Delineating species boundaries is crucial for understanding deep-sea sponge biodiversity and biogeographical
patterns, which aids in conservation efforts such as the establishment of Marine Protected Areas. The demo-
sponge Stylocordyla borealis (Lov´en, 1868) was historically thought to have a widespread distribution across
various ocean basins. However, recent studies using morphological and molecular tools revealed a complex of
several distinct species, each with specific biogeographical ranges. While a cosmopolitan distribution for
S. borealis has now been disproven, uncertainties about its wide-ranging records and many subspecies continue to persist. The present study focuses on the Arctic circumpolar distribution of the North Atlantic S. borealis and its
North Pacific subspecies S. borealis eous Koltun, 1966. Morphological traits, phylogenetic reconstruction with
genetic markers (COI, 28S), and 16S amplicon sequencing of microbial communities were used to explore
phenotypic and genetic diversity within S. borealis Atlantic and Pacific populations. Results showed that S. borealis is confined to the North Atlantic and Boreo-Arctic regions, while S. borealis eous inhabits the North Pacific and should be taxonomically elevated to a distinct species, named Stylocordyla eous Koltun, 1966 comb. nov. This distinction underscores the contentious nature of the subspecies concept in sponge taxonomy and highlights the value of molecular techniques. In this study, microbial fingerprinting enabled species-level resolution within the Stylocordyla genus, demonstrating the importance of 16S microbial amplicon sequencing as a complementary tool for Porifera taxonomy. These insights contribute to a deeper understanding of deep-sea sponge biodiversity and distribution