit I gene was determined for the oval species. Molecular phylogenetic analyses revealed both digeneans to be in the family Lepocreadiidae, a taxon not previously reported from staurozoans. The oval species was a member of the "<i>Diploproctodaeum</i> Clade." The elongate species was identified as <i>Prodistomum orientale</i> (Layman, 1930), known to use marine fishes in the genus <i>Scomber</i> Linnaeus, 1758 as definitive hosts. Second intermediate hosts of <i>P. orientale</i> known around Australia include five hydrozoan, one scyphozoan (Cnidaria), and one ctenophoran species. This is the first information on a second intermediate host from Japan; however, we cannot rule out the possibility that staurozoans may also be dead-end hosts. Including an opecoelid species previously reported, three digenean species in two families are now known from a single staurozoan species in the small region comprising the northwestern coast of Hokkaido. Future surveys for parasites in other staurozoan species and regions will likely detect additional digeneans and other parasite groups utilizing staurozoans.