Identifying Two Ectoparasite Species of Mackerel Icefish ( <i>Champsocephalus gunnari</i> ) based on Mitochondrial Gene and Morphology

As the most common ectoparasites on mackerel icefish (Champsocephalus gunnari Lönnberg 1905), which has great economic and ecological importance in the Southern Ocean, parasitic copepods and leeches can significantly affect fish health and survival and thus influence the sustainable use of this commercial fishery resource. Accurate identification of these parasites is essential for understanding their specific impacts on hosts and ecosystems, as well as for developing effective fisheries management. This study investigated 34 ectoparasites from 28 C. gunnari specimens at South Georgia in August 2023 using microscopy and mitochondrial cytochrome c oxidase I (CO I) markers. Morphological analysis tentatively identified 10 leeches as Notobdella nototheniae and 24 copepods as Eubrachiella antarctica. Microscopic examinations and morphological measurements revealed marked differences between male and female copepods. Additionally, CO I sequencing confirmed the leeches as N. nototheniae (98.81% similarity) but suggested the copepods were Parabrachiella merluccii (81.97-82.25% similarity). All sequences have been uploaded to Genbank. Maximum Likelihood phylogenetic trees were also constructed using the CO I sequence of both ectoparasites. The 10 leeches were identified as N. nototheniae, and the 24 copepods were E. antarctica in this study.