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The present study redescribes four species of Neanthes Kinberg, 1865 (Nereididae de Blainville, 1818) based on their type specimens collected from different worldwide localities: Neanthes chilkaensis (Southern, 1921) from India, N. galetae (Fauchald, 1977) from Panama, N. helenae (Kinberg, 1865) from St Helena Island, and N. mossambica (Day, 1957) from Mozambique. The morphology of the types was re-examined for the first time after the species were originally described, and incorporated the recent improvements in the standards and terminology for describing nereidid features. The arrangement of paragnaths on area VI stood out among the diagnostic features used to distinguish these four species. Neanthes chilkaensis and N. helenae are the unique nereidids bearing p-bar paragnaths on the area VI. Both species are also distinctive as the former species only exhibited p-bar paragnaths on the area VII–VIII and the latter ventrolateral projections on the apodous segment. Further examination revealed that N. nanciae (Day, 1949) from St Helena is a junior synonym of N. helenae. Moreover, N. galetae and N. mossambica are distinguishable from other species also by the development of dorsal cirri, neuropodial postchaetal lobe and ventral ligule, the presence/absence of merged paragnaths on area IV, paired oesophageal caeca, among other features. This study has further contributed to the morphological delimitation of the species in Neanthes as a first step towards revising the genus.
Seven species of Pseudopolydora are described and illustrated from the Arabian Gulf, Kuwait: P. achaeta Radashevsky & Hsieh, 2000, P. antennata (Claparède, 1868), P. arabica Radashevsky & Al-Kandari, 2020, P. auha sp. nov., P. kuwaiti sp. nov., P. melanopalpa sp. nov., and P. multispinosa sp. nov. The morphology of the developed planktonic larvae is described for P. antennata and P. kuwaiti sp. nov. Adults of all species live in tubes in soft sediments, while adults of P. kuwaiti sp. nov. also bore in shells of gastropods and dead corals encrusted by coralline algae. Pseudopolydora antennata and P. arabica form dense settlements up to 50 000 individuals per 1 m2, while other species are comparatively rare. The phylogenetic relationships between the examined species and other Pseudopolydora (18 species in total) were assessed in an analysis of sequence data of four gene fragments: mitochondrial 16S rDNA, nuclear 18S rDNA and 28S rDNA, and Histone 3 (2473 bp in total). Three species complexes are distinguished involving P. antennata, P. diopatra Hsieh, 1992 and P. paucibranchiata (Okuda, 1937). Sets of adult morphological features shared by species of each complex and an identification key to Pseudopolydora species from the Arabian Gulf are provided.