Cerebratulus sardonyx sp. nov. is described from a single specimen collected at Kita-Koho Seamount on the Kyushu-Palau Ridge, a major submarine arc extending from southern Japan into the Philippine Sea. The specimen was obtained at a depth of 454 m using the remotely operated vehicle KM-ROV during cruise KM24-03C of the R/V Kaimei. An integrative taxonomic approach-combining external morphology of the living specimen, histology, and molecular phylogenetic analyses-was used to characterize the species. The new species is readily distinguished from all congeners by its vivid reddish-brown body crossed by numerous sharply defined opaque white transverse rings, together with a uniquely patterned head bearing orangish patches and alternating white and brown transverse bands. Phylogenetic analysis places the species within a clade referred to as Lineage J in a previous study, corresponding to the Cerebratulus clade. This discovery represents the first record of Cerebratulus Renier, 1804 from Japanese seamounts and the first described nemertean from the Kyushu-Palau Ridge.
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