Dendronotus Alder & Hancock 1845

Genus Dendronotus Alder & Hancock, 1845

Type species. Dendronotusfrondosus (Ascanius, 1774)

Diagnosis.Dorsolateralappendageswithdistinct tertiary branches. Oral veil with branched appendages. Jaws commonly with set of tightly

packed ridge-like semi-discoid structures and also with denticles. Radula with at least more than eight lateral teeth in adult specimens. Central teethwith cusp integrated withinlateral denticles (or within teeth body if denticles absent), the central cusp is not protruding. Denticle-bear part of central teeth separated from lateral sides by usually distinct shoulder, thus central teeth with squarish base and triangular top.Prostatewith evident, conspicous alveolar glands, commonly represent by a discoid structure (varies from a flattened disk to a compressed spherical structure), or very rarely non-discoid, linear (in this case alveolar glands also present). Copulative organ basically conical, with various ratios of length and width anddegrees of apical tapering.

Natural history. Representaives of the genus Dendronotus inhabit a broad bathymetric range, from intertidal to depths over 3000 m, and a variety of substrates (including stony and soft benthic environments). Amajority of the Dendronotus species feed on hydroids (Robilliard, 1970; Thompson & Brown, 1984; Martynov & Korshunova, 2011). There as a few notable exceptions, such as D. iris Cooper, 1863 fromthenortheasternPacific,whichisanactive predator of ceriantharian cnidarians (Wobber, 1970; Shaw, 1991), whereas for the wide-bodied species D. robustus Verrill, 1870, D. velifer G.O. Sars, 1878 and D. bathyvela Martynov et al., 2020a less specific prey spectrum (including polychaetes and hydroids) has been recorded (Roginskaya, 1990, 1997).

Remarks. The genus Dendronotus is restricted here since some species have been moved to other genera. See discussions and remarks under the genera Cabangus gen. nov. and Pseudobornella.

Species composition. Diagnoses and remarksfor 27 included species are listed below. For each species, an extended diagnosis is provided when possible. The fine-scale diagnostics of the genus Dendronotus is a complex task since variation patterns of habitus and anatomical characters overlap in various species. By a careful listing it is possible to step-by-step fulfill the task and present a mutilelevel organismal diversity, though not in a strictly defined form, as was targeted previously (e.g., Odhner, 1936; Robilliard, 1970), but in a complex form that is best suited to the evolutionary mosaic in character evolution. There are also still uncertanties regarding some particular characters as well as previous overinterpretations. First of all, previously the pattern of branching in the dorsolateral appendages was claimed to be of considerable taxonomic importance (Ekimova et al., 2015: Fig. 8). In this respect, it is necessary to state, that while some trends exist in a few species, importance of these characters were considerably overestimated partly due to the use of preserved specimens in the analysis. This can be illustrated by the D. kamchaticus case, when according to Ekimova et al. (2015: Fig. 8D), D. kamchaticus has bulbous short branches that differ considerably from those in any other Dendronotus species. This resulted from the use of preserved specimens with considerably contracted branches, whereas the living adult specimens of D. kamchaticus have elongated branches of dorsolateral appendages, which are not fundamentally different from the type species of the genus, D. frondosus (Korshunova et al., 2016a). The description of D. zakuro (similar to D. kamchaticus by radula morphology) is based on recently collected specimens with both shorter “bulbous” (see Martynov et al., 2020a: Fig. 3A) and elongate dorsolateral appendages (Martynov et al., 2020a: Fig.3D, E), whichisdue to differences in physiological conditions. The preserved specimen of D.zakuro (see Martynov et al., 2020a: Fig. 3C) demonstrates very similar “bulbous” branches, which were incorrectly depicted as specific for D. kamchaticus in its original description (Ekimova et al., 2015: Fig. 8D) (see details in Synopsis under D. kamchaticus). Dendronotus lacteus apparently has shorter tertiary branches, and a “curly” appearance of the dorsolateral appendages. However, variation in D. lacteus does not only concern ontogenetic stages, but also occurs among different adult specimens. So, some large specimens of D. lacteus have quite long elongate branches of dorsolateral appendages, similar to those in D. frondosus (Korshunova et al., 2017a). Most species of the genus Dendronotus possess considerably branched dorsolateralappendages with distinct tertiary branches (MacFarland, 1966; Robilliard, 1970; Korshunovaet al., 2016a, b, 2017a, 2019b; Martynov et al., 2020a). Unbranched dorsolateralappendageswererecentlyreported only for D. claugei from the deep-sea (Valdes et al., 2018), but due to the most basal position of this species (fig. 1) the generic position of D. claugei needs to be evaluated with more data. For another deep-sea species, D. comteti, the unbranched dorsolateral appendages were also indicated (Valdes et al., 2018: 413), however, according to the original description of D. comteti, it possesses both branched and unbrancheddorsolateralappendages (Valdes & Bouchet, 1998: 320). Unbranched dorsolateral appendagescanoccurinposteriorpartsofsome other Dendronotus species (e.g., in D.robilliardi, see Korshunova et al., 2016a). For some species, e.g., for D. dalli in Ekimova et al. (2015: 862), the indication of absent tertiary branches of dorsolateral appendages is incorrect. Dendronotusdalli definitely has tertiary branches (Robilliard, 1970: 453; presentstudy, fig. 7). Instead, in the genus Pseudobornella the generalappearanceof dorsolateralappendages does not show as branches, but it is ctenidiumlike, because of very indistinct (or truly absent) tertiary branches (fig. 2). More details about the genus Pseudobornella are mentioned after the diagnoses of the species of the genus Dendronotus.

