Integration of molecular and morphological data
We discovered significant molecular differences between shallow-water and deep-water specimens of putative D. robustus. These molecular differences agree with the disparity in the colour features in the first description of true shallow-water D. robustus from the intertidal (Verrill, 1870) and the deep-water specimens. Verrill (1870: 406) described the colour of D. robustus as “pale greyish, thickly sprinkled with small yellow spots”, whereas the colour of the deep-water Norwegian specimens, previously described by G.O. Sars (1878: 315) from 109 –183 m under the name of Dendronotus velifer G.O. Sars, 1878, is a “bright reddish with varied white spots” (“Color læte rufescens, punctis albidis variegates”).
As shown in the present study, such molecular and morphological differences between shallow-water and deep-water specimens persist over a very large biogeographic range, from the American Eastern Atlantic to the Arctic Laptev Sea. Significantly, a molecular and morphological data set has recently become available for a deep-water specimen from off Newfoundland (Valdés et al., 2017). This specimen matches, both its external features (i.e. rather bright and uniform reddish colour) and its molecular data, with our deep-water specimens from about 100 m and deeper from the Barents, Kara and Laptev seas. It does not match our shallow water Barents Sea specimens, which represent a different species with more greyish to brownish-yellowish colour. Although these differences are somewhat subtle, the greyish colour mentioned in the first description of true D. robustus from the Canadian Atlantic fits with the shallow-water species from the Barents Sea (Figure 3a, b, d, e), and not with the more uniformly red colour of the deep sea D. velifer from the various Arctic seas, including the Laptev and Kara seas (Figure 2a, b, h-j).
Therefore, both the morphological and the molecular data clearly suggest that two species are actually confused under the name D. robustus in the North Atlantic Ocean – one lives in shallow water from the intertidal to approximately 20 m depth and the other lives deeper, at depths of 50–300 m, most commonly at 80-150 m. The integrative, molecular and morphological data unambiguously suggest that the shallow-water and the deep-water specimens belong to different species, and therefore we hereby restore the name Dendronotus velifer G.O. Sars, 1878 for the deep-water species and retain D. robustus for the shallow-water species. Both species share a similar body shape and a soft-bottom habitat, usually with an almost omnivorous diet (including cnidarians, which is typical of other dendronotoids, but also polychaetes and even sunken terrestrial beetles; see Roginskaya, 1990).