Further support for the hypothesis that the Swedish deep-water specimens belong to D. velifer is their reddish colour (Odhner, 1907: 19). However, in the same publication Odhner (1907) listed D. velifer as a synonym of D. robustus, without any specific discussion on the issue, probably due to the similarity of the broad body in both species. But Odhner evidently omitted noting the differences in colour (reddish versus greyish-yellowish colour), and importantly also the considerable differences in bathymetric distribution (100–300 m versus intertidal), and hence, in ecological patterns. Previously, Bergh (1900) had also uncritically synonymised D. robustus and D. velifer under the former name. Furthermore, Odhner subsequently reported D. velifer, mostly from some deep Norwegian fjords (e.g. Sandnessjøen, Nordland fylke, June 1938, coll. O. Björlykke, 200 m, see Odhner 1939, also Odhner, 1922, 1926), under the name D. robustus. Thus, it was accepted by most of the subsequent researchers and incorporated in regional lists and identification keys (e.g. Roginskaya, 1987; Martynov and Korshunova, 2011; Ekimova et al., 2015). Apart from the differences in coloration (which compared to some other Dendronotus species actually appear to be more stable in this species pair, but still may represent some difficulties for identification in the field) and considerable differences in the molecular data (Figure 4a, b), D. robustus and D. velifer can also be distinguished by radular features (Appendix; Figs. 1 2-3). In particular, specimens of D. robustus prove to have higher central (rachidian) teeth (Figure 3c, f) and considerably more denticulated lateral teeth, especially in smaller specimens (Figure 2c, f, k). Dendronotus velifer in contrast possesses lower central teeth and less denticulated lateral teeth. The lateral teeth in larger specimens of D. velifer are almost smooth with only traces of reduced denticles on some of them (Figure 1e), whereas smaller specimens of D. velifer may possess more denticulated teeth, which are considerably less denticulated (Figure 1j, k) than in D. robustus of comparable size (Figure 3g). These data on the Swedish Gullmar specimens are remarkably consistent with the original description of D. velifer by G.O. Sars (1878: 315), wherein he mentions of the lateral teeth: “uncini utrinqve 15, læves, vel vestigium modo indistinctum denticulorum hic et illic exhibentes” (= 15 lateral teeth, smooth with indistinct rudimentary denticles on the lateral teeth).
Dendronotus velifer also appears to have fewer dorsolateral appendages than D. robustus: even in quite a large specimen from the Laptev Sea (49 mm) there are only five pairs of dorsolateral appendages (Figure 2a, b), and likewise in specimens from the Gullmar Fjord, including a small specimen of 13.5 mm length (Figure 1a–h). In contrast, even in a subadult (19 mm) specimen of D. robustus (Figure 3d, e) from the Barents Sea, there are six dorsolateral processes. In the larger (35 mm) D. robustus from the Barents Sea, the number of dorsolateral appendages is seven. These data partly agree with the original descriptions of D. robustus and D. velifer: Verrill (1870) indicated six pairs of dorsolateral appendages in a 50 mm long specimen of D. robustus, whereas G.O. Sars mentioned five to six dorsolateral appendages for several specimens with a maximum length of 90 mm. According to these and our data, it is likely that D. velifer usually has no more than five dorsolateral appendages compared to the usual six appendages of D. robustus, even found in small specimens. As an exception, large specimens of D. velifer may have an additional sixth appendage, whereas D. robustus with half the size of the largest D. velifer possess up to at least seven pairs. Furthermore, the frontal digitate appendages (processes) on the oral veil are relatively much longer in D. robustus (Figure 3a-d) than in D. velifer (Figure 2a, b, h-j). Such relative differences, after additional testing, may prove to be reliable morphological features to distinguish D. velifer and D. robustus.
Dendronotus velifer differs from the three other congeneric species that can occur in the same geographic area (i.e. D. frondosus (Ascanius, 1774), D. lacteus (W. Thompson, 1840), and D. niveus Ekimova et al., 2015) by its wedge-shaped body, which is considerably expanded in comparison to the more slender body that is only slightly expanded anteriorly in the other three species. Furthermore, D. velifer has a considerably broader oral veil and it lacks lateral papillae on the rhinophoral sheaths, compared to the three congeners. Internally these three species also differ from D. velifer by their radular characters. Dendronotus velifer differs from a tropical Pacific Ocean species with a similarly wide anterior body, D. patricki Stout et al., 2011, by its reddish colour with scattered white dots and stripes (D. patricki is uniformly pinkish to reddish brown, without any white spots on the dorsum, except for the tips of the appendages) and D. velifer also differs from D. patricki by the less protracted cusp of the central tooth, more numerous lateral teeth per row, and by the presence of the reduced denticles on the lateral teeth (Figure 1e, j, k). Along the Swedish western coast D. lacteus and D. frondosus are common in the subtidal zone from the surface down to 30 m depth. None of these species have been confidently recorded from the deep section of the Gullmar Fjord, although the known bathymetric preferences of D. lacteus and D. niveus potentially allow these species to inhabit the deeper parts of the Gullmar Fjord. Should these species be found sympatrically with D. velifer in the Gullmar Fjord, it will be easy to distinguish them by body shape, colour and the radula. So far, true D. robustus has never been positively reported from the shallow areas of Norway or Sweden. Material from the Norwegian Trondheim Fjord, previously identified as D. robustus by Friele and Grieg (1901), was examined by Odhner and revised as D. frondosus.