First record of Argulus japonicus infestation on Cyprinus carpio in Hungary, and the first description of Argulus japonicus europaeus subsp. nov. Keve, 2025.

BACKGROUND: Species belonging to the genus Argulus are globally distributed fish parasites. Their veterinary significance primarily lies in their disruptive presence and their role as mechanical vectors. Although Argulus japonicus Thiele, 1900 is a widely distributed representative of this genus that feeds on freshwater fish, only Argulus foliaceus (Linnaeus, 1758) had previously been reported in Hungary. The aim of this study was to investigate the fish louse fauna in a local common carp (Cyprinus carpio Linnaeus, 1758) population. To the best of our knowledge, this is the first study to report the occurrence of A. japonicus in Hungary. METHODS AND RESULTS: Our detailed molecular analyses, including the complete mitochondrial genome, revealed for the first time that the A. japonicus specimens found in Hungary differ significantly from their Far Eastern counterparts. Furthermore, cytochrome c oxidase subunit I (cox1) sequence analysis-a region known to be stable within the species-showed that while our sequences were nearly identical to those of other European specimens, they differed markedly from the available Asian isolates. The phylogenetic analysis also confirmed this divergence. The European A. japonicus sequences form a clearly distinct sister group to the Asian lineages. CONCLUSIONS: In light of these findings, and on the basis of thorough morphological examinations, the authors propose that the specimens found in Hungary represent a new subspecies, Argulus japonicus europaeus subsp. nov. Keve, 2025.