A new species of monopisthocotylan parasite has been identified on the body surface of the Antarctic rockcod Notothenia coriiceps Richardson, in the coastal waters surrounding Galindez Island, West Antarctica. The newly described species, Pseudobenedenia galindezensis sp. n., is morphologically distinct from all previously recognized species within the genus Pseudobenedenia as evidenced by a combination of unique morphometric traits. These include the shape and size of opisthaptoral sclerotized elements, the presence of a particularly robust accessory sclerite, and a characteristic ratio between the anterior hamulus and accessory sclerite. These features collectively support its recognition as a separate species. A well-developed orifice of the common genital opening was observed and measured in the newly described species, an anatomical feature not previously documented in Pseudobenedenia. Measurements of the vagina are provided for the first time for all of the described Pseudobenedenia species. The presence or absence of a distinct gap between the testes represents a novel diagnostic character introduced in this description. Additional distinguishing features include the length-to-width ratios of the testes and body, as well as the body length-to-opisthaptor length ratio, all of which further support the species’ distinctiveness. Molecular analyses of 18S and 28S nuclear markers were conducted to study the phylogenetic position of this new species within the family Capsalidae. The results indicated consistent genetic divergences, which were accompanied by observed morphological differences, thereby substantiating the classification of the species as new to science. Molecular analysis was conducted to differentiate P. galindezensis sp. n. from the closely related Pseudobenedenia coriicepsi. This new species was found on a fish host that is morphologically distinct from most collected N. coriiceps, characterized by its larger size and reddish color; its identity was confirmed using mitochondrial cox1 sequences. A significant finding of our study is that one nototheniid species can host more than one species of Pseudobenedenia.
