The taxonomy, anatomy, and biological features of free-living flatworms of the phylum Plathelmintes from the White Sea (Russia) have been studied poorly, and their diversity is greatly underestimated. The main obstacle to this is their challenging identification and the miniature sizes of considerable number of representatives of the phylum, microturbellarians’ in particular. Microturbellarians of the genus Monocelis (Proseriata) are tremendously numerous in the littoral zone of the vicinity of the White Sea biological station of Moscow state university, and are represented, according to previous studies, by only two species - M. lineata (Müller, 1773) and M. fusca (Örsted, 1843). However, not all specimens examined during recent work match the characteristic features of the registred species.
Current research on Monocelis sp. specimens from the White Sea was carried out using an integrative taxonomic approach. We paied special attention to the significant variability of the worms’ coloration. Based on the molecular phylogenetic analysis of the 18S gene and the 28S fragment sequences of the Monocelis sp. specimens, five clades were identified. These groups also were confirmed as candidate species by morphological features, including coloration, body proportions, structural features, and relative arrangements of the reproductive system parts. Also, we have obtained the complete sequence of the mitochondrial genome for one group for the development of universal primers.
The structure of the copulatory organ turned out to be the most important taxonomic feature: each identified group has a unique structure to the others. This feature gives us reason to assume the existence of reproductive isolation, which allows for the coexistence of several closely related species within one local biotope.

This work was carried out with the support of an RSF grant (No21-74-20028).