Larvae in the Dark: Evidence of Diverse and Functionally Active Meroplankton Communities in an Arctic Fjord during the Polar Night.

AbstractIn the high Arctic polar night, the sun remains below the horizon for a period ranging from several weeks to 6 months, providing insufficient solar radiation for photosynthesis to support the marine ecosystem. Despite this, recent studies have found evidence of active feeding, growth, and reproduction in multiple marine organisms during this period. Here, we describe the polar night diversity and activity levels of early pelagic life stages of benthic invertebrates (meroplankton), a group that has historically been largely overlooked. We assessed the taxa richness, reproductive and developmental biology, feeding, and competence to settle of meroplanktonic larvae in January 2023 in Kongsfjorden, Svalbard, using plankton tows, experiments, and DNA bar coding. Although meroplankton densities were lower than in other seasons, DNA bar coding allowed us to assess richness, which was much higher than other studies and seasons, with 48 unique taxa belonging to six phyla observed. Most were planktotrophic (feeding) larvae. Using laboratory assays and direct observations, we documented feeding in several of these planktotrophic larvae, including bryozoan cyphonautes, bivalve veligers, and an ophiuroid pluteus. We found numerous embryos, suggesting that several taxa are actively reproducing in the polar night. We observed settlement in two polychaete taxa in the laboratory, as well as metamorphosed echinoderms in our samples, showing competency to settle across phyla. Overall, we show that early life-history stages of a diverse range of benthic invertebrates are actively feeding and developing during the polar night. Further targeted research should focus on the polar night to better understand seasonal patterns in reproduction and development.