Martynov et al. (2015a, b) described the nudibranch Dendronotus primorjensis from the north-west part of the Sea of Japan. The original description of D. primorjensis provided numerous diagnostic features including external characters, detailed scanning electron microscopy images of the radula, as well as remarks about its molecular phylogenetic relationships. The format of that publication precluded publication of lengthy molecular analyses, even though they were performed. Subsequently, Ekimova et al. (2016) described Dendronotus dudkai as new from exactly the same region in which the type material of D. primorjensis was collected. Nevertheless Ekimova et al. (2016) considered Dendronotus primorjensis a nomen dubium and presented weak evidence of its invalidity, including allegations that the type material of D. primorjensis “are lost if ever existed” (Ekimova et al. 2016: 31). Original type material of Dendronotus primorjensis (holotype and paratype) has been stored in the Zoological Museum of Moscow State University since 2014.

In this publication analysis of combined molecular and morphological data of the holotype and paratype of Dendronotus primorjensis demonstrate that it is a valid species and has taxonomic priority over D. dudkai, published one year later.

According to the morphological and molecular data presented above there are no species-level differences between D. primorjensis (Figs 1 and 2; Table 2) and D. dudkai (Ekimova et al. 2016: 33–37). Molecular analysis robustly places the holotype of D. primorjensis inside the clade of D. dudkai (Fig. 2). Both the external and internal morphologies, as well as radular and reproductive features, do not demonstrate any significant differences between the two. Other differences between the original descriptions of D. primorjensis and D. dudkai are discussed below.

Since D. frondosus is a closely related species to D. primorjensis (= D. dudkai syn. n.)(Fig. 2) and, according to Ekimova et al. (2016), co-occurs with D. primorjensis in the Sea of Japan, it is important to discuss distinguishing morphological characters between these two species as well as their biogeographical patterns. In the first description of D. primorjensis “an absence of the large amount of the white pigment” was described as a potential distinguishing external character from D. frondosus and the eastern Pacific related species D. venustus (Martynov et al. 2015a: 60). This statement was challenged by Ekimova et al. (2016: 31, 35) who claim that “all specimens possess well-visible white stripes between pairs of cerata.” However, surprisingly, on the figures of D. dudkai (Ekimova et al. 2016: fig. 9A, C) no white pigment is visible, thus in full agreement with the data on both the holotype and paratype of D. primorjensis (Martynov et al. 2015: 60, Fig. 5A–G and present study, Fig. 1A, C). The colour patterns of D. frondosus and D. venustus vary considerably (Robilliard 1970; pers. obs.) and D. primorjensis is potentially expected to have similar variations. However, to date no specimens of D. primorjensis with really large amounts of white pigment between the dorsolateral appendages have been reported (as previously documented for D. frondosus and D. venustus).

Ekimova et al. (2016: 37) claim that “D. frondosus is differentiated from D. dudkai by the absence of denticles on the masticatory process of jaws” but this is in error: in Robilliard’s review on the genus Dendronotus, the presence of denticles has been indicated for D. frondosussensu lato (“A small number of relatively large, black denticles adorns the masticatory margin”, Robilliard 1970: 443). Furthermore, since Robilliard’s study refers to a mixture of D. frondosuss. str. and D. venustus MacFarland, 1966 (but anyway not D. primorjensis) we have studied a topotype of D. frondosuss. str. from the North Atlantic (Norway) to clarify the situation. Species identity was confirmed using both morphological and molecular data: the topotype of D. frondosuss. str. demonstrated several small but distinct denticles on the jaws. In the original description of D. venustus, numerous denticles (between 27 and 40) on the masticatory processes were also described (MacFarland 1966: 277). Thus, the presence of denticles on the masticatory processes of the jaws cannot serve as a diagnostic character for D. primorjensis (= D. dudkai syn. n.) despite claims by Ekimova et al. (2016).

