Pygospio elegans Claparède, 1863, the type species of the genus Pygospio, was originally described from Normandy, France, and later widely reported from boreal waters in the northern hemisphere. Sequence data of four gene fragments (2576 bp in total) of the mitochondrial 16S rDNA, nuclear 18S and 28S rDNA, and Histone 3 have shown that individuals from California and Oregon, USA, Scotland and the White Sea, Russia were genetically similar (the average p-distances for the combined data between the four groups ranged from 0.04 to 0.16%, average p = 0.1%). These individuals are considered to be conspecific and the amphiboreal distribution of P. elegans is here confirmed. Adult morphology of the species is briefly described and illustrated. The molecular analysis revealed two genetically distant populations, Pygospio sp. 1 from the Sea of Okhotsk and Pygospio sp. 2 from Oregon. The morphological differences and high average genetic p-distances for the combined data (ranging from 3.06 to 3.18%, average p = 3.12%) between Pygospio sp. 2 and P. elegans suggest the presence of an undescribed Pygospio species co-occurring with P. elegans in Oregon. High morphological similarity and moderate genetic p-distances for the combined data (ranging from 1 to 1.11%, average p = 1.07%) between Pygospio sp. 1 and P. elegans indicate a comparatively recent genetic divergence of the Pygospio population in the Sea of Okhotsk. Taking into account the high genetic similarity of the remote European and North American populations of P. elegans and medial location of the Pygospio sp. 1 population, we suggest the latter to belong to a separate species. However, this conclusion should be verified in further studies on the morphology, reproductive biology and genetics of this population. The present findings show the need to re-examine Pygospio from the Asian Pacific and elsewhere that have been identified as P. elegans.

The jaw apparatus in several annelid families represents a powerful tool for systematic approaches and evolutionary investigations. Nevertheless, for several taxa, this character complex has scarcely been investigated, and complete comparative analyses of all annelid jaws are lacking. In our comprehensive study, we described the fine structure of the jaw apparatus and the ventral pharyngeal organ (VPO) in Histriobdella homari – a minute ectocommensal of lobsters putatively belonging to the Eunicida – using different comparative morphological approaches, including SEM, TEM, CLSM and subsequent 3D reconstruction. The H. homari jaw apparatus is composed of ventral paired mandibles and dorsal symmetrical maxillae consisting of numerous dental plates, ventral carriers and an unpaired dorsal rod, and the general assemblage and arrangement of the different parts are highly comparable to those of other eunicid families. The jaw ultrastructure of histriobdellids resembles that of the families Dorvilleidae and (juvenile) Onuphidae. Furthermore, our data reveal that in the process of development of the jaw apparatus, the mandibles, maxillae II and unpaired dorsal rod are formed first, and the remaining maxillae and ventral carriers appear later. Notably, the muscular apparatus differs from that in Dorvilleidae and Onuphidae in terms of the number and arrangement of muscle fibers encompassing the jaws – not only because of the very small size of Histriobdella but also because histriobdellid maxillary protraction occurs due to straightening of the dorsal rod and thus requires a different muscular scaffold. Based on our investigations, the arrangement of the muscular apparatus of the jaws, the presence of paired ventral carriers and the dorsal rod, and the morphology of the ventral pharyngeal organ represent a histriobdellid autapomorphy. Our datasets form a basis for further comparative analyses to elucidate the evolution of Eunicida and jaw-bearing Annelida.