Mediomastus species are widely distributed around the world. Hartman (1944) first established the genus Mediomastus for M. californiensis Hartman. Since then, 16 species (Table 1) have been reported from various parts of the world. These species share similar morphological characters: thorax with 11 segments, including one achaetous segment; segments 2–5 only with capillary chaetae in both rami; thereafter, with hooded hooks (Hartman, 1944; Day, 1967; Fauchald, 1977; Warren et al., 1994; Green, 2002). Capitellid morphology is simple and relatively few distinct morphological characters can be used for classification, which might cause confusion in the identification of some species (García-Garza and De León-González, 2011; García-Garza et al., 2012). It is also the case for the separation of Mediomastus species. Mediomastus exhibited high diversity in Japanese waters and four distinct Mediomastus species were recorded, M. californiensis in the Sagami Bay (Imajima, 2006), M. opertaculeus near Hokkaido (Tomioka et al., 2013), M. duobalteus and M. hanedaensis in the Tokyo Bay (Tomioka et al., 2014). By contrast, Chinese specimens were generally identified as M. californiensis, although specimens were collected from various parts of Chinese waters (Wang et al., 2006; Du et al., 2011; Chen et al., 2012; Ji et al., 2015). However, the original description showed that M. californiensis was mainly distributed to the east and west coasts of North America. Reports of M. californiensis in other parts of the world were unconfirmed. In this study, we examined the Mediomastus specimens from the southeast coast of China. Morphological observations and staining pattern with methyl green indicated that the specimens were not M. californiensis but rather a new species, M. chinensis sp. nov.

The Mediomastus specimens, from subtidal soft sediments at four localities along the southeast coast of China, were collected during marine environmental surveys conducted between 2015 and 2017. Water depths ranged from 6 m to 30 m. All specimens were fixed with 7% diluted formalin in seawater. Type materials of the new species and the additional specimens examined are deposited in the Third Institute of Oceanography, State Oceanic Administration, Xiamen, China. The methyl green staining pattern (MGSP) was used to observe some morphological structures presenting specific patterns of coloration for the species, following the protocol of Warren et al. (1994). Morphological terminology also follows that of Warren et al. (1994).

Class Polychaeta Grube, 1850

Order Capitellida Fauchald, 1977

Family Capitellidae Grube, 1862

Genus Mediomastus Hartman, 1944

Mediomastus chinensis sp. nov.

Type material. Holotype: TIO-BTS-Poly-1101, 1 specimen, Jieshi Bay, Lufeng City, Guangdong Province, 22°49.08′N, 115°36.96′E, 8 m depth, mud, incomplete; Paratypes: TIO-BTS-Poly-1102, 3 specimens, same collection site as holotype, 22°44.22′N, 115°46.32′E, 14 m, mud, all incomplete. All specimens collected in April 2017.

