Species differentiation, bathymetric distribution and biogeographic range
As shown above, the molecular analysis confirms that D. robustus and D. velifer are separate species, and that they are clearly differentiated by morphology. The bathymetric analysis also shows a clear separation in depth preference. The historical specimens from Gullmar Fjord have a morphology that is consistent with the recently collected Arctic D. velifer specimens and they show a clear preference to the greater depths around 100 m. Thus, these specimens can be confidently identified as D. velifer. A further finding relevant for this case, is the recent report of a single specimen identified as D. “robustus” collected from around 200 m depth off the North Atlantic coast of North America (Valdés et al., 2017) that fully agrees with D. velifer according to our molecular analysis. Therefore, it is possible that D. velifer might occur in more southward locations like the Gullmar Fjord if temperature and other ecological conditions permit. The isolated population of D. velifer in the Gullmar Fjord could therefore possibly have been a relict population from an earlier glacial period with colder water. At that time the species could have had a more southerly distribution.
The maximum depth of the Gullmar Fjord is about 118 m and many of the D. velifer specimens were taken from the deepest section of the fjord. The deeper parts of the fjord (90–118 m), inside the sill, have fully saline water and naturally stable conditions (Svansson, 1975; Filipsson et al., 2005). In the present study it is statistically proven (Figure 4c) that D. velifer differs significantly from D. robustus by its preference for greater depths and hence for lower seawater temperatures. Therefore, it can be supposed that small, statistically insignificant differences in the depth preferences between the Arctic specimens of D. velifer (Figure 4c) and the specimens from the Gullmar Fjord (Figure 4c) are due only to the depth limit of the Fjord. If the Gullmar Fjord were deeper, D. velifer would extend to depths of at least 200 m, and beyond. Two historical findings of single specimens in the fjords of Norway – the Oslo Fjord and the Hardanger Fjord – provide a link from the Gullmar Fjord in Sweden to the northwestern Norwegian coast, and further to the main range of D. velifer through almost all of the Arctic from the Barents to the Laptev Sea and further, to the Bering Strait.