Morphological and genetic diversity of ice-associated nematodes of the family Xyalidae Chitwood, 1951

The ice-associated nematode Theristus melnikovi has a long-distance dispersal throughout the Arctic. However, this species’ taxonomic position and phylogenetic relationships remain unresolved. We used an integrative approach of single-specimen barcoding of morphologically identified specimens of T. melnikovi with note on the phylogeny of two new ice-associated species of the family Xyalidae. DNA barcodes (18S and 28S gene markers) were obtained from 13 specimens of T. melnikovi collected from sea ice in the White and Kara Seas. Analysis of the D2D3 region found three genetically distinct lineages of T. melnikovi from the White Sea ice, suggesting that T. melnikovi specimens belong to the cryptic species, with genetic differentiation attributed to limited ice exchange between remote semi-isolated areas and open Arctic seas. The Kara Sea specimens belong to a single species. Our results highlight that using 18S rDNA data alone underestimates species diversity within sympagic nematodes. The barcoded specimens of T. melnikovi could not be distinguished morphologically, as the majority of the sequenced specimens were juveniles (92.3%). Additional comparative analysis of morphological traits was performed on 34 specimens from sea ice in the Barents, White, Kara, and East Siberian Seas, and from sediment in the East Siberian Sea. Our specimens differed from previous descriptions of T. melnikovi by variable body size, amphidial fovea shape, and absence of precloacal supplements and pigment spots. This study contributes to the understanding of the diversity, variability in habitat, and ecology of sympagic nematodes, confirming the need to put together the integration of molecular and morphological methodologies.