Additional materials examined. One hundred and fourteen specimens collected from the following four localities along the southeast coast of China: (1) 56 specimens, subtidal water off Xiaguan Town, Wenzhou City, Zhejiang Province: Sta. CNHD03, 4 specimens, 27°11.23′N, 120°32.37′E, 13 m, mud, 15 November 2016; Sta. CNHD14, 6 specimens, 27°14.35′N, 120°40.04′E, 17 m, mud, 16 November 2016; Sta. CNHD16, 4 specimens, 27°4.62′N, 120°34.04′E, 19 m, mud, 16 November 2016; Sta. CNHD18, 10 specimens, 27°6.56′N, 120°25.18′E, 10 m, mud, 17 November 2016; Sta. CNHD19, 7 specimens, 27°21.44′N, 120°36.80′E, 11 m, mud, 16 November 2016; Sta. CNHD26, 11 specimens, 27°12.90′N, 120°22.99′E, 18 m, mud, 17 November 2016; Sta. CNHD35, 11 specimens, 27°11.54′N, 120°33.89′E, 13 m, mud, 15 November 2016; Sta. CNHD42, 3 specimens, 27°55.63′N, 120°32.07′E, 26 m, mud, 17 November 2016. (2) 11 specimens, waters around Xiamen Island, Xiamen City, Fujian Province: Sta. XM3, 5 specimens, 24°31.40′N, 118°4.30′E, 10 m, mud, 13 August 2015; Sta. XM6, 5 specimens, 24°25.32′N, 118°0.48′E, 11 m, mud, 12 August 2015; Sta. XM9, 1 specimen, 24°26.45′N, 118°11.25′E, 24 m, muddy sand, 12 August 2015. (3) 32 specimens, Jieshi Bay and adjacent water, Lufeng City, Guangdong Province: Sta. 07, 1 specimens, 22°33.84′N, 115°56.34′E, 24 m, mud, 18 April 2017; Sta. 08, 3 specimens, 22°43.86′N, 115°51.42′E, 20 m, mud, 17 April 2017; Sta. 16, 3 specimens, 22°45.36′N, 115°47.16′E, 14 m, mud, 17 April 2017; Sta. 17, 4 specimens, 22°49.08′N, 115°42.6′E, 8 m, mud, 19 April 2017; Sta. 18, 3 specimens, 22°44.58′N, 115°39.78′E, 11 m, mud, 19 April 2017; Sta. 21, 4 specimens, 22°49.08′N, 115°36.96′E, 8 m, mud, 19 April 2017; Sta. 24, 5 specimens, 22°38.19′N, 116°12.71′E, 30 m, mud, 17 April 2017; Sta. 32, 3 specimens, 22°40.68′N, 115°46.8′E, 9 m, mud, 19 April 2017; Sta. 40, 2 specimens, 22°44.22′N, 115°46.32′E, 14 m, mud, 17 April 2017; Sta. 48, 4 specimens, 22°53.2′N, 116°17.06′E, 20 m, mud, 18 April 2017. (4) 15 specimens, Daya Bay, Shenzhen City, Guangdong Province: Sta. D3, 1 specimen, 22°35.55′N, 114°33.0′E, 9 m, mud, 16 April 2016; Sta. D5, 7 specimens, 22°37.5′N, 114°35.4′E, 10 m, mud, 16 April 2016; Sta. D6, 3 specimens, 22°34.7′N, 114°33.5′E, 12 m, sandy mud, 16 April 2016; Sta. D13, 4 specimens, 22°34.93′N, 114°36.84′E, 14 m, mud, 16 April 2016.

Description. Holotype incomplete, with 70 segments, measuring 39.3 mm long by 1.1 mm wide. Body thread-like. Thoracic segments solid and muscular. Color change from brown on first 10 chaetigers to whitish brown on following chaetigers (Fig. 1). Epithelium smooth. Prostomium short, conical with round-tipped palpode. Eyespots not observed. Eversible proboscis with numerous minute papillae. Peristomium achaetous, twice as wide as long, slightly longer than first chaetiger. Paratypes incomplete, with 37–83 segments measuring 22.0–46.7 mm long and 1.0–1.3 mm wide.

Thorax with 10 chaetigers, all more or less the same width; segmental length about 0.33–0.5 times width; chaetigers 6–9 longer (Fig. 2a). Thorax slightly flattened ventrally. Notopodia lateral in first chaetiger, moving dorsally to anterior abdomen, and neuropodia ventrolateral. Chaetigers 1–9 biannulate, each with clear inter-segmental grooves; chaetal fascicles inserted at midsegment. Chaetiger 10 transitional, shorter and posteriorly narrower, multiannulate, chaetal fascicles posterior to midsegment (Fig. 2a). Lateral organs present throughout the thorax, located between noto- and neuropodia, closer to notopodia. Chaetigers 1–4 only with narrow, bilimbate capillary chaetae; noto- and neuropodia each 12–25 in number per fascicle. Chaetigers 5–10 with hooded hooks only; 12–21 notopodial hooks per fascicle while 13–21 neuropodial hooks per fascicle. Shape of thoracic hooks similar between noto- and neuropodia (Figs 2d, e), with long and straight shaft, without constriction, indistinct shoulder. Short, triangular fang surmounted by several rows of teeth. Hood long, about 3–5 times width, tapering gradually to shaft, longer in notopodia.

Transition between thorax and abdomen marked by constriction and increase in segment length. All abdominal segments multiannulate, only with hooded hooks throughout. Anterior abdominal segments slightly longer but narrower than thoracic segments. Length of segments 0.7 times width in anterior abdomen (Fig. 2a), increasing to 1.5 times width in mid-abdomen (Fig. 2b), then decreasing to 0.8 times width in posterior abdomen (Fig. 2c). Posterior abdominal segments with parapodial ridges (evident from chaetiger 60). Chaetal fascicles positioned near posterior end of segments. Anterior abdomen with 18–22 hooks per fascicle, reducing to 5–8 hooks per fascicle in far posterior. Structure of abdominal hooks similar throughout abdomen, with shorter shaft, developed shoulder, distinct constriction (Fig. 2f), markedly different from those in thorax. Three rows of subapical teeth above main fang in lateral view. Hood shorter than in thoracic hook, length about 1.5 times width. Pygidium unknown, due to absence of posterior body. Branchiae not observed.

