Two new species and a remarkable record of the genus Dendronotus from the North Pacific and Arctic oceans (Nudibranchia)

Two new species of the nudibranch genus Dendronotus, D. arcticus sp. n. and D. robilliardi sp. n., are described from the Arctic and North Pacific oceans respectively, based on morphological and molecular data, and the North Pacific D. albus is revealed to be a species complex. The species D. robilliardi sp. n. is described from the northwestern Pacific (Kamchatka) differing from the northeastern Pacific D. albus by molecular and morphological data. The synonymy of D. diversicolor with D. albus is confirmed by analysis of their original descriptions. An endemic Arctic species D. arcticus sp. n. is also described here, differing substantially from all species of the genus Dendronotus using morphological and molecular data. An unusual record of the recently described D. kamchaticus Ekimova, Korshunova, Schepetov, Neretina, Sanamyan, Martynov, 2015 is also presented, the first from the northeastern Pacific, geographically separated from the type locality of this species in the northwestern Pacific by a distance ca. 6000 km; molecular data show them to belong to the same species.