Nieuw gevestigde soorten van het geslacht Polysiphonia (Rhodophyta, Rhodomelaceae) in Zuidwest-Nederland

Morphological descriptions are given of two species of Polysiphonia introduced into SW Netherlands’ coastal waters in recent years, namely: P. devoniensis, formally described only in 1993 and previously known from SW England, and P. senticulosa which is regarded as an “alien” and probably originates from the northern Pacific. The confusion surrounding the identity of P. “violacea” in the Netherlands is discussed: it is concluded that only a few old records (1940, 1951) belong to P. fibrillosa, but that all recent finds belong to P. harveyi, now a very common species; the latter’s introduction dates back to 1960. A revised key to the Dutch Polysiphonia species with four pericentral cells is given.