Molecular evidence of cryptic species diversity in the Perinereis nuntia species group (Annelida: Nereididae) with first records of P. nuntia and P shikueii in southern Japan

Taxonomic reexamination of Japanese populations of the Perinereis nuntia species group, which constitutes a major polychaete component in intertidal benthic communities, was carried out by analyzing the mitochondrial 16S rDNA and the nuclear ribosomal ITS sequences of 80 specimens, newly collected from 27 sites in Japan, together with 18 museum-preserved specimens collected from southern neighboring countries outside Japan. The Japanese populations of the Perinereis nuntia species group could be divided into four genetically different groups, which corresponded morphologically to four nominal species (P mictodonta, P wilsoni, P. shikueii, and P nuntia), with some exceptions. Perinereis nuntia and P shikueii were recorded as new to the Japanese fauna, with their distributions restricted to southern Japan. The Glade containing P shikueii was subdivided into two sister Glades (forms A and B), indicating that cryptic speciation has occurred within this Glade. Our results revealed a remarkable variability in the number of bars in area VI of the proboscis in form B of P shikueii, which appeared to be caused by breakage of the long bar into short bars during growth from a juvenile to an adult.