Paraopisthosyllis rufa Springer, Sato, Oguchi, Jimi, Miura & Aguado, 2025, sp. nov.

Paraopisthosyllis rufa sp. nov. Springer, Aguado, Sato, Oguchi, Jimi & Miura Figs 11–17, Table 2 urn:lsid:zoobank.org:act: EBEFAB46-9863-4144-9117-7013B664845B Material examined. Holotype (NSMT-Pol H-977) and seven paratypes (NSMT-Pol P-978): Japan, Misaki: Arahaima Beach, Sagami Bay, Misaki, 35°09'34"N 139°36'40"E, 2–5 m, coll. Sato, 2022. Paratypes (NSMT-Pol P-1006, ZMUG 32499) and additional material. (NSMT-Pol 113616): Japan, Misaki: Araihama Beach, Sagami Bay, Misaki Marine Biological Station, 35°09'36.1"N 139°36'40.1"E, 2–5 m, coll. Oguchi & Aguado 2019, Sato & Aguado 2024. Description. Large-sized body, largest specimen examined (holotype) 19.5 mm long, 0.8 mm wide, prostomium, 94 body segments and pygidium. Paratypes vary significantly in number of segments and body length. Body length ranges between 7.75–19.5 mm, number of segments between 58–94. Body cylindrical, broad (Figs 11A, 12A–B), flattened ventrally (Fig. 13 A, D). Main body colour red, anterior end reddish/pink, midbody section dark red, posterior region light red to white/opaque (Figs 12–14) (in live and fixed specimens).Appendages in anterior region faintly orange/red coloured (Fig. 12B), in midbody and posterior section white/opaque (Fig. 12A–B). Segments secondarily triannulated (Figs 11A, 12A, 13D, E). Segments thickest in anterior region, segments 1–8 enlarged (Figs 12A–B, 13A, 15A). Body surface, including cirri, covered by small papillae (Figs 15C–E, 16G). Prostomium with 4 lensed eyes arranged in trapezoidal shape, one thick and short median antenna and two lateral antennae (Figs 11A, 12A, 13A, D). Ciliation around the eyes present (Fig. 15C). Median antenna similar to prostomium length, lateral antennae longer than prostomium (Figs 13D, 15A, B). Palps broad, elongated, rounded, reddish in colour, basally fused (Fig. 13B, C). Peristomium with elongated, forward-pointing, cylindrical dorsal and ventral tentacular cirri (Figs 13D, 15A–B). Dorsal one longer than ventral one (Fig. 13D). Nuchal organs in ciliated grooves (Fig. 15B, E). Dorsal cirri digitiform (Fig. 12A, B), in a consistent pattern in their arrangement and length (Figs 12A, B, 13E). The dorsal cirri pointing upwards (U) arranged on a high position (closer to the dorsum) are double the length and thickness than those downwards (D) directed, whose arrangement is closer to the ventral side (Fig. 12A). The pattern of the first nine dorsal cirri is: U, D, D, U, D, U, D, D, U. Following ones alternate in a D, U, D, U, D, U pattern until they shift to the same position and continue at same arrangement height along the rest of the body. The shift from alternating to non-alternating dorsal cirri is consistent across all specimens, but the segment at which the alternation stops varies between individuals and ranges from segment 32–37. In some specimens, fourth dorsal cirri thickened compared to rest of upwards oriented dorsal cirri (Figs 11A, 15A). Ventral cirri of same length as parapodia, ovate, covered in small papillae (Figs 16A–E, 17A, C). Parapodia short, thick, distally bilobed, each with 12 compound chaetae (Fig. 17A–C). Compound chaetae bidentate with small serrations along the inner margin of the blade (5μm) (Figs. 11B, 17C, D). Shafts smooth and slender. Longest chaetae article ~43 µm, shortest ~18 µm. Simple chaetae not seen. Pygidium short and triangular, with two club-shaped and short anal cirri (Fig. 14A, 16F). Pharynx straight, occupying segments 1–6, followed by a cylindrical proventricle present in segments 7–11 in both live and fixed specimens (Fig. 12B). One single pharyngeal tooth located retarded on posterior dorsal outer side of the pharynx. Reproduction. Schizogamy by scissiparity. Paratypes adult specimens developing mature dicerous female stolons (Fig. 14B–H). The stolon head with two pairs of red eyes (Fig. 14E–F, H). Lateral antennae opaque, smaller than cirri, observed in developing and mature stolons (Fig. 14H). Female stolons with ovules visible by transparency, red/purple in colour (Fig. 14B–D, H). Dorsal cirri and ventral cirri present, elongated. Parapodia and compound chaetae present, notoaciculae present in mature stolons (Fig. 14H). Pygidium with two club-shaped anal cirri, similar to adult specimens (Fig. 14B, D). Stock forming a secondary tail ventrally while the stolon is still attached to the parental stock observed in paratype (Fig. 14F). Regenerating posterior end after detachment of stolon (Fig. 14G). Habitat. Rocky shorelines, shallow waters 2–5 m deep, on algae and coral rubble. Remarks. Paraopisthosyllis rufa sp. nov. is characterized by the distinctive alternation pattern of dorsal cirri and the distinct shape of the compound chaetae, together with its prominent colour in both live and fixed specimens (5 years storage). It is the first record of the genus Paraopisthosyllis in Japan (Jimi 2024). Chaetae of P. rufa sp. nov. are most similar to those of P. correiae Paresque et al. 2016, P. fusigera Augener, 1913, P. ornaticirra San Martín & Hutchings, 2006 and P. phyllocirra Hartmann-Schröder, 1991. They all have bidentate compound chaetae with serrations along the blade and spinulations on the chaetae shafts. Additionally, P. brevicirra Hartmann-Schröder, 1979, P. fusigera and P. phyllocirra all lack simple chaetae as P. rufa. sp. nov. Like in most Paraopisthosyllis species (except for P. fusigera and P. victoriae San Martín, López & Aguado, 2009) the body surface of P. rufa sp. nov. is covered with small papillae. P. rufa sp. nov. is further differentiated from P. victoriae by the continuous prominence of the intersegmental furrows and from P. fusigera by a longer pharynx and proventricle, as well as the red colouration. The regeneration of the pygidium, while the stolon is still attached, firstly reported herein for Paraopisthosyllis, is a feature in common with Megasyllis and Alcyonosyllis (Aguado & Glasby 2015; Nakamura et al. 2023; Sato et al. 2024). The three genera form a monophyletic group, potentially inheriting this ability from their common ancestor. Convergently, other genera within Syllinae regenerate the pygidium while the stolons are still attached, such as Ramisyllis kingghidorahi Aguado et al. 2022, R. multicaudata Glasby, Schroeder & Aguado, 2012, Parahaplosyllis kumpol and some species of Trypanobia Imajima & Hartmann, 1964, Trypanosyllis and Myrianida (Okada 1937; Glasby et al. 2012; Álvarez-Campos et al. 2013; Aguado et al. 2022). However, a detailed morphological comparison of this process in different groups has never been performed. Distribution. Misaki, Sagami Bay; Japan; NW Pacific. Etymology. This species is the first known species of Paraopisthosyllis with a striking red colouration and is therefore named “ rufa ” from Latin “rufus“ meaning red.