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 thatD.varicusrepresents a species complex and based on molecular evidence, four genetic lineages (labeled as “DV1–4”) have been designated within theD. varicusspecies 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 ofD. varicusin the stomach ofMerlangius merlangusoff the coast of Sweden, and inGadus morhuaoff the coast of Sweden and Norway; we compared them toD. varicusfrom the type-host, the Atlantic salmonSalmo salarfrom Norway, to verify their conspecificity. Newly generated sequences (28S rDNA, ITS2 andcox1) of Scandinavian and Arctic specimens consistent withD. varicusall formed a single clade, DV1. 28S sequences ofD. varicusfromS. salarfrom Norway, i.e., close to the Danish type locality, clustered within the DV1 clade along with sequences ofD. varicusfrom various hosts includingLimanda limanda,G. morhuaandMyoxocephalus scorpiusfrom the White Sea and the Barents Sea (Russia), without any host-related structuring. We thus consider that the lineage DV1 representsD. varicus sensu stricto. Additionally, specimens fromM. merlangushad a similar morphology and anatomy to those ofD. varicusfromL. limanda,G. morhuaandM. scorpiusfrom T. Odhner’s collection, supporting the presence of a single species in the DV1 lineage designated herein asD. varicus sensu stricto. We redescribeD. varicus sensu stricto, add new morphological characters and provide morphometric data. We infer thatD. varicustypes DV2–4 all relate to separate species. We also revise type-specimens ofDerogenes minorLooss, 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 ruberLühe, 1900. In light of their morphological distinctiveness relative toD. varicus sensu stricto,we reinstateD. parvusSzidat, 1950 andD. fuhrmanniMola, 1912.