The taxonomy of common northern nudibranch molluscs of the genus
Dendronotus in the vast cold regions of Eurasia remains largely unknown. Abundant material collected in many localities from the Barents Sea, via the Arctic region, to the north-west Pacific was analysed for the first time. An integrated approach combining morphological and ontogenetic data with molecular four-gene (
COI,
16S,
H3, and
28S) analysis reveals seven species, including three previously undescribed.
Dendronotus frondosus (Ascanius, 1774) and
Dendronotus dalli Bergh, 1879 were commonly considered as amphiboreal species; however, according to this study they are restricted to the North Atlantic and the North Pacific, respectively. In the north-west Pacific two new species were discovered,
Dendronotus kamchaticus sp. nov. and
Dendronotus kalikal sp. nov., that are externally similar to
D. frondosus, but that show significant distance according to molecular analysis and are considerably different in radular morphology. In the North Atlantic a new species
Dendronotus niveus sp. nov., sibling to North Pacific
D. dalli, is revealed. The separate status of North Atlantic
Dendronotus lacteus (Thompson, 1840) is confirmed, including considerable range extension. The essential similarity of early ontogenetic stages of radular development common for species with disparate adult radular morphology (such as
D. frondosus and
D. dalli) is shown, and its importance for taxonomy is discussed.