Specimens of shrimp Heptacarpus maxillipes (Thoridae) infected by parasitic barnacle Sylon hippolytes (Rhizocephala) were found for the first time along the Kuril Islands at a depth of 241–670 m. It is the second host from the genus Heptacarpus. Total number of hosts of S. hippolytes is currently 24. Carapace length of the infected shrimps ranged from 3 to 10 mm. Hosts had one or two parasitic externae of S. hippolytes. The size of the externa ranged from 1.5 to 9.0 mm in length and from 1.0 to 4.0 mm in height. Retinacula were not found. A single colleteric gland was present. Receptacles were absent. Regeneration of the ovary in the ovigerous externa was not observed. Sylon hippolytes produces only one larval generation. Larvae are hatched as small cyprids. Phylogenetic reconstruction confirmed the belonging of our specimens to the monophyletic clade of S. hippolytes with genetic distances between subclades from 6.96 to 8.4%. High values of intraspecific distances in S. hippolytes from different areas may point to cryptic speciation within this rhizocephalan.
