Hidden in the fog: morphological and molecular characterisation ofDerogenes varicus sensu stricto(Trematoda, Derogenidae) from Sweden and Norway, and redescription of two poorly knownDerogenesspecies

Abstract

Derogenes varicus (Müller, 1784) is widely reported as a trematode with exceptionally low host specificity and a wide, bipolar distribution. However, several recent studies have suggested that D. varicus represents a species complex and based on molecular evidence, four genetic lineages (labeled as “DV1–4”) have been designated within the D. varicus species complex. This possibility requires improved (ideally molecular) characterisation of specimens from the type-host (Salmo salar) and type-locality (off Denmark). During examination of trematode parasites of fish from Scandinavian and Arctic waters (Sweden and Norway), we found specimens of D. varicus in the stomach of Merlangius merlangus off the coast of Sweden, and in Gadus morhua off the coast of Sweden and Norway; we compared them to D. varicus from the type-host, the Atlantic salmon Salmo salar from Norway, to verify their conspecificity. Newly generated sequences (28S rDNA, ITS2 and cox1) of Scandinavian and Arctic specimens consistent with D. varicus all formed a single clade, DV1. 28S sequences of D. varicus from S. salar from Norway, i.e., close to the Danish type locality, clustered within the DV1 clade along with sequences of D. varicus from various hosts including Limanda limanda, G. morhua and Myoxocephalus scorpius from the White Sea and the Barents Sea (Russia), without any host-related structuring. We thus consider that the lineage DV1 represents D. varicus sensu stricto. Additionally, specimens from M. merlangus had a similar morphology and anatomy to those of D. varicus from L. limanda, G. morhua and M. scorpius from T. Odhner’s collection, supporting the presence of a single species in the DV1 lineage designated herein as D. varicus sensu stricto. We redescribe D. varicus sensu stricto, add new morphological characters and provide morphometric data. We infer that D. varicus types DV2–4 all relate to separate species. We also revise type-specimens of Derogenes minor Looss, 1901 from the A. Looss collection in the Swedish Museum of Natural History and provide redescriptions of it and of the type-species of the genus, Derogenes ruber Lühe, 1900. In light of their morphological distinctiveness relative to D. varicus sensu stricto, we reinstate D. parvus Szidat, 1950 and D. fuhrmanni Mola, 1912.

Untangling the Derogenes varicus species complex in Scandinavian waters and the Arctic: description of Derogenes abba n. sp. (Trematoda, Derogenidae) from Hippoglossoides platessoides and new host records for D. varicus (Müller, 1784) sensu stricto

“…[ 30 ] from the Barents Sea and White Sea; and those of Bouguerche et al . [ 3 ] from Northeast Atlantic, off Sweden and Norway and from Svalbard, Norway, Arctic Ocean.…”

“…[ 30 ] and by Bouguerche et al . [ 3 ]. Alignments for each gene region were constructed separately in AliView [ 32 ], then trimmed to the shortest sequence.…”

“… Hippoglossoides platessoides Svalbard, AO PP314018 PP314020 PP384389 Present study Derogenes abba n. sp. Hippoglossoides platessoides Svalbard, AO PP314019 PP314022 PP384390 Present study D. varicus s.s. Gadus morhua Sweden, NEA OQ916447 OQ916446 OR140896 [ 3 ] D. varicus s.s. Gadus morhua Norway, AO OQ916454 OQ916451 OR140779 [ 3 ] D. varicus s.s. Gadus morhua Norway, AO OQ916441 OQ916449 …”

“…Molecular techniques are indispensable for identifying cryptic species, as demonstrated in various trematode families [ 12 ]. The records of the derogenid Derogenes varicus (Müller, 1784), from 69 fish species across 33 families worldwide [ 3 ], displaying an exceptional euryxeny, hint that cryptic species may be present, and it has frequently been suggested that this species represents a species complex [ 19 , 28 ]. Krupenko et al .…”

“…By analysing 28S sequences of D. varicus from the type-host, the Atlantic salmon Salmo salar from Norway, Bouguerche et al . [ 3 ] demonstrated that the DV1 clade is D. varicus sensu stricto ( s. s .).…”