An accurate taxonomy is indispensable for evaluating the conservation status and biogeographic pattern of species (Bickford et al., 2006; Martynov et al., 2017). Modern molecular analyses are able to reveal considerable inconsistencies within morphology-based systematics and thus may have a direct influence on the conservation status of a species and on the perception of its distributional range. A potentially threatened species cannot be properly understood, or even recognised, without a thorough taxonomic background study. While for large well-known vertebrate animals a taxonomic placement is usually well established today, this is not the same for numerous invertebrates, especially marine ones. An incorrect taxonomy may affect the estimation of the actual number of species in a protected area, and thus may cause an underestimation of the conservation needs of that area.
Here we present a case from an outstanding group of marine invertebrates, the nudibranch molluscs (Gastropoda: Opisthobranchia), and discuss the importance of a correct taxonomic basis for the conservation perspectives and evaluation of biogeographic patterns within Scandinavian fjords.
Nudibranch molluscs are important model subjects for various disciplines, including phylogenetics, neurophysiology and also the emerging field of marine natural products (Nuzzo et al., 2012; Goodheart et al., 2015; Katz, 2016). They are now routinely included in well-illustrated field guides (Picton and Morrow, 1994; Malmberg and Lundin, 2015; Hayward and Ryland, 2017) and owing to their attractiveness they have recreational importance in national parks and other conservation areas (e.g., Bertsch, 2014; Garcia-Mendéz and Camacho-Garcia, 2016; Mehrotra and Scott, 2016; Nimbs et al., 2016). The species of the nudibranch genus Dendronotus occur in the Northern European and Arctic seas, generally on rocky and stony habitats in shallow waters less than 50 m deep (Thompson and Brown, 1984; Thollesson, 1998; Korshunova et al., 2016). Most have relatively slender bodies and they feed on hydroids. One of the most notable exceptions is a species that inhabits soft bottoms at depths greater than 50 m, which has a relatively broad body and an omnivorous diet (Roginskaya, 1987). This species has generally been called Dendronotus robustus Verrill, 1870, and has been assessed as having a very broad circumpolar distribution (Ekimova et al., 2015). In the northeast Atlantic Ocean, it occurs from the Vega archipelago in Hordaland on the northwestern Norwegian coast (Evertsen and Bakken, 2005), northwards to the Barents Sea in the Arctic Ocean, and across to the Kara Sea and the White Sea in Russia (Martynov and Korshunova, 2011). On the northwestern side of the Atlantic Ocean it occurs from Cape Cod to Greenland, and to the Bering Strait (Verrill, 1870; G.O. Sars, 1878; Odhner, 1907, 1939; Ekimova et al., 2015). On Arctic latitudes, as in the Barents Sea, it occurs in relatively shallow depths, but in the southernmost parts of its range it occurs at greater depths.
A species that is similar to D. robustus apparently had an isolated population in the Gullmar Fjord on the Swedish west coast at least until the early 1940s. This southernmost occurrence has not been appropriately described in the literature. There are no other reports or findings from any other areas in Swedish waters or the British Isles, where the nudibranch fauna is now well known (e.g., Thompson and Brown, 1984; Picton and Morrow, 1994; Malmberg and Lundin, 2015). The nearest present occurrence is on the northwestern Norwegian coast, some 1500 km from of the Gullmar Fjord, except for two historical findings - one specimen from the innermost part of the Oslo Fjord in 1912, and one specimen from the Hardanger Fjord, south of Bergen, collected sometime between 1860 and 1920. Both of these specimens are deposited at the Oslo Museum of Natural History. Dendronotus robustus was listed for Denmark (Jensen and Knudsen, 1995), but without a description that could confirm the identification. The isolated population in the Gullmar Fjord was apparently rather abundant if one considers how many specimens collected in the late 1800s were deposited at the Gothenburg Natural History Museum (GNM) and the Swedish Natural History Museum (NRM).
To re-evaluate the taxonomic status of the population in the Gullmar Fjord, we use an integrated analysis applying morphological and molecular methods for both very shallow water populations and the deep-water specimens currently known as “D. robustus” from across almost its entire geographic range. We compare the bathymetric distribution of the historical specimens from Gullmar Fjord with both shallow-water and deep-water populations using a statistical test. The reliability of an integrated molecular and morphological approach has recently been proved by the discovery in Norway and Great Britain of a large new nudibranch species within one of the world’s best studied European marine faunas (Korshunova et al., 2017). The resultant conclusions are highly relevant from both biogeographic and conservation perspectives, and important for the ongoing discussion regarding the conservation of the Gullmar Fjord.
In the present study we show that there are two distinct species previously mixed up under the name “D. robustus” (e.g. Ekimova et al., 2015). These species are D. robustus Verrill, 1870 and D. velifer G.O. Sars, 1878. According to our present analysis the Gullmar Fjord accommodates the species D. velifer G.O. Sars, 1878, and not the true D. robustus. The synonymy of the latter species, plus the designation of a lectotype, and a revised morphological description, are given in the Appendix to this paper.