Redescription and phylogenetic assessment of Helicometra antarcticae Holloway & Bier, 1968 (Trematoda, Opecoelidae), with evidence of non-monophyletic status of the genus Helicometra Odhner, 1902

Helicometra antarcticae Holloway & Bier, 1968 is one of the few Antarctic representatives of the species-rich genus Helicometra Odhner, 1902. It is traditionally attributed to morphological Group IV of Helicometra spp., characterized by a cup-shaped oral sucker with a terminal mouth opening. ­Helicometra antarcticae, similar to other Antarctic representatives of the genus, is quite rare and poorly studied. We redescribe this species based on newly collected material from the Antarctic toothfish, Dissostichus mawsoni Norman, 1937 (Nototheniidae Günther, 1861) caught in the Ross Sea (Antarctica) and evaluate its phylogenetic position using a fragment of the nuclear 28S rRNA gene. We provide new morphological details on H. antarcticae concerning the arrangement of the ventral part of the fields of vitelline follicles and the morphology of the proximal portion of the oviduct. Additionally, we describe the morphology of Helicometra sp. (morphological Group IV) from the Pacific zoarcid fish, Lycodes cf. brunneofasciatus Suvorov, 1935, caught off Simushir Island (Pacific Ocean) and Helicometra fasciata sensu lato (morphological Group I), from the black scorpion fish Scorpaena porcus Linnaeus, 1758 from the Black Sea. In our phylogenetic analysis, H. antarcticae clustered with a Pacific Helicometra sp. This clade has a sister relationship to the well-supported clade containing Helicometrina nimia Linton, 1910 and Helicometra spp. of Groups I and III respectively, including H. fasciata sensu lato from the Black Sea. Thus, our phylogenetic data indicate that Helicometra is not a monophyletic genus.