Holotype, MBM010010, Beibu Gulf, 21°15'N, 108°06'E, 91m, sandy mud, coll. Xiutong Ma, 11 Feb 1959. Paratypes, MBM200142, Beibu Gulf, 18°30'N, 107°00'E, 66m, sandy beach, coll. Zhengang Fan, 10 Sep 1960; MBM200143, Beibu Gulf, 18°00'N, 107°45'E, 90m, silty mud, coll. Ruiping Sun, 9 Apr1962; MBM200144, Beibu Gulf, 18°00'N, 108°00'E, 93m, sandy mud, coll. Xiutong Ma, 11 Dec 1959; MBM200145, Beibu Gulf, 18°30'N, 107°00'E, 66m, sandy beach, coll. Xiutong Ma, 9 Dec 1962; MBM200122, Beibu Gulf, 18°00'N, 107°00'E, sandy mud, coll. Zhengang Fan, 14 Feb1960.

Prostomium globular, posterior margin bilobed, with two pairs of eyes, anterior pair semicircular in shape. Medial caruncle conspicuous, arising from and confluent with posterior prostomial margin, medial lobe projecting above paired lateral lobes. Parapodia biramous, rami widely separated, with single dorsal and single ventral cirrus. Branchiae dendritic, present from chaetiger 3 onwards, with more than 40 pairs.

Holotype (MBM010010) incomplete, lacking posterior part, 17 mm long, 2 mm wide excluding chaeta, with 45 chaetigers. Body elongate, nearly rectangular in cross section, tapering posteriorly. Color in alcohol pale yellow, without color pattern.

Prostomium (Fig. 1A) divided into two parts by transverse groove. Anterior lobe rounded, with pair of cirriform antennae dorsally and similar pair of palps laterally, palps with three distinct articulations. Posterior lobe heart-shaped, bilobed along posterior margin, with median antenna and two pairs of reddish eyes. Anterior pair of eye spots semicircular in shape and large, posterior pair of eye spots rounded and small. Median antenna conical, slightly shorter than paired antennae, located at posterior margin of prostomium. Buccal opening occupying two chaetigers (Fig. 1B).

Figure 1. 

Linopherus beibuwanensis sp. n. A Anterior end in dorsal view B Anterior end in vental view C left parapodia of chaetiger 14 in frontal view D branchiae of chaetiger 28 E harpoon notochaeta F forked neurochaeta G slender capillary chaeta. Scale bars: A–B, 200 μm; C–D, 250 μm; E–F, 50 μm; G, 20 μm.

Caruncle rectangular and conspicuous, medial lobe projecting above paired lateral lobes. Caruncle restrict to chaetiger 1, equal in length to posterior prostomial lobe of prostomium, reaching to anterior margin of chaetiger 2, arising medially and confluently with posterior prostomium (Fig. 1A).

All parapodia biramous, with chaetiger 1 greatly reduced, chaetiger 2 forming first dorsally complete ring. Parapodia with slender cirri (Fig. 1A, C), both notopodia and neuropodia well-developed, chaetal sac forming low rounded lobe. First two parapodia with longer and more conspicuous cirri than other parapodia; ventral cirri of chaetiger 2 longest, each with four articulations (Fig. 1A). Succeeding parapodia with tapering and rather short ventral cirri.

Branchiae present from chaetiger 3, located on posterior face of each notopodia (Fig. 1A), arising as tufts, dendritically branched, decreasing along body and disappearing at approximately chaetiger 42; chaetiger 1 branchiae with five terminal filaments, chaetiger 17 branchiae with 29 or 26 terminal filaments, chaetiger 28 branchiae with 15 filaments (Fig. 1D), chaetiger 42 branchiae with 6 or 7 filaments.

Notochaetae of three types: stout harpoon chaetae, numbering 13–24 per fascicle (Fig. 1E); slender capillary chaetae (Fig. 1G), faintly serrated, without basal spur, many broken; notoacicula, numbering 3–4 per fascicle, hastate (Fig. 1C). Neurochaetae of three kinds: forked chaetae, short limb reduced as spur, with thin shafts and long, distally serrated cutting margins (Fig. 1F); slender, capillary chaetae, smooth; neuroacicula, numbering 2–3 per fascicle, slightly hastate, slender than notoacicula (Fig. 1C).

All specimens examined are incomplete lacking the posterior part. Specimen size varies from 1.9 to 2.5 mm in maximum width. Branchial chaetiger range varies from 3 to 31–40. The maximum number of branchial filaments varies from 20–29. One female paratype (MBM200143) with eggs in posterior coelomic cavity, ranging in diameter from 71μm to 90 μm.

The species is named after Beibuwan, the Chinese name for the Beibu Gulf (Tonkin Gulf), South China Sea.

Only known from Beibu Gulf, South China Sea.

Linopherus beibuwanensis sp. n. is referred to Linopherus because of the arrangement of branchiae and the reduced but characteristic caruncle. Linopherus beibuwanensis sp. n. is unique in this genus for the combined shape of its prostomium and the caruncle. While the posterior prostomial margin is straight (not bilobed) in most Linopherus species, their caruncles are also either absent or reduced. In the latter case, species with reduced caruncles typically exhibit a transverse groove that appears to isolate the caruncle from the prostomium, which is usually partly or completely concealed by the second chaetiger (see Langerhans 1881; Fauvel 1932; Monro 1933, 1937; Treadwell 1941; Wesenberg-Lund 1949; Knox 1960; Fauchald 1972a, b; Kudenov 1975; Wu et al. 1975; Kudenov and Blake 1985; San Martín 1986; Salazar-Vallejo 1987).

Linopherus beibuwanensis sp. n. is similar to L. abyssalis in having the posterior prostomial margin bilobed. The new species can be distinguished from the latter by the presence of eyes and arrangement of branchiae; the former has two pairs of eyes and more than 40 pairs of branchiae, while the latter has no externally detectable eyes and only 5-6 pairs of branchiae.

Caruncle morphology appears to be an essential character which can be used to distinguish the species of Linopherus. However, Linopherus species are small in size, and so their caruncle morphology is imperfectly known since it is usually concealed by constriction of the anterior chaetigers. Clearly, further research on this taxon is necessary.