A 50-year conundrum is conclusively solved: nudibranchs Dendronotus albus (= D. diversicolor) and Dendronotus robilliardi are valid species with compelling evidence from type materials, bibliographic sources, and molecular data

Abstract

A persistent confusion in the identification of two similar species of nudibranch molluscs of the genus Dendronotus Alder and Hancock, 1845, which inhabit shallow Canadian, Russian, and US waters of the northern Pacific Ocean, has caused a considerable taxonomic conundrum for over 50 years. In this study, we present compelling evidence for the first time, including morphological and molecular data as well as a study of type materials, to definitively conclude that Dendronotus albus MacFarland, 1966 was misidentified in the publication of G.A. Robilliard (1970; Veliger, 12: 433–479). This caused the long-term incorrect application of the characters of the real D. albus sensu F.M. MacFarland (1966; Mem. Calif. Acad. Sci. 6: 1–546) (which is still circulated in the literature) to a completely different, though externally similar, species incorrectly named “D. albus” in G.A. Robilliard (1970; Veliger, 12: 433–479). This species was properly described only 3 years ago as Dendronotus robilliardi Korshunova, Sanamyan, Zimina, Fletcher and Martynov, 2016. Using multiple evidence in this study, we firmly conclude that D. robilliardi is a well-supported valid species, whereas “Dendronotus diversicolor” Robilliard, 1970 is a junior synonym of D. albus. We also present rare bibliographic materials that shed light on the source of the misidentification of these species and also confirm the designation of the lectotype for D. albus. Biogeographical data, which are important for conservation and ecological studies, are also presented for D. albus and D. robilliardi in the Canadian and American Pacific Ocean.