Dendronotus frondosus

Dendronotus frondosus (Ascanius, 1774)

Fig. 7

Amphitrite frondosa Ascanius, 1774: 155–158, Pl. 5, Fig. 2 (neotype selected in Ekimova et al. 2015).

Dendronotusfrondosus (Ascanius, 1774) – Odhner, 1936: 1105–1109, Fig. 39 (mixture of several species); Robilliard, 1970: 441–446, Pl. 63, Fig. 29, textand Figs 4, 7, 8, 9 (partim,

mostly referred to D. venustus); Ekimova et al. 2015: 848–857, Figs 1–5, 6A, 7A, 8A.

Doris arborescens Müller, 1776:229 (mixture of several species).

Doris cervina Gmelin, 1791: 3105, no. 12.

Tritonia reynoldsii Couthouy, 1838: 74–80, Pl. 2, Figs 1, 2, 3, 4.

Tritonia felina Alder & Hancock, 1842: 33.

Tritonia pulchella Alder & Hancock, 1842: 33–34.

Tritoniaascanii MØller, 1842: 5.

Amphitridea fabricii Kröyer, 1847: 114.

Campaspe pusilla Bergh, 1863: 471–478, Pl. 12, Figs 28–35.

Campaspe major Bergh, 1886: 21–24; Pl. 1, Figs. 23–26; Pl. 2, Figs 1 –11.

Dendronotus luteolus Lafont, 1871: 267.

Dendronotus junior Mörch, 1875: 125.

Dendronotus arborescens var. aurantiaca Friele, 1879: 284.

NB: Mixture of several species under the name Dendronotus frondosus in the literature before 2015.

Extended diagnosis. Body narrow. Fiveto six pairs of branched dorsolateral appendages. Four to seven appendages of oral veil. Four to five appendages (middle and posterior ones can be longer) of rhinophoral stalks. Lateral papilla of rhinophoral sheaths present. Rhinophoreswith six to 12 lamellae. Lip papillae four to 12. Basal colour brownish to reddish-brown, often with small white and yellow specks, but usually without opaque white stripes between dorsolateral processes to completely white translucent specimens. Dorsal processes of jaws inclined posteriorly at approximately 60° to the longitudinal axis of the jaw body and 0.4 of its length. Masticatory processes apparently bear ridgelike structures and denticles. Radula with up to 42 rows of teeth. Central tooth with deep furrows and with up to 14 (common range 8–12) distinct denticles. Upto 10 (usually upto eight) lateral teeth with upto seven denticles. Ampulla voluminous, folded. Bursa copulatrix large, oval to rounded. Seminal receptaculum small placed distally at a moderately short distance from the vaginal opening. Prostate discoid with range 16–30 alveolar glands. The vas deferens is moderate in length, penis relatively long, curved. Common body length no more than 50 mm.

Distribution. North Atlantic, both east and west parts, not distributed to the Arctic further than the easternmost border of the Barents Sea.

Bathymetry. Intertidal to 20–30 m depth.

Remarks. Three common North Atlantic shallow water species, namely D. europaeus Korshunovaet al., 2017b, D. frondosus (Ascanius, 1774), and D. lacteus, show an extremely similar range of habitus variation (see Korshunova et al., 2017b) and have been confused in older literature. Therefore, the verified distribution of D. frondosus can only be assessed using recent data (Korshunova et al., 2017b) thatcoversatleastthe North East Atlantic from Norway and also the Barents Sea and the White Sea to possibly France/ northern Spain. The range of D. frondosus does not extend further than the the true Arctic. Also, several deep-water records of D. frondosus (e.g., Odhner, 1939; Swennen, 1961; Thompson & Brown, 1984) most likely refer to D. lacteus, since according to our data D. frondosus was usually found shallower than 20–30 m depth. We have examined an extensive collection of various Dendronotus from the NW Pacific and NE Pacific and have so far not found any specimen from the North Pacific for which the identity could be confirmed as D. frondosus despite the presence of a few published records (e.g., Ekimova et al., 2016). In this respect, the common occurrence of true D. frondosus in the North Atlantic and the extreme rarity of D. frondosus in the North Pacific, with the simultaneous common presence in the North Pacific of other species formerly considered as “ D. frondosus ”, (e.g., D. kalikal, D. kamchaticus, D. primorjensis, D. venustus), clearly suggests that true D. frondosus does not occur naturally in the North Pacific. Here, it is largely substituted by other species fromthe D. frondosus species complex. In particularly, D. primorjensis endemic to the Sea of Japan, is morphologically similar to D. frondosus and sister to it according to our molecular phylogenetic analysis (fig. 1). D. primorjensis has several traceable diagnostic features in radula and reproductive system (see Korshunova 2016a and Synopsis below). By such features and molecular data D. primorjensis could be excellent candidate for an example of a relict species, with close relatives with ranges extending to the Arctic, which afterwards formed a separate species, D. frondosus, in the North Atlantic. As a result of such an evolutionary history, D. primorjensis and D. frondosus are evidently naturally separated from from closely related North Pacific and North Atlantic species, because true D. frondosus does not occur in the Arctic. On the contrary, the potentially disrupted populations of D. frondosus in the North Atlantic and the North Pacific do not reveal morphological and molecular differences. Most reliable explanations for such an occurrence of few of so far known specimens of D. frondosus in the North Pacific can bean anthropogenic introduction or a mistake in the sorting or processing of collection material. Dendronotusfrondosus does not occur naturally in the North Pacific.

In this study we also evidently show that the North Pacific D. kalikal and the North Atlantic and Arctic D. yrjargul sp. nov. are invariably placed in two distinct clades (figs 1–3) with stable distinct differences in external and internal morphology (fig. 4) and without any gene flow between each other. Dendronotus kalikal thus does not occur naturally in the North Atlantic. Apotential future introduction of some North Pacific species from the “ D. frondosus megacomplex” may take place, but so far there are no reliable records of any North Pacific species in the North Atlantic. At least recently, a case of evident anthropogenic transportation of the dendronotid Pseudobornella orientalis was reported from Japan or China to NE Pacific (Agarwal et al., 2017) (for taxonomy of Pseudobornella see below). Thus, most previous records of this species from the Northwest Pacific refer to one of three recently described species, D. kamchaticus Ekimova et al., 2015, D. kalikal Ekimova et al., 2015, or D. primorjensis Martynov et al., 2015, and in the Northeast Pacific, previous records refer to D. venustus Stout et al., 2010 or D. kamchaticus Ekimova et al., 2015 (see also Korshunovaet al., 2016a, b).