The phylogeography and ecology of Oligobrachia frenulate species suggest a generalist chemosynthesis-based fauna in the arctic

**Background:** Chemosynthesis-based ecosystems (CBEs) such as hydrocarbon seeps and hydrothermal vents are typically thought to host specialist fauna adapted to high sulfide and methane conditions. In the Arctic, however, chemosymbiotic animals like siboglinid frenulates are also found in fjords and sounds where organic matter accumulation generates reduced compounds. The taxonomic identity of the fjord species Oligobrachia webbi, described from Kvalsund (northern Norway) in 1966, has been debated since the seep species O. haakonmosbiensis was described from the Haakon Mosby Mud Volcano (HMMV) in 2000, as the two are morphologically nearly identical. No DNA sequence data existed for O. webbi prior to this study. The authors aimed to resolve this taxonomic question and investigate whether Arctic chemosymbiotic fauna are seep specialists or habitat generalists, and to explore the nutritional ecology of these animals in non-seep environments.

**Methods:** The authors applied ancient DNA (aDNA) extraction methods (QIAamp DNA FFPE Tissue Kit) to a museum voucher specimen of O. webbi from the Arctic University Museum of Norway (collection number P 746). New samples were collected from the type locality in Kvalsund in October 2020 using box cores, a gravity core, and a Van Veen grab, guided by approximate coordinates from the original 1966 description. Additional samples were collected from Nordfjord (a non-seep fjord) and the Lofoten-Vesterålen (LV) canyon seep. The mitochondrial COI gene was amplified and sequenced using universal primers HCO2198/LCO1490 and three overlapping primer sets designed for fragmented DNA. Phylogenetic analysis was performed using Bayesian inference (MrBayes v3.2.6) with a GTR model. Sediment porewater sulfide was measured by iodometric titration, methane by gas chromatography, and total organic carbon (TOC) and total nitrogen (TN) by elemental analysis. Stable carbon, nitrogen, and sulfur isotope ratios were measured via CF-EA-IRMS at the University of Liège.

**Key Results:** The museum specimen of O. webbi yielded a 574 bp mtCOI sequence that was 99% identical (two mutations) to O. haakonmosbiensis voucher V VI-143-1-HMMV. The single multi-tentacled Oligobrachia specimen collected from Kvalsund in 2020 was also 99% identical (one mutation) to the same voucher. All other Kvalsund specimens (11 individuals) were identified as Siboglinum fiordicum (100% match with GenBank MK673144.1). Nordfjord specimens comprised both S. fiordicum and S. ekmani. Sediment porewater sulfide at Kvalsund reached concentrations as high as 735 μM in the gravity core, with bottom water concentrations in the tens of μM. Methane concentrations reached mM levels at ~5 m sediment depth. TOC at Kvalsund was mostly above 10 mg/g, reaching >100 mg/g, with TOC/TN mass ratios averaging 17.4 (range 2.4–121.4), indicating substantial terrestrial input. Stable isotope analysis showed that O. haakonmosbiensis from the LV seep had δ¹³C values from −52.2‰ to −45.7‰. S. fiordicum from Kvalsund had δ¹³C values mostly below −35.0‰, but two individuals had values of −26.9‰ and −25.6‰, within the photosynthetic range. Similarly, two Nordfjord specimens had δ¹³C values of −21.4‰ and −21.3‰. These photosynthetically influenced individuals also had higher δ¹⁵N values (7–8‰ at Nordfjord; −3.7‰ and 1.8‰ at Kvalsund) compared to chemosynthesis-dominated specimens (−9‰ to −7‰ at Nordfjord; around −5‰ at Kvalsund). One LV seep O. haakonmosbiensis had a δ³⁴S value of 15.7‰, distinct from other LV samples (−1‰ and lower).

**Clinical Implications:** While this is not a clinical study, the findings have significant implications for understanding Arctic marine ecosystem functioning and carbon cycling. The synonymization of O. webbi and O. haakonmosbiensis demonstrates that a dominant chemosymbiotic species at Arctic seeps is not a seep specialist but also inhabits fjord environments with naturally reducing sediments. This challenges the global paradigm that seep ecosystems host specialist fauna. The presence of chemosymbiotic animals (frenulates and thyasirid bivalves) across both seep and non-seep Arctic habitats suggests that chemosynthesis-based carbon fixation is a widespread but overlooked component of Arctic benthic food webs. The stable isotope evidence for mixotrophy (supplementing chemosynthesis with uptake of photosynthetically derived organic matter) indicates trophic plasticity that may be an adaptation to the extreme seasonality of the Arctic, where sulfide generation rates likely decline during dark winter months. The authors propose that seasonal shifts in reliance on chemosynthetic versus photosynthetic carbon sources may be a common strategy among Arctic chemosymbiotic fauna, with implications for carbon and energy transfer pathways that have not been previously considered in Arctic marine ecosystem models.