Cryptic diversity of the tube-dwelling polychaete Phyllochaetopterus in the Shinkai Seep Field, Mariana Trench

: Phyllochaetopterus (Annelida: Chaetopteridae) is a diverse genus of tube-dwelling polychaetes found in a wide range of marine environments from subtidal to abyssal depths, including chemosynthesis-based ecosystems. The Shinkai Seep Field (SSF) is a serpentinite-hosted system in the Mariana Trench, where the deepest-known Phyllochaetopterus polychaetes inhabit the surfaces of brucite/carbonate chimneys. Despite all specimens collected from SSF being morphologically consistent with P. polus originally described from a deep-sea hot vent on the Mid-Atlantic Ridge, molecular barcoding using the mitochondrial cytochrome c oxidase subunit I (COI) gene revealed at least three cryptic lineages, none of which corresponded to P. polus . Phylogenetic reconstruction re covered P. polus embedded among the three SSF lineages, confirming their close relationship. These results warrant careful examina-tion of Phyllochaetopterus from other regions using integrative taxonomy in order to understand its true diversity and pinpoint further taxonomically informative morphological characters.