Paraopisthosyllis rufa, Springer & Sato & Oguchi & Jimi & Miura & Aguado, 2025

Paraopisthosyllis rufa sp. nov. Springer, Aguado, Sato, Oguchi, Jimi & Miura Figs 11–17 View FIGURE 11 View FIGURE 12 View FIGURE 13 View FIGURE 14 View FIGURE 15 View FIGURE 16 View FIGURE 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 View FIGURE 11 , 12A–B View FIGURE 12 ), flattened ventrally ( Fig. 13 A, D View FIGURE 13 ). Main body colour red, anterior end reddish/pink, midbody section dark red, posterior region light red to white/opaque ( Figs 12–14 View FIGURE 12 View FIGURE 13 View FIGURE 14 ) (in live and fixed specimens).Appendages in anterior region faintly orange/red coloured ( Fig. 12B View FIGURE 12 ), in midbody and posterior section white/opaque ( Fig. 12A–B View FIGURE 12 ). Segments secondarily triannulated ( Figs 11A View FIGURE 11 , 12A View FIGURE 12 , 13D, E View FIGURE 13 ). Segments thickest in anterior region, segments 1–8 enlarged ( Figs 12A–B View FIGURE 12 , 13A View FIGURE 13 , 15A View FIGURE 15 ). Body surface, including cirri, covered by small papillae ( Figs 15C–E View FIGURE 15 , 16G View FIGURE 16 ). Prostomium with 4 lensed eyes arranged in trapezoidal shape, one thick and short median antenna and two lateral antennae ( Figs 11A View FIGURE 11 , 12A View FIGURE 12 , 13A, D View FIGURE 13 ). Ciliation around the eyes present ( Fig. 15C View FIGURE 15 ). Median antenna similar to prostomium length, lateral antennae longer than prostomium ( Figs 13D View FIGURE 13 , 15A, B View FIGURE 15 ). Palps broad, elongated, rounded, reddish in colour, basally fused ( Fig. 13B, C View FIGURE 13 ). Peristomium with elongated, forward-pointing, cylindrical dorsal and ventral tentacular cirri ( Figs 13D View FIGURE 13 , 15A–B View FIGURE 15 ). Dorsal one longer than ventral one ( Fig. 13D View FIGURE 13 ). Nuchal organs in ciliated grooves ( Fig. 15B, E View FIGURE 15 ). Dorsal cirri digitiform ( Fig. 12A, B View FIGURE 12 ), in a consistent pattern in their arrangement and length ( Figs 12A, B View FIGURE 12 , 13E View FIGURE 13 ). 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 View FIGURE 12 ). 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 View FIGURE 11 , 15A View FIGURE 15 ). Ventral cirri of same length as parapodia, ovate, covered in small papillae ( Figs 16A–E View FIGURE 16 , 17A, C View FIGURE 17 ). Parapodia short, thick, distally bilobed, each with 12 compound chaetae ( Fig. 17A–C View FIGURE 17 ). Compound chaetae bidentate with small serrations along the inner margin of the blade (5μm) ( Figs. 11B View FIGURE 11 , 17C, D View FIGURE 17 ). 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 View FIGURE 14 , 16F View FIGURE 16 ). Pharynx straight, occupying segments 1–6, followed by a cylindrical proventricle present in segments 7–11 in both live and fixed specimens ( Fig. 12B View FIGURE 12 ). 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 View FIGURE 14 ). The stolon head with two pairs of red eyes ( Fig. 14E–F, H View FIGURE 14 ). Lateral antennae opaque, smaller than cirri, observed in developing and mature stolons ( Fig. 14H View FIGURE 14 ). Female stolons with ovules visible by transparency, red/purple in colour ( Fig. 14B–D, H View FIGURE 14 ). Dorsal cirri and ventral cirri present, elongated. Parapodia and compound chaetae present, notoaciculae present in mature stolons ( Fig. 14H View FIGURE 14 ). Pygidium with two club-shaped anal cirri, similar to adult specimens ( Fig. 14B, D View FIGURE 14 ). Stock forming a secondary tail ventrally while the stolon is still attached to the parental stock observed in paratype ( Fig. 14F View FIGURE 14 ). Regenerating posterior end after detachment of stolon ( Fig. 14G View FIGURE 14 ).

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.