Ramisyllis kingghidorahi n. sp., a new branching annelid from Japan

Abstract Among over 20,000 species of Annelida, only two branching with a highly modified body-pattern are known until now: the Syllidae Syllis ramosa McIntosh, 1879, and Ramisyllis multicaudata Glasby et al. (Zoological Journal Linnean Society, 164, 481–497, 2012). Both have unusual ramified bodies one head multiple branches live inside canals host sponges. Using an integrative approach (combining morphology, internal anatomy, ecology, phylogeny, genetic divergence, complete mitochondrial genome), we describe new from Japan, kingghidorahi n. sp., inhabiting undescribed Petrosia (Porifera: Demospongiae) shallow waters. We compare its closest relative, R. ; emend diagnosis discuss previous reports S. . This study suggests much higher diversity syllids than currently known. Finally, possible explanations for feeding behaviour in relation to ciliated wall digestive tubes (especially at distal anus), provide hypothesis evolution body patterns as result adaptation sponge labyrinthic canal system.