Synonymy of the Scale Worm Hesperonoe urechis with
Arctonoella sinagawaensis (Annelida: Polynoidae), Newly
Recorded from the Seto Inland Sea, Western Japan, with
Remarks on Symbiosis with the Spoon Worm Urechis
unicinctus (Annelida: Thalassematidae)

Masanori Sato1,7,8, Naoto Jimi2,3, Gyo Itani4, Yumi Henmi5, and Shuji Kobayashi6

The monotypic polynoid genus Arctonoella Buzhinskaja, 1967 comprises solely the type species A. sinagawaensis
(Izuka, 1912), which was originally described from Tokyo Bay (central Japan), and subsequently recorded from China
and the Russian Far East. The six specimens we collected together with the burrowing filter-feeding spoon worm Urechis
unicinctus (Drasche, 1880) from three intertidal-flat sites in the Seto Inland Sea represent a new report for the western
Japan, and the second for the country. Our morphological observations reveal that the shape of the cephalic peaks in the
frontal prostomial margin is variable even within a local population, although this character has been considered as diagnostic
for Arctonoella. This genus is closely related with Hesperonoe Chamberlin, 1919, both morphologically and phylogenetically.
Hesperonoe urechis Marin and Antokhina, 2020, collected inside a burrow of U. unicinctus in the Russian Far
East, is hereby deemed a junior synonym of A. sinagawaensis. This species morphologically resembles Hesperonoe adventor
(Skogsberg in Fisher and MacGinitie, 1928), which inhabits spoon worm (Urechis caupo Fisher and MacGinitie, 1928 and
Echiurus echiurus alaskanus Fisher, 1946) burrows along the northeastern Pacific coast of America. Therefore, we conclude
that the Arctonoella-Hesperonoe species complex requires to be reviewed to determine the proper generic arrangement. Additionally,
we revise the available data on the symbiotic fauna living inside the burrows of U. unicinctus and U. caupo.
Key Words: commensalism, echiuroid, molecular phylogeny, polychaete, taxonomy, tidal flats.

Species Diversity

28

1

17