Fimbriarioides intermedia and F. lintoni (Cestoda: Hymenolepididae): molecular phylogeny, species status, and transmission in Arctic coastal ecosystems.

Cestodes of the genus Fimbriarioides (Hymenolepididae, Fimbriariinae) parasitise waterfowl and shorebirds in the Holarctic. Using molecular and morphological methods, we studied two Fimbriarioides spp. from sea ducks of the European North of Russia: the type species F. intermedia from eiders Somateria mollissima and S. spectabilis 1758 and a closely related species F. lintoni from a previously unrecognised host, Velvet Scoter Melanitta fusca 1758. Our analysis also involved metacestodes Fimbriarioides sp. from barnacles Semibalanus balanoides of the Barents Sea and the Sea of Okhotsk. Metacestodes from S. balanoides matched the adults of F. intermedia by the molecular markers. In the phylogenetic tree based on 28S rRNA and cox1 genes, isolates from the two Fimbriarioides spp. formed distinct monophyletic groups in a clade sister to Fimbriaria spp. In the 16S rRNA tree, Fimbriaria sp. was positioned as a sister clade to F. intermedia, probably due to the marker's limited resolution. We provided emended morphological descriptions of F. intermedia and F. lintoni and diagnostic characters for their discrimination. The haplotype network based on the 16S rRNA gene, which included the isolates from the Barents Sea, the Pechora Sea, the White Sea, and the Sea of Okhotsk, indicated that F. intermedia has a trans-Arctic distribution and is represented by the same population in all these regions. This is due to the high vagility of its definitive hosts, sea ducks, which make long-distance migrations along the coasts of the northern Palaearctic seas, and the almost circumpolar distribution of its intermediate hosts, barnacles.