The north-west Pacific nemertean Cephalothrix simula has been introduced to the Pacific coast of North America, to the Mediterranean Sea and to the Atlantic coast of the Iberian Peninsula. This paper describes the first records of Cephalothrix simula from northern Europe, revealing a broader range for this non-endemic palaeonemertean species. A tree-based species delimitation method (Poisson Tree Process) employing the mitochondrial cox1 gene fragment reveals that four of five Cephalothrix specimens collected from The Netherlands correspond to Cephalothrix simula and only one to the endemic Cephalothrix rufifrons. The Cephalothrix simula sequences from The Netherlands exhibit the highest similarity with sequences from the native area and those from the invaded area in southern Europe. Its co-occurrence with the native Cephalothrix rufifrons in the same habitat is reported. External characters potentially distinguishing live Cephalothrix simula from Cephalothrix rufifrons are given. This finding raises the number of recorded species in The Netherlands to 20.
