Becoming worldwide: first record of the invasive fan worm <i>Branchiomma luctuosum</i> (Annelida: Sabellidae) in the Pacific, based on a specimen from Japan

The present study documents the first record of the fanworm Branchiomma luctuosum [new Japanese name: Koukai-murakumo-keyarimushi] (Annelida: Sabellidae) from the Pacific, based on a specimen collected from the tidal flat at Torinosu, Wakayama, Japan. The species was originally described from the Red Sea and has been reported as a non-indigenous species in the Mediterranean, Brazil, and Angola. It has also been recorded from Pakistan, although its status there, whether native or introduced, remains uncertain. Mitochondrial COI and cytb gene sequences of the specimen exhibited 99.9–100% identity with sequences from Italy, France, and Pakistan deposited in GenBank. Although the taxonomic validity of diagnostic characters within the genus Branchiomma has been questioned, B. luctuosum can be morphologically distinguished from other congeneric species recorded in Japan—Branchiomma cingulatum, Branchiomma sp. B sensu Capa et al. (2013), and Branchiomma sp. I sensu Kobayashi et al. (2024)—by the combination of its large size, the absence of macrostylodes, the presence of slender stylodes, including basal paired ones, and uniformly colored radioles. Given its type locality in the Red Sea and the absence of prior records from the Pacific, Branchiomma luctuosum is considered non-indigenous in this region.