Dibothriocephalosis in salmonids from Iceland: A more complex taxonomic problem than assumed until now?

The diphyllobothriid tapeworm <i>Dibothriocephalus ditremus</i>, one of the three <i>Dibothriocephalus</i> species native to Europe, parasitises exclusively piscivorous birds and has not yet been detected in mammals. This is probably the reason why there is much less molecular data on this tapeworm. The aim of our study was to determine the genetic structure of the <i>D. ditremus</i> populations from Europe, namely Iceland, for the first time. To exclude any possible misidentifications between sympatrically occurring <i>D. ditremus</i> and <i>D. dendriticus</i>, <i>D. dendriticus</i> from Iceland was also analysed. Great genetic diversity of <i>D. ditremus</i>, displayed by a large number of cytochrome <i>c</i> oxidase subunit 1 (<i>cox</i>1) haplotypes and three distant clusters, contrasted sharply with the lower genetic variation in <i>D. dendriticus.</i> Previously published <i>cox</i>1 sequences of <i>D. ditremus</i> from different localities in Europe (UK - Scotland), Asia (Russia and Japan) and North America (USA - Oregon) were also included in the analysis in order to determine the genetic architecture of <i>D. ditremus</i> at a broader geographical scale. While the sequences of tapeworms from Scotland and Russia were placed in <i>D. ditremus</i> Clusters 2 and 3, the sample from USA (Oregon) displayed a unique position distant from the Icelandic tapeworms. Japanese samples of <i>D. ditremus</i> and <i>Diphyllobothrium hottai</i> formed a common clade, indicating their conspecificity. The unexpected output of the analysis was a unique position of the currently detected Haplotype 31 of a tapeworm from Iceland, which was distant from all other <i>D. ditremus</i> individuals from Iceland, but showed close relationships with the Japanese <i>D. ditremus</i>/<i>D. hottai</i> cluster. Further studies are needed to reveal if <i>D. ditremus</i> represent a complex of genetically diversified populations, or more species occur in salmonids in the Northern Hemisphere.