Two blue mussel species Mytilus trossulus and M. edulis co-exist in Northern Europe, in particular in Northern Russia and along the Norwegian coast (in the latter area the minor presence of the third pecies M. galloprovincialis was recently reported). These species ultimately are separate genetic orms that have distinct evolutionary histories. On the other hand, according to the conventional nowledge M. trossulus and M. edulis has no clear morphological differences (yet multivariate morphometric analysis permits to discriminate them probabilistically), has no clear ecological differences in the areas of co-existence (data on habitat segregation is limited and controversial) and hybridize easily while the extent of hybridization is usually reported in relative, not absolute terms in genetic papers. Therefore marine biologists quite often (and quite reasonably) regard M. trossulus and M. edulis as intangible, virtual entities that are of importance for geneticists only. Here, on the example of the Whi