| Summary: | <i>Ophiura sarsii</i> is a common brittle star species across the Arctic and Sub-Arctic regions of the Atlantic and the Pacific oceans. <i>Ophiura</i><i>sarsii</i> is among the dominant echinoderms in the Barents Sea. We studied the genetic diversity of <i>O.</i><i>sarsii</i> by sequencing the 548 bp fragment of the mitochondrial COI gene. <i>Ophiura</i><i>sarsii</i> demonstrated high genetic diversity in the Barents Sea. Both major Atlantic mtDNA lineages were present in the Barents Sea and were evenly distributed between the northern waters around Svalbard archipelago and the southern part near Murmansk coast of Kola Peninsula. Both regions, and other parts of the <i>O.</i><i>sarsii</i> range, were characterized by high haplotype diversity with a significant number of private haplotypes being mostly satellites to the two dominant haplotypes, each belonging to a different mtDNA clade. Demographic analyses indicated that the demographic and spatial expansion of <i>O.</i><i>sarsii</i> in the Barents Sea most plausibly has started in the Bølling–Allerød interstadial during the deglaciation of the western margin of the Barents Sea.
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