Arabella gouqi sp. nov.

Figs 2 A – F, 3 A – H, 4 A – D

Material examined.

Holotype. • 1 complete specimen (CMSTT 7): China: Zhejiang: Zhoushan; Gouqi Island, 30°42.4546'N, 122°45.7676'E, 20 m depth, 13 Oct 2025, Anton YE leg. Paratypes. 7 specimens: • 1 complete specimen (CMSTg 13), 30°42.2679'N, 122°45.7230'E, 1 Dec 2024, other data same as holotype. • 3 complete specimens (CMSTT 1, CMSTT 2, CMSTT 3), 30°42.2963'N, 122°46.1492'E, with other data the same as the holotype. • 3 specimens (CMSTT 4, CMSTT 5 incomplete, CMSTT 6) 30°42.4181'N, 122°46.1410'E; other data same as holotype.

GenBank accession numbers.

COI gene: PV 548907, PX 495939, PX 495940, PX 495941, PX 495942, PX 495943, and PX 495948. 16 S gene: PX 601586, PX 610608, PX 610609, and PX 601587.

Etymology.

The specific name “ gouqi ” is obtained from the location where the specimens were collected, Gouqi Island, found in Zhejiang Province, China.

Diagnosis.

Prostomium tapers anteriorly, with four eyes in a concave arrangement. Peristomium forms double rings. Maxillae with five plate pairs. Shapes both symmetrical and asymmetrical; MxI left bifid robust, right gracile falcate. Long right MxII, short left MxII. MxIII and MxIV with longest anterior-most tooth. MxV one tooth both sides. Maxillary carriers long and slender. Dorsal carriers longer than ventral. Short notopodial cirri. Prechaetal lobe rounded, shorter than chaetae. Posterior postchaetal lobe shorter than chaetae. Chaetae bilimbate, length decreases from median to both dorsal and ventral or dorsal to ventral only. Chaetae coarsely or finely serrated; ventralmost chaetae taper gradually to guards. Pygidium with one pair of distinct swollen pads.

Description.

Holotype, complete specimen, 58 mm long with 308 chaetigers. Width, 2 mm at chaetiger 10 and 2.5 mm at the central part, including parapodia. Paratypes. 1 incomplete specimen, 103 mm long with 364 chaetigers. Width, 3 mm at chaetiger 10 and 3.2 mm at the central part, including parapodia. 3 complete specimens, 63–125 mm long with 330–524 chaetigers. Width, 1–3 mm at chaetiger 10 and 1.5–3.5 mm at the central part, including parapodia.

Body long, slender, dorsoventrally rounded, dorsal surface more convex. Widest centrally, maintaining width through most of its length, then gradually tapering to the pygidium. Preserved holotype and paratypes pale brown with dark spots forming dorsal lines throughout the body (Fig. 2 A – C). Prostomium slightly shorter than peristomium. Posterior deeper than anterior end, dorsally in a slope, ventrally flattened with median groove from posterior to anterior (Fig. 2 C, F). Prostomium eyes arranged in a concave shape. Median pair smaller than lateral (Fig. 2 E). Peristomium of two distinct rings, both wider than prostomium (Fig. 2 B, C, F).

Mandibles black, connected by short ligament, positioned anterior to maxillae when retracted, cutting plates shorter than the mandibular carriers, inner edge shorter than lateral (Fig. 3 G). Dorsal maxillary carriers paired widest anteriorly, poorly defined posteriorly; ventral carrier unpaired, widest anteriorly, tapers posteriorly. Ventral carrier is about 2 / 3 the length of dorsal carriers (Figs 3 H, 4 A, 4 C). Maxillae with five pairs of plates, symmetrical (MxIII, MxIV and MxV) or asymmetrical (MxI and MxII). MxI left bifid robust, right gracile falcate. Long right MxII, short left MXII. MxIII and MxIV with the longest anterior-most tooth, and MxV with one tooth on both sides. Maxillary formula: MxI teeth (2, (6–7)) + (1, (8–9)); MxII teeth (8–9) + (17–18); MxIII teeth 6 + 6; MxIV teeth 5 + 5; MxV teeth 1 + 1. (Fig. 4 A, B).