There are also various uncertainties and misinterpretations regarding internal characters that need to be addressed before diagnoses can be provided. For example, Robilliard (1970) suggested to apply several coefficients of jaw proportions. However, it is easy to incorrectlyapply respective angleof measurements and receive very different results, as for example in Ekimova et al. (2015) compared to the same species which jaws by Robilliard (1970). As an example, for D. dalli in Ekimova et al. (2015: 863) an angle of dorsal processes to the longitudinal axis of 25° was indicated, whereas in Robilliard (1970: 454) this was 60°. In the present study we re-checked jaw proportions andpresented updated values for the majority of species with sufficient available information. This character still needs further testing, but some characteristic patterns for different species are likely to occur. Similarly, the number of the prostatic alveoles once was thought to be a character that well characterizes different species (Robilliard, 1970; Ekimova et al., 2016). However, our previous (Korshunova et al. 2016a, b) and presentstudy show that this character is much more variable, and while there are clear trends in some species, it should generally be used with care. In the species diagnoses below, differences in shape, length and proportions of the copulative organ (penis) are indicated. While in some species the differences in penial morphology are clear, in other species this can be partly due to different degrees of contraction after preservation. This character needs further investigation.

In the presentstudy we show that the shape andsize of the bursa copulatrix, atleastamong several Dendronotus species (e.g., between D. kalikal and D. yrjargul), are rather constant and can be used as a reliable diagnostic character. However, the exact position of the commonly very small receptaculum seminis can be very difficult to determine, but despite that, it will be given further attention in the diagnoses below.

Different terms were applied for the structures on the masticatory edge within different species of the genus Dendronotus. For example, MacFarland (1966) used “denticles” or “ridge-like” denticles and Robilliard (1970) applied “denticles”, whereas Ekimova et al. (2015) used “rodlets”. In this respect, the term “rodlets” was used for rod-like structures that constitute the body of jaws in other nudibranch groups. In the genus Dendronotus, the elements on the masticatory edges are usually termed as “denticles” when the bodies of jaws are entire. However, in the genus Dendronotus there is a special set of structures in addition to the denticles that appears as broad semidisks or ridges, which can additionally be covered with smaller spine- or denticle-like structures. It is likely that most species of the genus Dendronotus possess such structures. In addition to the semi-disks/ridges at the masticatory edges, more normallylooking denticles can occur. Transitive denticles on masticatory edges werealsooccasionally reported forjuveniles of D. frondosus s.l. (Colgan, 1914; Wägele & Willan, 2000). Therefore, in the present study we use two different terms “ridge-like structures” and “denticles” when characterizing the masticatory edges of Dendronotus species. The few denticles seen in many cases are likely a precursor that further transits into semi-disk structures on the masicatory edges (see e.g., this clearly for D. kalikal, fig. 4Q). Because thefull row of denticles and/or ridges is not documented for all species due to difficulties during preparation, the exact number for these structures can be uncertain. Though whenever possible, we already presented the number of denticles/ridges, for example the recently described Dendronotus species (e.g., Martynov et al., 2020a), in the diagnoses below we do not provide the number because it can be misleading, because only a part of the row was counted in various species. True rodlets can occur in addition, as part of the thin labial cuticle, which is separated from the masticatory edges. Such true rodlets have been well documented for the tropical representatives of the family Dendronotidae (Pola & Stout, 2008), and possibly also occur in many Dendronotus species, since the labial cuticle with rodletswere already described for several representatives (see e.g., MacFarland, 1966). However, these characters remain largely untested by modern methods and we do not include it in the diagnoses below. As an additional complication, denticles and especially ridge-like structures on the masticatory edges can be easily discarded during preparation, and therefore in some previous descriptions it was inappropriately indicated that masticatory edges are devoid of any denticles or ridges. For example, the absence of the denticles on the masticatory edges of D. frondosus were incorrectly mentioned in Ekimova et al. (2015), which was further used as a putative difference between D. frondosus and D. primorjensis (Ekimova et al., 2016). However, in both species denticles on the masticatory edges are definitely present (Korshunova et al., 2016a). The same is true also for other species, such as D. kamchaticus and D. kalikal. For D. kamchaticus the absence of any structures on the mascticatory edges was indicated in the original description. For D. kalikal the presence of merely “rodlets” was reported in the original description (Ekimova et al., 2015). However in reality denticles and ridge-like structures are present in both of these species (Korshunova et al., 2016a, 2019b; for D. kalikal see fig. 4Q). Therefore, when the original descriptions denticles or ridge-like structures was not indicated for species that we could not check, we suppose the presence of at least one of these structures and indicate that in the diagnoses.

