Seamounts are ecological oases nurturing abundant fisheries resources and epibenthic megafauna in the vast oligotrophic ocean. Despite their significance, the formation mechanisms underlying these seamount ecological oases remain uncertain. To shed light on this phenomenon, this study conducted interdisciplinary in situ observations focusing on a shallow seamount in the oligotrophic ocean. The findings show that the seamount’s topography interferes with the oceanic current to generate lee waves, effectively enhancing the nutrient supply to the euphotic layer downstream of the seamount. This continuous supply enhances phytoplankton biomass and subsequently the grazing and diurnal vertical migration of zooplankton, rapidly transporting the augmented phytoplankton biomass to the aphotic layer. Unlike the cyclonic eddies that move in the upper ocean, seamounts stand at fixed locations creating a more efficient and steady active transport loop. This active transport loop connects the euphotic and twilight zones, potentially conveying nourishment to benthic ecosystems to create stereoscopic oases in the oligotrophic ocean.

Deep-sea sponges create complex biogenic structures and attract a wide array of deep-sea organisms, including symbionts. In this study, we describe Eunice siphoninsidiatorsp. nov., a new eunicid species living in the central cavity of deep-sea farreid glass sponges found on northwest Pacific seamounts. The new species closely resembles the Atlantic Eunice norvegica both morphologically and molecularly, but it differs in the relative length of palp compared to peristomium, starting points of subacicular hooks, and shape of pectinate chaetae. A 13% COI genetic distance between the two species further supports the establishment of E. siphoninsidiator as a distinct species. Gut content analyses reveals fragments of barnacles and brittle stars, suggesting a carnivorous diet and a sit-and-wait predatory strategy. The eunicid gains protection from living inside the sponge, which consistently harbored the polychaete in all specimens examined, while the sponge benefits from the cleaning of epibionts, pointing to a potentially mutualistic relationship.