Sabaco Kinberg, 1867 is one of seven genera within the subfamily Maldaninae of family Maladanidae Malmgren, 1867. The other genera are Asychis Kinberg 1867, Maldane Grube 1860, Bathyasychis Detinova 1982, Chirimia Light 1991, Metasychis Light, 1991 and Paramaldane Wang and Li, 2016. Sabaco was first erected by Kinberg (1867) and synonymized with Asychis Kinberg, 1867 by Arwidsson (1907). Light (1991) revised subfamily Maldaninae and resurrected Sabaco as a valid genus. The genus Asychis is restricted as having the following characters: cephalic rim with two lateral notches, nuchal organs J- or U-shaped, without collar on chaetiger 1, one preanal achaetigerous segment, pygidium well developed, foliaceous. Sabaco is characterized by crescentic nuchal grooves and well-developed collar on the first chaetiger. Now, four species in this genus have been described from Indo-West Pacific regions: S. maculatus Kinberg, 1867, from Indonesia; S. javanicus (Augener, 1934), from Java; S. gangeticus (Fauvel, 1932), from Ganges Delta; and S. steineri Light, 1991, from the Gulf of Thailand.

Only one species, Sabaco gangeticus (Fauvel, 1932), has been recorded from Chinese coast waters (Yang and Sun, 1988; Uschakov and Wu, 1962, 1963). After reexamining the specimens identified as S. gangeticus collected in 1958–2016, we recognized them as a new species, which is a common polychaete species from sea area south of the Changjiang Estuary.

The materials were collected during the “National Comprehensive Oceanography Survey” (1958–1960), “Sino-Vietnam Joint Oceanographic Survey in the Beibu Gulf (1959–1960, 1962)” and other surveys carried out in 2015–2016. All specimens were deposited in the Marine Biological Museum (MBM), Chinese Academy of Sciences in Qingdao, China. Sampling sites are shown in Fig. 1.

The morphological observations and morphometric measurements were taken using a Zeiss Stemi 2000-C stereo microscope equipped with AxioCam MRc 5 microscopic camera. Methyl green was used for staining in order to describe the glandular pattern of the epidermis. For observations by scanning electron microscope (Hitachi S-3400 N), chaetae were dissected from the body, rinsed in distilled water for 12 h to remove salt crystals, run through a series of ethanol concentrations, and stored in 90% ethanol until observation.

(Figs 2–4)

Asychis gangeticus—Uschakov and Wu, 1962: 89–109, Pl. I, h, misidentification; 1963: 154–164, misidentification.

Asychis cf. gangeticus—Yang and Sun, 1988: 264–265, Fig. 125A–E.

Material examined. Holotype. MBM285975, East China Sea, Sta. 12300–5, 30.5°N, 123.0°E, mud sediment, 55 m, 16 October 2015.

Paratypes. MBM285976, Yellow Sea, Sta. I3, 33.0°N, 123.0°E, mud sediment, 33 m, 22 June 2016; MBM285977, Yellow Sea, Sta. I3, 33.0°N, 123.0°E, mud sediment, 32.8 m, 1 February 2015; MBM007985, East China Sea, Sta. 4059, 29°N, 122°E, mud sediment, 42 m, 26 October 1959.

Other materials examined. MBM201448, South China Sea, Sta. 7204, 20.61°N, 108.5°E, mud sediment, 49 m, 24 April 1962; MBM201447, South China Sea, Sta. 6285, 19°N, 105.75°E, 9 December 1959; MBM201446, South China Sea, Sta. 6279, 19.75°N, 106°E，mud sediment, 6 July 1960; MBM201445, South China Sea, Sta. 6214，20.75°N, 108.5°E, mud sediment, 42 m, 14 December 1959; MBM201444, South China Sea, Sta. 6211, 21.5°N, 108.5°E, silt sediment, 12 m, 18 July 1962; MBM201439, South China Sea, Sta. 6213, 20.75°N, 108.5°E, silt sediment, 27.5 m, 17 July 1960; MBM201534, South China Sea, Sta. 7101, 21.5°N, 109°E, silt sediment, 10 m, 26 August 1962; MBM007989, East China Sea, Sta. 4059, 29.25°N, 122.5°E, mud sediment, 42 m, 26 October 1959; MBM197508, East China Sea, Sta. G5, 29.333°N, 122.667°E, 46 m, 25 May 1986; MBM012649, South China Sea, Sta. 6074, 21.75°N, 113°E, 18 m, 22 April 1959; MBM012612, South China Sea, Sta. 7204, 20.667°N, 108.5°E, 45 m, 27 August 1962; MBM012601, South China Sea, Sta. 6200, 20.25°N, 109°E, 34.5 m, 15 April 1960; MBM012508, East China Sea, Sta. 6245, 17°N, 108°E, 27 m, 23 February 1962; MBM012519 South China Sea, 6074, 21.75°N, 113°E, 19 m, 6 July 1959; MBM012662, East China Sea, Sta. 4061, 29°N, 122.25°E, 27 m, 21 October 1959; MBM012592, South China Sea, Sta. 6200, 20.25°N, 109°E, mud sediment, 32.5 m, 13 July 1960; MBM012661, South China Sea, Sta. 6106, 20.5°N, 112°E, silt sediment, 72 m, 2 April 1959; MBM012750, East China Sea, Sta. 4133, 29.5°N, 122.75°E, silt sediment, 10 July 1959.