Prechaetal lobe rounded, shorter than chaetae. Posterior postchaetal lobe elongate but shorter than chaetae (Figs 3 A, 3 B, 4 D). Short notopodial cirri present, observed clearly at the anterior and central chaetigers (Fig. 3 A, B). 3 notoaciculae present shorter than prechaetal lobe, neuroaciculae not observed (Fig. 4 D). 5 to 7 bilimbate chaetae present (the number varies between specimens, but consistent throughout the body), dorsal or mediodorsal chaetae longer than others. Chaetae coarsely serrated (mediodorsal chaetae) or finely serrated (other chaetae), ventralmost chaetae taper gradually to guards (Figs 3 A – F, 4 D). Chaetal length decreases from median to both dorsal and ventral or dorsal to ventral only (Figs 3 A, 3 B, 3 F, 4 D). Pygidium with two distinct swollen pads (Fig. 2 D).

Distribution.

Currently known only from mussel farms in the sea off the coast of Gouqi Island in Zhejiang, China.

Habitat.

Specimens of A. gouqi sp. nov. were obtained from the ropes used to hold mussels in mussel farms. The ropes were submerged in seawater to a depth of approximately 20 m.

Remarks.

Arabella gouqi sp. nov. is characterized by a combination of the following features: a pygidium with two distinct swollen pads; short notopodial cirri; gradually tapering ventralmost chaetae; a bifid robust left MxI; a gracile falcate right MxI; a short left MxII; and a long right MxII.

The gradually tapering ventralmost chaetae have also been reported in A. iricolor Montagu 1804, A. semimaculata Moore, 1911, Arabella turbidiricolor (Kang et al. 2024), A. protomutans Orensanz, 1990, Arabella aracaensis Steiner & Amaral, 2009, Arabella pulvinata Zanol & Ruta, 2015, Arabella logani Crossland, 1924, Arabella longicirrata Hartmann-Schröder, 1979, Arabella pectinata Fauchald, 1970, A. umgazanae, and A. ampulliformis Darbyshire & Kara, 2024.

However, A. iricolor, A. semimaculata, A. aracaensis, A. pulvinata, A. longicirrata, and A. turbidiricolor are easily distinguished from A. gouqi sp. nov. by having a unidentate left MxI (vs. a bidentate left MxI in A. gouqi sp. nov.) (Moore 1911; Hartmann-Schröder 1979; Blake et al. 1995; Steiner and Amaral 2009; Zanol and Ruta 2015; Darbyshire and Kara 2024; Kang et al. 2024). Other Arabella species reported as having a unidentate left MxI are as follows: Arabella cincta Hartmann-Schröder, 1962, from Chile; Arabella coeca Fauvel, 1940, from the Adriatic Sea; Arabella monroi Colbath, 1989, from Ecuador; Arabella robusta Zanol & Ruta, 2015, from Australia; Arabella atlantica Crossland, 1924, from the Cape Verde Islands; Arabella endonata; and Arabella longipedata Monro, 1931, from Australia.

Of the species introduced above, A. logani Crossland, 1924, reported from the Gulf of Suez, Egypt, is very close to A. gouqi sp. nov. in the shapes of MxI and MxII, the pygidium, the ventralmost chaetae, and the length of the posterior postchaetal lobe in relation to the chaetae. However, the original description by Crossland (1924) shows clearly the presence of five notoaciculae in A. logani that differ from A. gouqi sp. nov., which has three notoaciculae. Moreover, A. logani has 8 chaetae in anterior chaetigers, which outnumber the 5 to 7 chaetae in A. gouqi sp. nov. (Darbyshire and Kara 2024). Arabella umgazanae Darbyshire & Kara, 2024, (described from Mngazana, South Africa) is closer to A. gouqi sp. nov. in having gradually tapering ventralmost chaetae, short notopodial cirri, a bifid left MxI, and a pygidium with one pair of swollen pads. However, the two species can be primarily distinguished by the shape of the pygidial lobes: in A. umgazanae, the lobes are semicircular (vs. circular / round lobes in A. gouqi sp. nov.). There are also some differences in the number of teeth on MXI, MXII, and MXIV. In addition, in A. umgazanae, chaetae decrease in length from dorsal to ventral in all parapodia (vs. chaetal length decreases from median to both dorsal and ventral, or from dorsal to ventral, in A. gouqi sp. nov.).