Instead, it is suggested here that the number of denticles on the central teeth is a better diagnostic, and is higher in D. primorjensis (commonly more than 12, in holotype 14, reported range 12–18) than in D. frondosus (commonly up to 12, rarely up to 14 denticles in the topotype specimens from North Atlantic, pers. obs., reported range 8–12). The original description of the D. primorjensis is accompanied by two detailed SEM images of the radula (see Martynov et al. 2015a: fig. 5). On these images up to 14 denticles of the central radular tooth can be recognized, whereas according to Ekimova et al. (2016), the central tooth of D. frondosus from the Sea of Japan bears a maximum of 12 denticles. Therefore, a larger number of denticles on the central teeth of D. primorjensis distinguishes it from the Japan Sea’s D. frondosus, but was not discussed by Ekimova et al (2016). No other species of Dendronotus with a similar radula have been so far reported from the Sea of Japan. The discussion of the origin of D. frondosus in the Sea of Japan in the Ekimova et al. (2016: 38–39) mainly concentrates on the idea that D. frondosus could be a natural amphiboreal species. However, the fact that the only Japan Sea D. frondosus specimens recorded in Ekimova et al. (2016) were found exclusively in the vicinity of the Institute of Marine Biology FEB RAS (Vladivostok) indicates a high probability of anthropogenic introduction of D. frondosus. On the contrary, D. primorjenisis (= D. dudkai syn. n.) has a broad distribution in the Sea of Japan.

The number of lobules present in the prostate of D. dudkai are described by Ekimova et al. (2016: 31) as being among the “most important diagnostic characters” for the genus Dendronotus. This is inaccurate, as Ekimova et al. (2016: 32) indicate 16–30 prostatic lobules for D. frondosus, whereas 12–14 lobules are described for D. dudkai(Ekimova et al. 2016: 37). However, the holotype of D. primorjensis possesses no less than 19 lobules in the prostate clearly implying that variability of numbers of prostatic lobules in the genus Dendronotus can be significant, and therefore cannot be species-diagnostic. The number of prostate lobules has recently been shown in Korshunova et al. (2016) to be an unreliable character in several Dendronotus species.

Unfortunately, the description of D. dudkai contains a number of problems that prevent the repetition of their results obtained by molecular phylogenetic analysis. Despite the presence of a molecular analysis that was based on “additional material” (i.e. non-type specimens) no molecular data is available for the type material. According to their Table 1 and “Type material” section (Ekimova et al. 2016:19–21, 33-35) five specimens were listed as type material for D. dudkai, but the molecular data for only one paratype (ZMMU Lc-40366) was registered at GenBank; however, this voucher number for this paratype of D. dudkai does not exist in GenBank: specimen W203 was registered at GenBank instead of specimen ZMMU Lc-40366, but specimen W203 is completely absent in the “Material” list or Table 1. In addition, no illustrations of the holotype of D. dudkai were provided nor were characters of the holotype mentioned in the description. Thus, the analysis of Ekimova et al. (2016) leads to the conclusion that there is no evidence that description of D. dudkai is based on the primary type, i.e. the holotype. According to the Recommendations 73A, 73B, and 73C of ICZN (1999) the “author who establishes a new nominal species-group taxon should designate its holotype in a way that will facilitate its subsequent recognition”, “an author should designate as holotype a specimen actually studied by him or her, not a specimen known to the author only from descriptions or illustrations in the literature”, and “an author who establishes a new nominal species-group taxon should publish at least the following data concerning the holotype, if they are relevant and known to the author, including: 73C.1. its size or the size of one or more relevant organs or parts.” Thus, the absence of the holotype’s morphological (and molecular data), and even its measurements in the original description of Ekimova et al. (2016) do not allow anyone to recognize it.

The morphological and molecular data provided in this publication and two previous studies (Martynov et al. 2015 a, b) conclusively demonstrate that D. primorjensis is a valid species as per current ICZN regulations (ICZN 1999). The integrative taxonomy (Dayrat 2005) presented here indicate that D. primorjensis and D. dudkai are one and the same species, and that D. primorjensis has taxonomic priority over D. dudkai published one year later. The species epithet D. dudkai is therefore to be considered a junior synonym of D. primorjensis.