Flabelligena daitoensis Jimi, 2024

Flabelligena daitoensis Jimi , sp. nov.

[New Japanese name: Daito-hime-kumanoashitsuki] ( Figs 2 View Fig , 3 View Fig )

Material examined. Holotype (NSMT-Pol H-968): 5 mm in length, 0.5 mm in width, 21 chaetigers, incomplete body, off NE Minamidaito Island , Japan (25°50.945′N, 131°16.936′E), 644 m depth, 1 May 2024, collected by Naoto Jimi. GoogleMaps Paratypes: 1 specimen (NSMT-Pol P-969), collected with holotype, 3 mm in length, 0.5 mm in width, 18 chaetigers, incomplete body, used for SEM, used for total DNA extraction; 1 specimen (NSMT-Pol P-970), 4 mm in length, 0.5 mm in width, 20 chaetigers, incomplete body, Kita-Koho Seamount (26°43.092′N, 135°16.274′E), 774 m depth, 16 May 2024, collected by Naoto Jimi GoogleMaps . GoogleMaps

Description. Body cylindrical ( Fig. 2A, B View Fig ), milky whitish in life and after fixation, without ventral center line, surface papillated, slightly swollen in chaetigers 5–9. Body papillae presented along several areas. Papillae of body surface globular ( Fig. 3A View Fig ), each about 20 µm long and 20 µm wide (n = 10), densely packed, not arranged in transverse rows, scattered. Papillae around branchial scars ( Fig. 3B, C View Fig ) subcylindrical, smooth, with a subdistal constriction, each papilla about 10 µm long and 5 µm wide (n = 10), 2–3 papillae per branchial scar. Papillae on mouth subcylindrical, smooth, irregularly constricted, each papilla about 20 µm long and 10 µm wide (n = 3), scattered ( Fig. 3D View Fig ). One papilla present between noto- and neurochaetae, short (1/10 of neurochaetal length), digitate, rugose, about 10 µm long and 10 µm wide (n = 10). Genital papillae paired, large, conical, in ventral side of parapodium in chaetiger 7 (40 µm long and 40 µm wide).

Prostomium subtriangular ( Figs 2D View Fig , 3B View Fig ), eyes absent ( Fig. 2D View Fig ). Palp scars on anterior margin of prostomium, one pair ( Fig. 3B View Fig ). Branchiae three pairs ( Fig. 3B View Fig ). Nephridial lobes one pair, consist of a nephriodiopore papilla and globular base ( Fig. 3B, C View Fig ). Glandular lobes two pairs, two glandular papillae per globular base ( Fig. 3C View Fig ).

Notochaetae capillaries, spinulose ( Fig. 3E View Fig ), 1–2 notochaetae per fascicle. Neurochaetae compound ( Fig. 3F View Fig ), blades sickle shaped, bidentate, barely spinulose, handle barely spinulose, inner edge without teeth, tip of blade forked, 1–2 neurochaetae per fascicle.

Posterior end lost; pygidium unknown.

Etymology. The specific name daitoensis derives from the type locality, Minamidaito Island.

Phylogenetic results. In the resulting phylogenetic tree ( Fig. 4 View Fig ), Flabelligena daitoensis sp. nov. is sister to Flabelligena sp. with a 100% bootstrap support. The Flabelligena clade is sister to a clade including the swimming acrocirrids ( Swima , Teuthidodrilus , bomber worms, and Juanita worms) with high support.

Distribution and habitat. Found in sand accumulated on steeply structured seabed at depths of 644–744 m, western Pacific Ocean.

Remarks. The new species was placed in Flabelligena due to the presence of branchiae and spinulose notochaetae ( Aguirrezabalaga and Ceberio 2006), further supported by our phylogenetic analyses ( Fig. 4 View Fig ). It closely resembles F. gascognensis and F. hakuhoae due to the presence of three pairs of branchiae ( Aguirrezabalaga and Ceberio 2006; Jimi et al. 2020b). However, the new species can be distinguished from the two other species by the presence of a pair of ventral genital papillae on the ventral side of chaetiger 7, 1–2 notochaetae, and 1–2 neurochaetae. In comparison, F. gascognensis has a pair of ventral large papillae between chaetigers 6–7, 2–5 notochaetae, and 2–4 neurochaetae ( Aguirrezabalaga and Ceberio 2006); F. hakuhoae has a pair of ventral large papillae on the ventral side of the parapodium in chaetiger 6, 1–2 notochaetae, and 1–4 neurochaetae ( Jimi et al. 2020b).

Osborn and Rouse (2008) provided DNA sequences of an unidentified species of Flabelligena ( Flabelligena sp. ) from the Pacific-Antarctic Ridge without any morphological information. Our molecular identification confirmed that the new species is distinct from the unidentified species, as the Kimura 2-parameter (K2P) genetic distance in the COI region between the two species is 24.9%. This distance is larg- er than the typical interspecific genetic distance observed in polychaetes (e.g., Jimi et al. 2022), thereby supporting the classification of the new species as a separate entity.

Molecular phylogenetic analysis revealed that Flabelligena formed a clade with pelagic genera such as Swima and Teuthidodrilus , a relationship previously suggested by the phylogenetic analyses of Osborn et al. (2011) and Jimi et al. (2020a). Evolution of pelagic traits in Acrocirridae remains unclear while our phylogenetic tree indicated that a pelagic lifestyle evolved at least once within the family ( Fig. 4 View Fig ).

Within Acrocirridae , the polyphyly of several interstitial genera such as Actaedrilus , Macrochaeta , and Flabelligena is notable. While Actaedrilus and Macrochaeta cluster together, Flabelligena is grouped with larger-bodied pelagic species. This suggests that the interstitial lifestyle might also have been independently acquired multiple times within the family. This complexity highlights the adaptive flexibility and evolutionary diversity observed in Acrocirridae .