Dendronotus yrjargul, Korshunova & Bakken & GrØtan & Johnson & Lundin & Martynov, 2021

Korshunova, Tatiana, Bakken, Torkild, GrØtan, Viktor V., Johnson, Kjetil B., Lundin, Kennet & Martynov, Alexander, 2021, A synoptic review of the family Dendronotidae (Mollusca: Nudibranchia): a multilevel organismal diversity approach, Contributions to Zoology 90 (1), pp. 93-153 : 100-104

Dendronotus yrjargul sp. nov.

Figs 1–4 , 6 , 7

ZooBank: http:// urn:lsid:zoobank.org:act:D44EDB5D-9C27-4D38-A18B-AD6DA5ACE43D

Etymology. From Norwegian yrjar (= old name for the type locality in Ørland) and gul (= yellow) meaning “yellow/golden of Ørland” in reference to the remarkable habitus of this species.

Type locality. Norwegian Sea, Norway, Ørland.

Material examined. Holotype. NTNU-VM- 76306 , live length 95 mm (48 mm preserved), Norwegian Sea, mouth of Trondheimsfjord, Ørland, Breivika, 12.03.2018, depth 20–25 m, collector Viktor V. GrØtan. Paratypes with same provenance data: NTNU-VM-76307, 1 specimen, live length 75 mm (40 mm preserved) ; NTNU-VM-76308, 1 specimen, live length 60 mm (40 mm preserved) ; paratypes with same provenance data but collected 08.01.2018, depth 20–25 m, collector Viktor V. GrØtan, Kjetil B. Johnson: NTNU-VM-76302, 1 specimen, preserved length 18 mm; NTNU-VM- 76305, 1 specimen, preserved length 17 mm; NTNU-VM-76303, 1 specimen, preserved length 14 mm; NTNU-VM-76304, 1 specimen, preserved length 19 mm; ZMMU Op-718, 1 specimen, preserved length 19 mm .

Description. Body elongate, living specimens up to 95 mm in length (fig. 4). Five to six main branched appendages on oral veil. Five appendages on rhinophoral stalks; 18–30 rhinophoral lamellae.Branchedlateralpapillaon rhinophoral sheath present. Seven to eight pairs of dorsolateral appendagespresentin largeadult specimens. Lip papillae 10 to 20. Dorsolateral

appendages with relatively short primary stalk, strongly branched secondary branches and elongate tertiary branches (fig. 4A–D). Basal colour is semitransclucent white to greyish, with characteristic two thin wavy subparallel brownish-orange to brownish lines running from the head nearly to the tail. There are no specimens with brownish basal colour. Upper part of the dorsolateral appendages covered with bright golden yellow to yellowish pigment. Foot bordered with thin brownish-orange line. Dorsal processes of jaws inclined posteriorly at approximately 40–48° to the longitudinal axis of the jaw body and 0.4 of its length (fig. 4E–I). Masticatory border of jaw with evident ridge-like denticles (fig. 4H, I). Radular formula: 38–50 × 0–9.1.9–0. Central (rachidian) tooth moderately denticulated and bearing up to 16 distinctdenticles(fig.4J–M).Denticleswithdeep furrows. Lateral teeth elongate, slightly curved, bearing up to 7 denticles (fig. 4J–N). Ampulla thickened kidney-shaped (fig. 6A). Prostate discoidal, consisting of up to 50 alveolar glands. Distal part of vas deferens moderate in length, transitioning to long, thick, curved copulatory organ. Bursa copulatrix large, rounded, with seminalreceptaculum placeddistally.

Distribution. From Norwegian Sea to Kara Sea.

Habitatandecologicalobservations. Dendronotus yrjargul sp. nov. is closely associated with the hydroid Abietinaria fusca (Johnston, 1847) (see Vervoort,1942)whichformsdarkbrowncolonies that resemble small dry fir branches (personal observationsof VGand KJ).The hydroid usually inhabits somewhat deeper water, but currents and other conditions in the Norwegian waters makeit reachable by scuba diving at circa 20 m. We have only rarely observed the new species on other hydroids. Dendronotus yrjargul sp. nov. appears quite abundant in this location, up to 10 individualsper m 2. Thenewspecies appears to have a one-year life cycle. Mating and egg laying occurs around March, andthe large adult individuals eventually disappear throughout the spring, as does the hydroid they live on. During the summer, small individuals begin to reappear, along with the new growing phase of A. fusca . The new generation of D. yrjargul sp. nov. grow throughout autumn andwinter.

Remarks. Externally, D. yrjargul sp. nov. readily differs from all known species of the genus Dendronotus (synopsis given below), including thesisterspeciesfromthe North Pacific, D.kalikal (description forcomparison isgiven below), by a combination of off-white semitransparent body with two thin dorsal subparallel brownish lines and dorso-lateral appendages golden-yellow to yellowish(fig.4A–D).Instead, D.kalikal invariably possesses a basal body colour covered with numerous brownish speckles and small spots, whichmakesubparalleldorsallinesconsiderably blurred and indistinct (fig. 4O). Importantly, we investigated more than ten D. kalikal specimens from very distantly placed localities in the Bering Strait, Kamchatka and Kurile Islands (fig. 3A) and not one specimen possessed a mostly whitish semitransparent body with distinct thin subparallel dorsal lines, as in D. yrjargul sp. nov. (fig. 4A–D). The whitish colouration in D.kalikal consists of a background colour represented by areas in between the numerous basal brownish spots and stripes. Furthermore, considerable differences in the reproductive system and the maturation process between D. yrjargul sp. nov. and D. kalikal were revealed in course of the

presentstudy. Allstudiedspecimensof D.kalikal have an elongated bursa copulatrix (fig. 6B), whereas in all studied specimens of D. yrjargul sp. nov. (fig. 6A) thebursa copulatrix is rounded.

