What happens when Arctic sea-ice melts? Macroalgae as a summer refuge for sympagic nematode Theristus melnikovi

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.