Though a pair of salivary glands indeed inserts into the pharynx on the sides of the oesophagus, the knowledge of these is limited only to a few species (e.g., MacFarland, 1966; Valdes & Bouchet, 1998; Pola & Stout, 2008). The salivary glands are usually not figured in the original descriptions (an exception: Valdes and Bouchet, 1998) and were never consistently used for taxonomic diagnostics. We therefore do not include salivary glands in the diagnoses of the species of the genus Dendronotus.

The radular patterns within the genus Dendronotus still offer one of the best diagnostic characters (Korshunova et al., 2017a). However in several species groups quite similar patterns of the central tooth can be observed. These similarities occur between groups/clades of closely related species (for example between species that closely related to D. frondosus, e.g., D. primorjensis and D. venustus, between D. lacteus and D. rufus, between D. dalli and D. elegans), but also among apparently more distantly related species suchas D. kamchaticus and D. zakuro, and between D. albus and D. robilliardi (see fig. 1). Despite these similarities, in a majority of these cases fine-scale differences in morphology of radula can be detected. At the same time, different species which are grouped in the same subclade (fig. 1) can demonstrate considerably different radular patterns, as for example the clade consisting of D. jamsteci, D. subramosus and D. albus (Robilliard, 1970; Pola & Stout, 2008; Korshunova et al., 2016a; Martynov et al., 2020a). Furthermore, the radulae of the morphologically very different D. robilliardi and D. gracilis demonstrate similar patterns in the presence of a widened central tooth with numerous small lateral denticles without furrows (Baba, 1949; Korshunova et al. 2016a, 2019b). The radula undergoes remarkable transformation during the development from early juvenile to adult. The central tooth in earlier juveniles is low and bears fewdistinct lateral denticles. The lateral teeth in early juveniles are very few in number (1–3) or absent, showing an indistinct denticulation or they are almost smooth. In individuals approaching the adults stage, the central teeth become elongate-triangular with distinct shoulders, lateral denticles become relatively smaller and numerous or completely reduced. Meanwhile, the lateral teeth increase in number and the denticles become more conspicuous. Since parts of juvenile radula may persist in some adults, this may create a considerable confusion regarding the taxonomic importance of radular characters. For instance, D. dalli usually has a smooth central teeth without denticles in adult stage, whereas for adults of D. dalli also denticles on central teeth were reported, which led to the incorrect synonymyzation of D. dalli with D. frondosus (Odhner, 1936; MacFarland, 1966). This decision was later changed in favour of the validity of D. dalli, but uncertanties over the taxonomic application of the putatively variable radula remained (Robilliard, 1970; Roginskaya, 1987). Such “variations” can be overcome by a consistent application of general ontogenetic principles in taxonomy, using the attribution of a particular set of characters to a particular ontogenetic stage (Martynov et al., 2015b, 2020b). Thus, the application of a multilevel diagnostic within the genus Dendronotus (one that combines morphological characters of various levels and molecular data) instead of a search for immediate and clearly distinctive differences, requires a fine-scale, complex approach.