Redescription of the Indo-Pacific polychaete Neanthes pachychaeta (Fauvel, 1918) n. comb. (Annelida, Phyllodocida, Nereididae) and its synonyms

Type specimens of three Indo-west Pacific nereidid polychaetes are redescribed: Ceratonereis pachychaeta Fauvel, 1918, Nereis (Ceratonereis) ramosa Horst, 1919 and Nereis (Lycoris) anchylochaeta Horst, 1924. No significant morphological differences were found between the three species. The latter two names are therefore relegated to junior synonymy with the oldest, C. pachychaeta. A lectotype is identified from among the syntypes of C. pachychaeta in order to stabilise the name and type locality of the species. Further, C. pachychaeta is newly transferred to the genus Neanthes, based on a comparison of generic level features, especially the presence of paragnaths on the oral ring. Newly collected specimens of Neanthes pachychaeta n. comb. from Indonesia, Philippines, northern Australia, Japan and French Polynesia are described and the living coloration reported for the first time. Apart from its striking red-orange colour, the species may be recognised by having paragnaths atop a plate-like basement and thick, hammer-headed fused falcigers in posterior parapodia. The new material and new synonymies give N. pachychaeta n. comb. a widespread Indo-Pacific distribution.