Moreover, A. protomutans is similar to A. gouqi sp. nov. in the shape of MxI and MxII, the pygidium, the ventralmost chaetae, and the length of the posterior postchaetal lobe in relation to the chaetae. However, the two species differ in the denticulation of maxillae II and III (Orensanz 1990). Arabella pectinata Fauchald, 1970, (from Mexico, eastern Pacific) has all teeth evenly long in maxillae III and IV; in addition, each tooth has a short blunt tip; however, A. gouqi sp. nov. has alternately long and short teeth, with sharp tips and the longest anterior-most tooth in MxIII and MxIV (Fauchald 1970; Kang et al. 2024). On the other hand, A. ampulliformis Darbyshire & Kara, 2024, (reported from the UK) is distinguished from A. gouqi sp. nov. in possessing a pygidium with two pygidial lobes extended into short, stout cirri, ampulliform in lateral view (vs. a pygidium with two swollen pads in A. gouqi sp. nov.)

Of A. iricolor described outside of its original Europe (Montagu 1804) and before redescription by Darbyshire and Kara (2024), including those from America (Colbath 1989), Brazil (Ribeiro et al. 2018), Pakistan (Mustaquim 2000), Australia (Blake et al. 1995; Zanol and Ruta 2015), and Mexico (Fauchald 1970), all were described as having gradually tapering ventralmost chaetae, similar to A. gouqi sp. nov. However, with the exception of specimens from Pakistan for which the pygidium shape is unknown, the rest were described with a pygidium bearing at least two cirri (vs. two swollen pads in A. gouqi sp. nov.). Other specimens from Caribbean (Treadwell 1921) and Japan (Imajima and Hartman 1964; Uchida 1968) were also described with a pygidium of at least two cirri, which differs from the two swollen pads in A. gouqi sp. nov.

Comparisons of A. gouqi sp. nov. with other Arabella species reported from China, such as Arabella renierii Grube, 1877, are challenging due to the lack of detailed morphological descriptions from its original description. Other samples were reported as A. iricolor (Wang and Huang 1993; Zhou et al. 2010), but currently no updated morphological features are available for comparison. Arabella mutans Chamberlin, 1919, was listed as one of the species found in the East China Sea by Liu (2008). If the material from China resembles the original description, then Arabella mutans is easily distinguished from A. gouqi sp. nov. in having abruptly tapering ventralmost chaetae and a pygidium with four cirri (vs. gradually tapering ventralmost chaetae and two swollen pads in A. gouqi sp. nov.) (Chamberlin 1919; Zanol and Ruta 2015).

Arabella zonata Moore, 1903, reported from Japan, can easily be distinguished from A. gouqi sp. nov. by having a pygidium with a pair of short lateral cirri (Moore 1903; Hartman 1942) (vs. a pygidium forming two swollen pads with no cirri in A. gouqi sp. nov.). Other Arabella species, including Arabella mutans, Chamberlin, 1919, from Chile, and Arabella panamensis Colbath, 1989, from Costa Rica, were all described as having abruptly tapering ventralmost chaetae (Colbath 1989; Steiner and Amaral 2009; Zanol and Ruta 2015). This feature differs from A. gouqi sp. nov., which is characterized by gradually tapering ventralmost chaetae. Arabella iridescens Treadwell, 1906, reported from Hawaii (Hartman 1942), is easily differentiated from A. gouqi sp. nov. through the shape of right MXI and MXII. Arabella iridescens has a right MXI roughly triangular in outline (vs. gracile falcate right MXI in A. gouqi sp. nov.).