The description of Metacanthocephalus (Bimuscularis) rennicki inflatus n. subgen., n. subsp. (Acanthocephala: Leptorhynchoididae) from Notothenia coriiceps off Galindez Island, West Antarctica, with the erection of two new subgenera and subspecies.

The study of many acanthocephalans of the genus Metacanthocephalus Yamaguti, 1959 (Leptorhynchoididae) from the black rockcod Notothenia coriiceps Richardson in the Argentine Islands revealed their close affiliation to Metacanthocephalus rennicki (Leiper and Atkinson, 1914, 1915). Metacanthocephalus rennicki was originally described from the emerald rockcod Trematomus bernacchii Boulenger in Cape Evans, Ross's Sea, McMurdo Sound, and is herein split into two subspecies in two different primary fish host species and separate geographical locations. These specimens are herein described as Metacanthocephalus rennicki inflatus n. subsp. which is similar to the traditional Metacanthocephalus rennicki rennicki n. subsp. by having similar eggs (with polar prolongation of fertilization membrane) of the same size (75-93 X 20-25), ellipsoid proboscis with 5-7 hooks per row, and subterminal gonopore in both sexes. Specimens of M. rennicki inflatus from N. coriiceps off Galindez Island, Argentine Islands are, however, distinguished from those of M. rennicki rennicki n. subsp. from a different host, T. bernacchii, in McMurdo Sound by having markedly wider measurements of their fusiform trunk, elliptoid proboscides, and more rows of proboscis hooks (14-16) compared to the subcylindrical trunk and cylindrical proboscis with 12-13 (usually 12) hook rows of M. rennicki rennicki. In addition, the cephalic ganglion of specimens of M. rennicki inflatus is found at or just posterior to the middle of the proboscis receptacle, compared to being in the anterior half of the receptacle as in M. rennicki rennicki. Specimens of both subspecies are clearly distinguished from the 3 other species of Metacanthocephalus by characters of size and shape of trunk, proboscis size and armature, egg size, and position of cephalic ganglion and of the female and male gonopore. We regard all species to be independently valid as originally described. We have also included the two subspecies of M. rennicki with the 3 other species of Antarctic Metacanthocephalus (M. campbelli Leiper and Atkinson, 1914, 1915, M. dalmori Zdzitowiecki, 1983; M. johnstoni Zdzitowiecki, 1983) in a new subgenus Bimuscularis n. subgen (with 2 muscular sphincters). The other subgenus, Unimuscularis n. subgen. includes the two other Japanese species, M. ovicephalus (Zhukov, 1960) and M. pleuronichthydis Yamaguti, 1959, that have only one sphincter. We now also provide the first molecular analysis of a species of Antarctic Metacanthocephalus, M. (B.) rennicki inflatus n. subgen., n. subsp. The molecular characterization of the nuclear gene 18S of the ribosomal subunit and cox1 of the mitochondrial gene of Metacanthocephalus, M. (B.) rennicki inflatus n. subgen., n. subsp is provided. Furthermore, in regard to the phylogenetic relationships of the genus Metacanthocephalus and the other genera of Echinorhynchida, phylogenetic analyses were performed using maximum likelihood (ML) and Bayesian inference (BI). The phylogenetic results showed that Metacanthocephalus, M. (B.) rennicki inflatus n. subgen., n. subsp. has a sister relationship with Metacanthocephalus and other species from different genera are also representing sister to it. In view of the morphology, morphometry differences with previously described species alongside with its phylogenetic arrangement, the present acanthocephalan is described as a new species.