Biodiversity is considered one of the most significant parameters for characterizing aquatic environments. The value of species diversity is taken into account when assessing the ecological state and quality of the ecosystem. However, erroneous taxonomic identification distorts biodiversity signification and, consequently, prevents a clear understanding of ecological signals and traits. In this study, we presented the first data on Piscicola leech species diversity in the vast area of the Eastern Palaearctic. To avoid misidentifications, an integrated approach was applied, combining modern DNA-based and classical morphological methods. Previously, only one species of the genus, Piscicola geometra, was recorded in the whole of Siberia. We discovered three new species: Piscicola sibirica sp. nov., Piscicola khubsugulensis sp. nov. (Eastern Siberia) and Piscicola nordica sp. nov. (European Northeast). The species Piscicola pojmanskae was reduced to synonymy with Piscicola geometra, whose description was updated. Taxonomic adjustments allowed us to ascertain the eastern edge of the Piscicola geometra distribution and to understand the biogeography of the group as a whole. The Piscicola geometra range is limited to Western Siberia, while the widespread Piscicola sibirica sp. nov. and the purely Khovsgolian Piscicola khubsugulensis sp. nov. inhabit Eastern Siberia.

In this study, we assembled a complete mitochondrial genome of Codonobdella sp. sample from Lake Baikal, Russia and reassembled third party raw data of Piscicola geometra . Mitogenomes of both freshwater piscine leeches consist of 37 genes, including 13 protein-coding genes (PCGs), two ribosomal genes (12S and 16S), 22 transport RNA genes, and one control region. The complete mitogenome of Codonobdella sp. is 14,486 bp long and A + T biased (75.51%). The complete mitogenome of Piscicola geometra is 14,788 bp long and A + T biased (78.27%).

The omul, Coregonus migratorius (Georgi, 1775), an endemic fish of Lake Baikal, is the main commercial species of essential economic importance for the extensive region of Eastern Siberia. Despite the continuous 250-year study of Baikal omul and intensive commercial fishing, neither fishers nor biologists have ever mentioned or found external annelid parasites in numerous omul catches. Targeted parasitological studies have also never detected these ectoparasites on the body of this fish. In this study, we present the first reliable information on the existence of specific annelid parasites infecting the pelagic population of Baikal omul and report a 4% infestation of spawners in the Selenga River, the largest tributary of Lake Baikal. We also obtained the main morphometric parameters of the infected fishes, the morphology of the unknown parasite and its feeding strategy. DNA-based analysis was applied to ascertain the taxonomy of the leeches and revealed no closely related taxa existing in international genetic databases (e.g., GenBank) to date. A maximum genetic identity of 89–91% was detected with European Caspiobdella fadejewi. The genetic distances in 9–11% of the substitutions from the closest relatives were very high for a reliable molecular identification of a taxon. The Selenga omul leech may represent a potentially new species and genus.

Molecular delimitation methods have revealed that the morphological homogenous species of the Baikal endemic fish leeches, Baicalobdella torquata (Grube, 1871), is a cryptic complex. The identified level of genetic differences allows us to assume the existence in Lake Baikal of two species within this complex, which are clearly separated geographically.