The ‘ normani -group’ within Onisimus is characterized by a partly cleft telson, a completely constricted uropod 2, and a peculiar 1 st epimeral plate, with an anterodistal hook (Vader et al. 2005). The group consists of O. normani, O. turgidus, and 2 or 3 taxa closely related to O. turgidus, but strongly deviating in the arrangement of the setal teeth on the outer plate of maxilla 1. Material belonging to these taxa has been found in the W Atlantic Ocean, near Greenland and in the NW Pacific (Johnsen 2001). Most (or possibly all) species in this group appear to be obligate associates of sea anemones, a trait absent in all other Onisimus species. In Norwegian waters there are two species in this group, O. normani in W Norway north to Andfjorden, and O. turgidus in N Norway and the Barents Sea. They can be distinguished as follows:
1. Pereopod 1 palm oblique, propodus subrectangular. Uropod 2 inner ramus with small setae in constriction; uropod 1 outer ramus without spines on inner margin............................................................................................................................................................... Onisimus normani Sars, 1891
– Pereopod 1 palm strongly oblique, propodus triangular. Uropod 2 inner ramus with large spine in constriction; uropod 1 outer ramus with spines on inner margin.... Onisimus turgidus (Sars, 1879)