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

Abstract

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.