Irina Ekimova, Dmitry Fedorov, Darya Grishina, Dimitry Schepetov
The Cadlina laevis (Linnaeus, 1767) species complex in the White Sea exhibits a high mitochondrial DNA polymorphism, with two divergent haplogroups (HA and HB) showing ~2.5% COI divergence, consistent with minimal interspecific mtDNA differences within this group. This study integrates complete mitochondrial genomes of both haplogroups of the White Sea Cadlina laevis, nuclear markers (18S, ITS1, ITS2, 28S, H3), and species-delimitation analyses to test whether mitochondrial lineages correspond to distinct species. Comparative mitogenomics revealed conserved gene order but heterogeneous divergence, with protein-coding genes showing p-distances from 1.0% (COX2) to 5.1% (ATP6) between haplogroups. Phylogenetic reconstructions based on nuclear markers consistently collapsed HA and HB into single, undifferentiated clade, demonstrating mitonuclear discordance. Similar patterns emerged for other mitochondrial lineages: C. laevis and North Pacific C. vavilovi Korshunova & Martynov, 2024, as well as C. koltzovi Korshunova & Martynov, 2024 and Cadlina sp. 3 showed no nuclear differentiation despite substantial COI p-distances. Species-delimitation methods yielded conflicting results, failing to provide consistent support for mitochondrial-defined species. The COI-wide distance distribution placed the HA-HB comparison within the "grey zone" of ambiguous species boundaries. The study demonstrates that mitochondrial data alone are unreliable for species delimitation in this group, suggesting that the current taxonomy may over-split mitochondrial lineages and require genome-wide nuclear evidence for robust taxonomic revision.