What You Can See From here: A Critical Role of Integrative Approach and Sample Size in Defining Species Boundaries for Trans‐Arctic Nudibranchs (Gastropoda: Heterobranchia)

ABSTRACT Recent integrative taxonomic revisions of the amphiboreal nudibranch Diaphoreolis viridis (Forbes, 1840) suggest that it represents a complex of cryptic or pseudocryptic species. However, species delimitation within trans‐Arctic taxa remains challenging due to the reliance on potentially variable morphological traits (such as coloration) and the limitations of using mtDNA data alone. Therefore, the taxonomic status of several North Pacific populations traditionally attributed to the Diaphoreolis viridis complex ( Diaphoreolis midori (Martynov, Sanamyan & Korshunova, 2015), Diaphoreolis viridis emeraldi Korshunova, Fletcher, Bakken & Martynov, 2023, and the nominal form) requires rigorous re‐evaluation using integrative approaches. The aim of this study is to clarify the identity of chromatic variants of D. viridis across its broad Pacific distribution and to assess the genetic connectivity of its distant populations. By combining detailed morphological examination with mtDNA genotyping and sequencing of nuclear ribosomal markers across a wide geographic range of the complex, this study provides a robust assessment of species boundaries within Diaphoreolis viridis and contributes to a broader understanding of the challenges and solutions in trans‐Arctic nudibranch taxonomy. The phylogeographic analysis included a TCS‐based network analysis, an analysis of molecular variance (AMOVA) and divergence time estimations. The results demonstrate the conspecificity of D. v. emeraldi and D. midori , as well as the conspecificity of the entire complex under the name D. viridis due to the absence of significant morphological and molecular differences. We also identified previously undescribed species from the Kuril Islands, Diaphoreolis shinkaii sp.nov., which differs markedly from other Diaphoreolis species in its molecular, morphological, and bathymetric characteristics.