The presence of the non-indigenous species, the black-pygmy mussel
Xenostrobus securis, is reported here for the first time in an intense shellfish farming
area off Galicia (NW Spain). Very high concentrations of this mytilid bivalve have
colonized estuarine waters located at the inner part of the Ria de Vigo. The invasive
role of X. securis is discussed in the context of the wide ecological tolerance of the
species and the recent finding of settlements of this species on numerous colonies of
the economically-important blue mussel Mytilus galloprovincialis. The mode of
introduction of the black-pygmy mussel is also discussed in relation to human
management activities.