The littoral/sublittoral hoplonemertean (phylum Nemertea) species Oerstedia dorsalis (Abildgaard, 1806) is described as polymorphic in external characters (Örsted, 1844; Burger, 1895; Brunberg, 1964). Sundberg (1984) grouped specimens in three classes based on external morphology and asked, using multivariate morphometrics, if there was a consistent correlation between external and internal characters. Such a correlation would have indicated a restricted gene flow between the externally different morphs, and thus the possibility of a group of species instead of one, polymorphic, species.
One of the forms in Sundberg (1984) appeared to be different from the other two morphs but the morphometric analysis was inconclusive. Later, Sundberg & Janson (1988) established by enzyme electrophoresis that this form was genetically different and it was later described as Oerstedia striata (Sundberg, 1988). A problem both in Sundberg (1984), and Sundberg & Janson (1988) was that one of the groupings (class ‘C’ in both studies) of specimens subsumed a number of morphs, all speckled or dotted but to varying degrees. These morphs may in fact represent different species, which would have been concealed in the electrophoretic and morphometric analysis as merely increased intraspecific variation. One of the ‘C’ morphs in Sundberg (1984) and Sundberg & Janson (1988) is light brown with numerous small dark brown/black dorsal spots. Based on one specimen from Anglesey, North Wales, Gibson (1988) described Oerstedia (Paroerstedia) nigrimaculata which is externally similar to this variety. Envall & Sundberg (1993) could not find any differences in internal characters between the type specimen of O. nigrimaculata and the brown form of O. dorsalis but hesitated to conclude that O. nigrimaculata is a variety of O. dorsalis.