Vanessa Pitusi

Sea ice-associated (sympagic) nematodes are particularly vulnerable to ongoing Arctic sea-ice loss, yet the capacity of these sympagic species to persist in ice-free habitats remains poorly understood. We investigated the summer habitat and taxonomic identity of the sympagic nematode Theristus melnikovi , in Svalbard, using an integrative taxonomic approach. We barcoded 17 morphologically identified nematodes from macroalgae (5), sea-ice (9), and sediment (3). Morphological analysis and phylogenetic reconstruction based on 18S rRNA gene analyses revealed that the macroalgae-associated T. melnikovi represented a habitat-associated phenotypic variation of T. melnikovi rather than a morphotype of the benthic nematode T. acer . Macroalgae-associated specimens were generally shorter and stouter, with proportionally longer tails and shorter anterior structures than their ice-associated counterparts, resulting in a superficial resemblance to T. acer. Despite these morphological differences, molecular analyses consistently placed macroalgae-associated individuals within the T. melnikovi clade and haplotype. Our results highlight that T. melnikovi may have a greater ecological flexibility than previously assumed and macroalgae may offer an alternative (summer) habitat, or refuge, during ice-free periods. This capacity could become increasingly important as Arctic sea-ice duration and extent continue to decline.

HHUMTBT26 (Helmer Hanssen, University Museum, Tromsø–Bergen–Tromsø, 2026) was a research cruise conducted aboard the R/V Helmer Hanssen, a multipurpose research vessel owned by UiT The Arctic University of Norway (Fig. 1). The cruise was organised and carried out by The Arctic University Museum of Norway, in cooperation with the UArctic Thematic Network on Arctic Marine Biodiversity and Artsdatabanken, and with participation from several research projects aimed at advancing our understanding of marine biodiversity, taxonomy, and ecosystem history in northern and central Norway.

R/V Helmer Hanssen is a multipurpose research vessel owned by UiT The Arctic University of Norway. The cruise ‘HHUMTKT25’ was organized and executed by The Arctic University Museum of Norway (units ‘UM’ and ‘UMAK’ at UiT). The six participating projects in this cruise were (1) Scalidophora of Norway, (2) Microfauna and macrofauna in Northern Norway, (3) Reconstruction of past ecosystems using sedimentary ancient DNA, (4) ArKoBi studying the population connectivity of Arctica islandica in Northern Norway, (5) DragonNest studying population genetics and distribution patterns of Kinorhyncha in the Arctic region and (6) NUDIST studying biogeography and taxonomy of nudibranchs in northern Norway and the Arctic. Projects (1) ‘Scalidophora of Norway’ and (5) DragonNest aimed at sampling scalidophoran fauna in Northern Norway, primarily the phylum Kinorhyncha, using a box corer. Projects (2), (4) and (6) relied on a triangular dredge to obtain specimens, and project (3) utilized a gravity corer to capture a 5-m long sediment core. The cruise leader was Andreas Altenburger (UiT).

R/V Helmer Hanssen is an ice class research vessel owned by UiT The Arctic University of Norway. The HHUMLT24 cruise onboard R/V Helmer Hanssen was an initiative by The Arctic University Museum of Norway (UMAK, UiT). Sampling was conducted for the following four projects: 1. The Scalidophora of Norway, 2. The role of microRNAs in animal evolution, 3. The diversity of benthic fauna around Svalbard and the Barents Sea, 4. Reconstruction of past ecosystems using sedimentary ancient DNA. For the first project, which aimed to sample scalidophoran fauna, primarily kinorhynchs, for the Scalidophora of Norway project funded by Artsdatabanken, box cores were taken. The second project employed plankton nets, box cores, and a triangular dredge. The third project used samples from box cores, plankton nets, and a triangular dredge to collect marine macrofauna for the museum collections. All macrofauna present in plankton nets, box cores, and triangular dredge samples were photographed, and records were uploaded to GBIF. The fourth project took gravity cores. The cruise leader was Andreas Altenburger (UiT).

Svalbard is one of the fastest warming regions in the Arctic including massive loss in fjord sea ice both in terms of area coverage, ice thickness and duration. Sea ice is a habitat for a wide variety of microscopic flora and fauna, and we know little about the impact of accelerated loss of sea ice on this unique sea ice community. Here, we present the first study on the seasonal progression and spatial distribution of the sympagic meiofauna community, in a Svalbard fjord. Further, the meiofauna community in sea ice versus the water column below were compared to investigate the link between the two habitats. In total, we found 12 taxa associated with the sea ice and 15 taxa in the water column below with 11 taxa occurring in both habitats. However, a Canonical-analysis (CA) showed that despite similarities in taxa the two mediums were distinctly different (potentially) due to the low abundance of ice nematodes and polychaete juveniles, in pelagic samples. Temporally, ice meiofauna abundances ranged from 9.7 to 25.3 x 10³ind m^-2from beginning of March to end of April, following the seasonal build-up of ice algal biomass from 0.02 to 15.99 mg Chlam^-2during the same time span. For the transect stations, the lowest ice meiofauna abundance was recorded at the outermost station (VMF2) with 1.6 x 10³ind m^-2and the highest abundance at the mid-station MS with 25.3 x 10³ind m^-2. Our results indicate that fjord ice harbors most ice algae and sympagic meiofauna in its lower 10-cm with highest values in the lowermost 2-cm, at the sea ice water interface. Sympagic meiofauna communities were mostly dominated by nematodes or polychaete juveniles. We observed the phenology of ice nematodes through the maturation of females and hatching of juveniles from eggs. Polychaete larvae developed (quickly) into juveniles and grew morphological features indicative of readiness for settlement. Thus, we propose, that as with other parts of the Arctic, sea ice in Svalbard fjords plays an important role in the life cycle of ice nematodes and for accelerating the growth of polychaete larvae. Loss of coastal sea ice may therefore negatively impact coastal biodiversity and affect recruitment for some benthic infauna in Svalbard.

Understanding the diversity and functioning of Arctic sea ice ecosystems is vital to evaluate and predict the impact of current and future climate change. In the microscopic communities inhabiting the brine channels inside sea ice, nematodes often dominate numerically and act as bacterivores and herbivores. Despite nematodes great abundances and known ecological roles, molecular tools have not been applied to investigate their species diversity in sea ice. In an attempt to begin establishing a molecular baseline for species diversity of sea ice nematodes, we Sanger sequenced 74 specimens from four locations around Svalbard (European Arctic), using the 18S rRNA barcode. Currently available nucleotide reference databases are both underpopulated with representative marine nematode taxa and contain a substantial number of misidentified organisms. Together, these limitations inhibited the ability to identify marine specimens collected in this study with certainty. Nevertheless, our molecular data indicate the presence of two genera in sea ice on Svalbard—TheristusandHalomonhystera.While it is possible that the latter represents a novel ice nematode species, future studies, including morphometric analysis, are needed to verify our molecular findings. We leverage the assignment of molecular information to robustly identify nematodes and provide the first insight into the diversity of sea ice nematodes in the European Arctic. We advocate for an intensified cooperation between molecular and morphological taxonomists to expedite the establishment of baseline surveys that are required to predict biological consequences of the diminishing sea ice habitat in the future.