Diagnosis. Body with 19 chaetigers. Anterior part of prostomium broad. Nuchal grooves crescent and transverse. Preanal achaetigerous segments lacking. Pygidium trumpet-shaped with a disc-shaped dorsal lobe. Notochaetae two kinds: simple capillaries and spirally fringed notosetae. Dark-brown pigmentation spots on nuchal grooves and first three chaetigers.

Description. Holotype about 130 mm long and 4.5 mm wide. Body cylindrical, consisting of 19 chaetigers and a trumpet-shaped pygidium. Head obliquely truncate with an elliptical cephalic plate. Cephalic rim smooth, with two deep lateral notches (Fig. 2d). Lateral parts of the rim low and anteriorly fused with prostomium, while posterior part of the rim well-developed and forming a deep pocket coving the posterior part of the cephalic keel. The keel prominent, anteriorly narrow and getting broad posteriorly. Two nuchal grooves deep, crescent and transverse with dark-brown pigmentation spots on its distal ends (Figs 2d and 3c–e).

The first four chaetigers obviously biannulate in dorsal view (Fig. 2a). Two remarkable dark-brown pigmentation spots on each anterior part of first three chaetigers (Fig. 2b). Collar on chaetiger 1 with two deep lateral slits (Fig. 2a and 3d). Dorsal margin of collar smooth or slightly crenulate. Ventral margin crenulate with a projecting swelling (Fig. 3b). Anterior chaetigers with epidermal glands and heavily stained with methyl green. The glandular pattern on the dorsum of chaetigers 3–6 noteworthy (Figs 2a, b). Chaetigers 3 having a large glandular pad with inward contraction on its middle (Fig. 2b). Glandular pad V-shaped on chaetiger 4 and 5, crescent on chaetiger 6.

The last two chaetigers shorter than preceding ones (Fig. 2c). Preanal achaetigerous segments lacking. Pygidium well developed, trumpet-shaped (Figs 2e, f). Dorsal lobe of anal plate disc-shaped with smooth margin, separated from ventral lobe by deep lateral notches. A large broad mid-ventral notch on the ventral lobe (Fig. 2f).

Notochaetae two rows, including long capillaries with stout proximal shafts and slender companion notochaetae. Long capillary notochaetae simple on anterior chaetigers (Fig. 4a), posteriorly becoming spirally fringed in distal ends with spirally pectinate bands imbricated over the main shaft (Figs 4e, f). Companion notochaetae on anterior chaetigers short and slender (Figs 4a, b), on middle and posterior chaetigers getting longer and keel-shaped with lateral projection and long tips (Figs 4c, d, g).

Uncini present from second chaetiger, arranged in a row on neuropodial tori; rostrate in shape and similar on all chaetigers (Figs 4h–j). Capitium of uncinus with 3–5 transverse arcs of small teeth (Figs 4h–j). First arc with about 8 small teeth larger than on other arcs. A tuft of bristles under the main fang.

Etymology. The species name “sinicus” is a Latin word meaning “Chinese”, and refers to the type location of this new species.

Remarks. This species was previously misidentified as Asychis gangeticus Fauvel, 1932, which was later accepted as Sabaco gangeticus by Light (1991). The two species are similar on shape of cephalic plate and collar but differ in the pigmentation spots. Sabaco sinicus has very stable pigmentation spots on cephalic plate and first three chaetigers, which is absent in S. gangeticus. Besides, the two species differ in the form of the pygidium. The dorsal lobe of pygidium in Sabaco sinicus sp. n. is disc-shaped, but that of S. gangeticus is triangular and has two lateral lobes rolled inwards. An identification key to the species of Sabaco recorded in Indo-West Pacific regions is provided below.

Geographic distribution. Significant distribution change of this species has occurred since the 1950s. Specimens were collected during the “National Comprehensive Oceanography Survey” (1958–1960) range from Beibu Gulf (17°N) northward to 30°N. Studies of Uschakov and Wu (1963) also showed that this species was distributed in the warm-temperate waters not beyond the south border of the Changjiang Estuary. However, recent surveys indicate that its distribution extend northward by 2°–3° latitudes (Fig. 1). This distribution change is perhaps a response to global climate change. There is growing evidence that warm-water species are currently extending their range northward under global climate change (Sun et al., 2016).

1. Collar on chaetiger 1 with a wide lateral indentation (see Light 1991, Figs 2I–J); ventral lobe of pygidium without a midventral notch ........................................................................S. steineri

– Collar on chaetiger 1 with a narrow lateral slit; ventral lobe of pygidium with a distinctive midventral notch ...............................2

2. Dark-brown pigmentation spots on nuchal grooves and first three chaetigers; Dorsal lobe of pygidium reduced, disc-shaped ....................................................................S. sinicus sp. nov.

– Dark-brown pigmentation spots lacking; dorsal lobe of pygidium reduced or expanded................................................................3

3. Dorsal lobe of pygidium reduced, disc-shaped ......S. maculatus

– Dorsal lobe of pygidium expanded, with ventrallateral flaplike projections ..................................................................................4

4. Dorsal lobe of pygidium with a longitudinal keel on midventral surface ......................................................................S. gangeticus

– Dorsal lobe of pygidium without a longitudinal keel on midventral surface........................................................................S. javanicus

The authors are grateful to José Eriberto De Assis for his suggestions and providing many important references. Thanks to all the managers of the MBMCAS for their help with specimens sorting.