Genetically, Krupenko et al. [30] split D. varicus species complex into 4 groups: DV1–DV4. Bouguerche et al. [3] strongly showed that the DV1 clade is the “real” D. varicus and proposed to recognise it as D. varicus s. s. through the analysis of the 28S sequences of D. varicus from the type-host, the Atlantic salmon S. salar. Krupenko et al. [30] did not find any adult stages of DV 2 in fish and hence could not provide further morphological comparison of DV1 and DV2. Additionally, the only available sequences of D. varicus s. s. from the type-host had been made available only recently [3]. Herein, the sequences of D. abba n. sp. from Arctic H. platessoides were identical to those of D. varicus s. l. from the same host in the North Sea [50] and those of D. varicus DV 2 as rediae from gastropods in the White and Barents seas [30]. They were also identical to D. varicus of Olson et al. [50] (AY222189) ex H. platessoide s from the North Sea. On the other hand, they differed from the sequence of D. varicus s. s. from Norwegian S. salar by 3% as p -distance. This level of divergence is comparable to that observed between the well-established Mediterranean species D. ruber ex C. lastoviza from the Western Mediterranean, off Algeria (3% as well). Thus, the genetic divergence supports that D. abba n. sp. is a separate species from D. varicus s. s. This species uses a different first intermediate host, and produces cercariae that can be morphologically distinguished from those of D. varicus s. s. [30].
Krupenko et al. [30] speculated that D. varicus DV 2 is the species that Shulman and Shulman-Albova (1953) called D. crassus Manter, 1934. Type localities for D. crassus sensu Manter (1934) and of D. abba n. sp. are so distinct (Tortugas, Florida, western Atlantic for D. crassus [42] vs. Svalbard, Arctic Norway, for D. abba n. sp.) that we consider this unlikely. However, it remains possible that D. crassus sensu Shulman and Shulman-Albova (1953) represents D. abba n. sp.
The lineage D. varicus DV 4 corresponds to an 18S rDNA sequence (AJ287511) of D. varicus of Littlewood and Olson [37] from the same host as D. abba n. sp., H. platessoides from the North Sea. DV4 was labeled only based on the genetic divergence of 0.13% (p -distance) from D. abba n. sp. (DV2) in the 18S rDNA sequence [30]. In light of the available data, it is premature to consider D. varicus DV 4 as a distinct species from D. abba n. p. considering that our study does not include the 18S rDNA analysis. Hence, we cannot estimate the taxonomical position of DV4 further. Meanwhile, the possibility that D. varicus (s.s.) also occurs in H. platessoides is not ruled out.
Overall, the evolution of understanding of this genus is such that while most combinations of species can be distinguished on the basis of morphology, some are presently morphologically cryptic with respect to each other. On this basis, it remains critical that further study is based on both morphological and molecular data. A key area of uncertainty relates to patterns of host-specificity. Some species, e.g. D. varicus s.s., appear to be genuinely euryxenic, whereas several others apparently have highly restricted host ranges. The extent to which this is a genuine reflection of the true nature of these species is uncertain. Certainly, for clearly closely related species, it is not obvious why host-specificity patterns should differ so dramatically.