Cryptic species complex or an incomplete speciation? Phylogeographic analysis reveals an intricate Pleistocene history of Priapulus caudatus Lamarck, 1816

We detected five divergent COI lineages in Priapulus caudatus Lamarck, 1816: four from the Arctic and one from the Sea of Okhotsk. K2P distances ranged from 6.87% to 14.35% with a mean distance of 10.03%, which typically corresponds to inter-species distances. However, four of five lineages (excluding the Okhotsk one) were found in syntopy, and no morphological differences between potential species were found. Single-gene phylogenies for the mitochondrial 16S, as well as nuclear 28S, 18S, and FoxQ2 markers, had different topologies and were non-congruent with the COI tree. This may point to an incomplete speciation due to a series of long-time geographical isolations during Pleistocene glaciations in different parts of range of P. caudatus. The 28S and 18S sequences fell into two genotypes, one being found mainly in the intertidal, and the other exclusively in the subtidal, which indicates the prevalence of unidirectional migrations from the intertidal into the subtidal. Because Arctic intertidal populations disappear during glaciations and recover in interglacial periods, and due to their partial reproductive isolation from each other, they give rise to new genetic lineages. Subtidal populations are permanent and have less significant isolation boundaries, therefore they accumulate genetic diversity.