Furthermore, we discovered that there is considerable difference in the patterns of maturation (and hence, ontogenetic patterns) between D. yrjargul sp. nov. and D. kalikal . Dendronotus yrjargul sp. nov. reaches a big size of 95 mm (live) in the adult state, and in specimens of 20–30 mm (live) the reproductive system was still very weakly developed. Instead, D. kalikal has a maximum length of 17 mm when preserved and already at lengths of 7–10 mm live specimens show a well-developed reproductive system. The molecular differences between D. yrjargul sp. nov. and D. kalikal areas follows. Regarding the COI marker, the maximal intragroup distance within D.kalikal is 1.74% andwithin D.yrjargul sp. nov. it is 0.85% (mean intragroupdistances are 0.80% and 0.39%). The mean intergroup distance between D. kalikal and D. yrjargul sp. nov. groups is 2.1% (Appendix, table A2 ). Thus, intragroup distances within D. yirargul sp. nov. are smaller than the distance between D. yirargul sp. nov. and D. kalikal . Thus, D. yrjargul sp. nov. and D. kalikal demonstrate considerable differences in morphological, molecular and, to a very large degree, ontogenetic levels. Dendronotus yrjargul sp. nov. thus represents a well-separated species, although possibly recently diverged, as indicated by the slight molecular divergence.

Figures

FIGURE 1 Phylogenetic relationships of the species of the genus Dendronotus based on COI + 16S + 28S + H3 concatenated dataset inferred by Bayesian Inference (BI). Numbers represent posterior probabilities from BI and bootstrap values for Maximum Likelihood.Some branches are collapsed at species level.Dendronotus elegans from the type region (North Atlantic) are highlighted in dark purple. Abbreviations:h – holotype; p – paratype.Summary of species delimitation results from ABGD analyses for the COI dataset is shown

FIGURE 2 Phylogenetic relationships of the species of the genera Dendronotus, Pseudobornella and Cabangus in single COI, 16S, 28S, and H3 gene trees inferred by Bayesian Inference (BI). Some branches are collapsed. Positions the Pseudobornella orientalis highlighted green; positions the Cabangus regius highlighted orange.Numbers represent posterior probabilities from BI. PHOTOGRAPHS BY A. MARTYNOV AND T. KORSHUNOVA

FIGURE 3 A.Map of the Northern Hemisphere showing the distribution of D.elegans (including “D.niveus”), D.kalikal, and D.yrjargul sp. nov.The type locality of D.elegans is indicated by crossed lines. B. The haplotype networks based on cytochrome c oxidase subunit I (COI), 16S rRNA (16S), and 28S rRNA (28S) molecular data showing genetic mutations occurring within D.elegans (“D.niveus”), D. kalikal, and D.yrjargul sp. nov. specimens.Abbreviations:BeSt – Bering Strait; BS – Barents Sea; Gr – Greenland; Km – Kamchatka; KS – Kara Sea; Ku – Kuril Islands; LS – Laptev Sea; Nf – Newfoundland; NS – Norwegian Sea; WS – White Sea.Type locality of D.elegans is marked on the map by “x”.

FIGURE 4 Dendronotus yrjargul sp.nov.(a–n) and D.kalikal (o–t), external and internal morphology.a–c: living holotype of D.yrjargul NTNU-VM-76306 (a) and its habitat (b, c), dorsal,ventral and lateral views with egg mass and associated hydroid Abietinaria fusca, respectively; d: Dendronotus yrjargul, habitat of living paratype NTNU-VM-76302;e: jaw of holotype; f:denticles on the masticatory edge of the holotype; g: jaws of paratype NTNU-VM-76302; h:discoid ridge-like structures on the masticatory edge of paratype NTNU-VM-76302;i: details of discoid structures on the masticatory edge of paratype NTNU-VM-76302; j: anterior part of radula of holotype; k:central teeth of radula of holotype;l:lateral teeth of radula of holotype; m:central teeth of radula of paratype NTNU-VM-76302; n: lateral teeth of radula of holotype; o: living holotype of D.kalikal; p: jaw of D.kalikal, specimen from Kurile Islands;

FIGURE 6 Reproductive systems. a, Dendronotus yrjargul sp. nov.; b, D.kalikal; c, D.elegans; d, D. nordenskioeldi sp. nov. Scale bars:a–d 1mm.Abbreviations:am, ampulla; bc, bursa copulatrix; fgm, female gland mass; fo, female opening; p, penis; pr, prostate; rs, receptaculum seminis; ud, uterine duct; vd, vas deferens; vg, vaginal duct.

FIGURE 7 Representaives of the genus Dendronotus (living specimens). PHOTOGRAPHS BY T. KORSHUNOVA, A. MARTYNOV, K. FLETCHER, D. MILLER, Y. FUJIWARA, K. HASEGAWA, K. SANAMYAN, N. SANAMYAN, AND O. ZIMINA