Fourteen species representing four genera Boccardiella, Neoboccardia, Polydora and Pseudopolydora of the polychaete family Spionidae from the North West Pacific are described on the basis of author's collections as well as material deposited in five museums. The study includes eight previously described species, two species raised in rank from subspecies, four new species, and six synonyms. Descriptions, figures and ecological data of the species covered, keys to the species of Polydora and Pseudopolydora from the North West Pacific, and diagnoses of the genera covered are included.

The spionid mudworm Polydora cornuta Bosc, 1802 (formerly Polydora ligni Webster, 1880) is redescribed based on museum and new material collected in temperate and subtropical zones worldwide. Previously unobserved features are noted, including arrangement of lateral ciliated organs on all chaetigers but 4 and 5, metanephridial organs and glandular pouches beginning from chaetiger 7. Larval morphology is described and illustrated based on material from Brazil. The larvae are characterized by middorsal vesiculate melanophores from chaetiger 3 or 4, dorsal paired melanophores band-shaped from chaetiger 3 and ramified from chaetiger 7 or 8, large ramified yellow chromatophores on ventral side from chaetigers 5–7, specific modified chaetae in notopodia of chaetiger 5, and hooks in neuropodia from chaetiger 7 not accompanied by any other kind of chaetae. Some differences from earlier descriptions of larvae from Europe and North America are highlighted. Consistent morphological differences between adults from distantly separated populations, as suggested in earlier studies, were not revealed and all the examined materials are referred to one species.

This is the third guide in a series aimed to help in the identification of northern European samples through the NationalMarine Biological Analytical Quality Control (NMBAQC) Scheme. A review is presented of general morphology andbiology of one of the most common groups in marine and estuarine communities worldwide, spionid polychaetes(Annelida: Spionidae). An identification key to 20 genera and a list of 80 species likely to occur in waters around Britainand Ireland are provided based on the taxonomic literature, collection records and the author’s examination of various collections. The names are updated but some records need verification.

A Polydora species was found boring in shells of the abalone Haliotis discus hannai cultivated in landbased tanks in Coquimbo, Chile.