A new species of Phalacrostemma (Annelida, Sabellariidae) is described based on the specimens collected at a depth of 675 m on the Ritto Seamount of the West Mariana Ridge, northwestern Pacific. This record represents both the deepest occurrence of Sabellariidae in Japan and the first record of the genus Phalacrostemma from the region. Phalacrostemma ritto sp. nov. dwells in a tube made of small sand particles and foraminiferans, which is attached to the spines of a cidarid sea urchin. The new species is characterized by having 22-30 pairs of outer paleae arranged in a spiral, outer paleae with acute tips and compact thecae, one or two inner paleae in each row, 8-13 opercular papillae on each side, five pairs of nuchal hooks with curved tips, a pair of slender ventral lobes on the first chaetiger, a single pair of conical papillae on the second chaetiger, bilobed notopodia in the middle to posterior abdomen, and abdominal uncini bearing three rows of teeth. A phylogenetic analysis, which included the new species and the newly sequenced Idanthyrsus okudai and Sabellaria isumiensis and was based on sequences of four gene fragments (COI, 16S, 18S, and 28S), was conducted. The new species was nested within the fully supported Phalacrostemma clade and recovered as the sister taxon to Gesaia csiro, whereas support values for phylogenetic relationships among sabellariid species were generally low.
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