Methyl green staining pattern. Light green staining from peristomium to prechaetal part of chaetiger 5 with minute spots, denser on chaetigers 2–3; dark green staining uniformly extending from postchaetal half of chaetiger 5 to anterior of chaetiger 10; posterior of chaetiger 10 stained light green, with sparsely coarse spots (Figs 1 and 3). Anterior abdominal segments stained light green, with sparsely minute spots.

Distribution. Mediomastus chinensis sp. nov. was found in several localities along the southeast coast of China, from Zhejiang Province to Guangdong Province. This species inhabits soft sediments of shallow coastal waters. We are not sure whether Mediomastus chinensis sp. nov. lives in northern China seas, since no specimens from those areas were examined in this study.

Etymology. The specific name refers to the geographical region in which this species lives.

Remarks. Mediomastus chinensis sp. nov. is characterized by its body shape and the structure of its chaetae. Among all described Mediomastus species, M. chinensis sp. nov. is similar to the other five known species: M. australiensis, M. calliopensis, M. capensis, M. cirripes, and M. fragilis by the segmental annuli (thoracic chaetigers biannulate and abdominal ones multiannulate); by the position of chaetal fascicles, which are situated at midsegment on chaetigers 1–9 but more posteriorly on the subsequent chaetigers; and by the possession of parapodial ridges on the posterior abdomen. However, M. chinensis sp. nov. can be readily distinguished from M. capensis and M. cirripes by the shape of the capillary chaetae, which were hook-tipped in M. capensis and broad-winged in M. cirripes, unlike those of the new species. When compared to M. fragilis and M. australiensis, abdominal hooks of M. chinensis sp. nov. have a distinct constriction, which was not observed in the former two species. In M. calliopensis, capillary chaetae are unilimbate and hood length of thoracic hooks are shorter than width, whereas M. chinensis sp. nov. has bilimbate capillaries and hood of thoracic hooks was 3–5 times as long as wide. Furthermore, staining pattern with methyl green is unique in M. chinensis sp. nov., different from other species. The new species also resembles N. warrenae reported from the Andaman Sea in chaetal structure and methyl green stain on chaetigers 5–10, but differs from the latter by the location of chaetae on chaetiger 9, the shape of chaetiger 10, and methyl green stain on first 4 chaetigers. The new species can be distinguished from M. californiensis reported from various seas around China. The distinct difference between the two species might be that the structure of thoracic and abdominal hooks is similar in M. californiensis, and that chaetal fascicles are inserted posterior to midsegment on chaetigers 1–9 and capillary chaetae on chaetigers 1–4 are unilimbate, all of these different from the new species. Thus, the records of M. californiensis in Chinese waters need confirmation.

The genus Mediomastus differs from other described capitellid genera in its body shape and the thoracic chaetal formula. According to Hartman’s (1944) original description of Mediomastus, there are 10 thoracic chaetigers, of which the first four have only capillary chaetae and the following six have hooded hooks. However, juvenile Heteromastus also conforms to the above definition, which could cause confusion. For this reason, it is important to collect adult individuals for correct identification. Mediomastus mostly resembles Parheteromastus in that both have four thoracic chaetigers with capillary chaetae. However, the latter genus has 11 thoracic chaetigers rather than 10. This indicates that the number of thoracic segments is an important diagnostic character that can be used to identify capitellid genera. One of the most useful diagnostic features in the differentiation of species within Mediomastus is chaetal characteristics, including the structure of hooks and the size of the hood, which should be carefully examined under oil immersion. Although Mediomastus specimens are usually incomplete, they can be identified with anterior fragment that most of diagnostic characters are located on. Dental formula of hooks might be useful for identification, yet they are difficult to observe. Along with morphological characters, MGSP is also a useful tool to determine whether the specimens belong to the same species.

The authors are grateful to Wang Jianjia and Xiang Peng for their assistance in photographing type materials and additional specimens. The authors also thank the members of Benthos Laboratory, Laboratory of Marine Biology and Ecology, Third Institute of Oceanography, State Oceanic Administration, for helping to collect specimens.