An integrative redescription of the copepod Tracheliastes polycolpus (Lernaeopodidae) based on ultrastructure, COI barcoding, and Palearctic distribution.

Tracheliastes polycolpus is one of the most widely reported parasitic copepods of Palearctic freshwater fishes. Despite nearly two centuries of citations in fish-disease literature, its morphology, taxonomy, and genetic identity have remained insufficiently resolved and summarized. We provide an integrative redescription based on scanning electron microscopy (SEM) and mitochondrial cytochrome c oxidase subunit I (COI) barcoding of specimens collected from the type host Leuciscus idus in Lake Dąbie, Poland. Molecular identity was confirmed by PCR using universal COI primers, with phylogenetic reconstruction under the GTR + I + G4 model. SEM examination revealed fine-scale features of all cephalothoracic appendages and genital processes. Secondary denticles on the antennal endopod and exopod are demonstrated to constitute reliable diagnostic characters within the genus Tracheliastes. Vestigial thoracic legs, each surrounded by concentric cuticular patches of distinct ultrastructure, are documented for the first time in this species. This study presents the first COI sequences for T. polycolpus (GenBank: PQ520502-PQ520503), only the second for the genus, confirming its distinct genetic identity and close relationship to T. maculatus. A maximum-likelihood phylogeny places T. polycolpus firmly within the Lernaeopodidae. Based on morphological reassessment and historical evidence, T. chondrostomi and T. mourkii are synonymized with T. polycolpus, reducing the number of valid species in the genus. A revised genus diagnosis is provided.