Sternaspis radiata, Wu, Xuwen & Xu, Kuidong, 2017

Wu, Xuwen & Xu, Kuidong, 2017, Diversity of Sternaspidae (Annelida: Terebellida) in the South China Sea, with descriptions of four new species, Zootaxa 4244 (3), pp. 403-415 : 406-409

Sternaspis radiata sp. nov.

Figures 1 , 3 , 4

Type material. South China Sea. Holotype: MBM 020950 , R/V 220, Sta. 6034, 22°15’N, 115°00’E, 41 m, soft mud, coll. Jingzuo Qu , 12 Apr. 1960. Paratypes: MBM 020952 (1 spec.), R/V 220, Sta. 6034, 22°15’N, 115°00’E, 41.6m, soft mud, coll. Shaozong Wu, 8 Jan. 1960. Body 22.6 mm long, 9.2 mm wide; abdomen 12.4 mm long; left shield plate 3.20 mm long, 3.37 mm wide. MBM 020957 (1 spec.), R/ V Hanggong, Sta. 6051, 21°45’N, 114°00’E, 43.5m, soft mud, coll. Weiquan Zhang , 9 Jul. 1959. Body 28.4 mm long, 12.5 mm wide; abdomen 17.2 mm long; left shield plate 3.5 mm long, 3.8 mm wide. MBM 283395 (3 spec.), Xinying, Hainan Island, HBR-3, approximate co-ordinate: 19°54’N, 109°30’E , 15 Dec. 2010. Three anterior ends retracted. Body 7.9–10.5 mm wide; abdomen 13.2–16.0 mm long; left shield plate 2.89–3.70 long, 3.01–3.84 mm wide.

Additional material. South China Sea. MBM 283397 (3 juv.), same collecting information as the holotype. One anterior end retracted. Body 5.6–13.5 mm long; left shield plate 1.60–2.51 mm long, 1.66–2.12 mm wide . MBM 020953 (1 juv.), R/V 220, Sta. 6022, 22°00’N, 116°00’E, 87m, muddy sand, coll. Jingzuo Qu, 19 Jul. 1959. Anterior end retracted, body 7 mm wide; abdomen 10 mm long; left shield plate 2.2 mm long, 2.1 mm wide. MBM 020955 (1 spec.), R/V 220, Sta. 6044, 22°00’N, 114°30’E, 42m, soft mud, coll. Weiquan Zhang, 20 Mar. 1959. MBM 020974 (2 spec.), R/V 220, Sta. 6034, 22°15’N, 115°00’E, 42m, sandy mud, coll. Weiquan Zhang, Song, 11 Dec. 1959. MBM 021252 (2 spec.), R/V 131, Sta. 6192, 21°00’N, 109°24’E, 16 m, soft mud, coll. Zhican Tang, 19 Apr. 1959. Body 21.0–31.0 mm long, 8.5–12.0 mm wide; abdomen 13.0– 20.5 mm long; left shield plate 2.46–3.60 long, 2.56–4.19 mm wide. MBM 021247 (1 spec.), R/V 131, Sta. 6131, 20°00’N, 111°15’E, 43 m, soft mud, coll. Zhican Tang, 12 Jul. 1959. MBM 021248 (1 spec.), R/V S, Sta. 6045, 21°45’N, 114°30’E, 64.5 m, coll. Wu, 9 Apr. 1959. Body 28.0 mm long, 9.5 mm wide; abdomen 16 mm long; left shield plate 3.26 mm long, 3.66 mm wide. MBM 021249 (1 spec.), R/V 171, Sta. 6131, 20°00’N, 111°15’E, 52 m, soft mud, coll. Xiutong Ma, 25 Apr. 1959. Left shield plate 2.46 mm long, 2.71 mm wide. MBM 021251 (1 spec.), R/V 228, Sta. 6023, 21°45’N, 116°00’E, 105.5 m, sandy mud, coll. Yongliang Wang, 9 Jan. 1960.

Description. Holotype (MBM 020950) complete, 21.7 mm long, 9.9 mm wide; abdomen 12.6 mm long; left shield plate 3.01 mm long, 3.21 mm wide. Body pale to whitish, introvert fully exposed, with similar pigmentation as abdomen, constriction or waist segments relaxed ( Fig. 3 A).

Prostomium hemispherical, projected, whitish. Eyespots not seen. Boundary distinct in side and back of prostomium ( Fig. 3 B). Peristomium round, with few papillae, barely extending laterally and ventrally to anterior margin of first chaetiger. Mouth circular, projected, slightly wider than prostomium, covered by minute papillae ( Figs 3 B; 4F).

First three chaetigers each with 14–17 bright bronze hooks per side. Hooks tapering to sharp points, subdistally darker, gradually shortened towards ventrolateral side ( Figs 3 B, C; 4F). Introvert covered by fine papillae, papillae partly eroded in some specimens ( Fig. 3 B, C). Genital papillae digitate, protruding ventro-laterally from inter-segmental groove between segments 7 and 8. Anterior abdomen with 7 segments, covered with minute papillae on ventral side ( Fig. 3 A). Capillaries not seen.

Ventro-caudal shield brick red, darker in central areas; radial stripes well-defined on lateral plates, concentric lines distinct near margin ( Fig. 3 E–H). Shield width two times larger than length (width-length ratio: 2.06–2.32). Anterior margins angular, anterior depression deep; anterior keels half as long as anterior depression, covered by translucent integument layer. Suture visible throughout shield or only fused in posterior region. Lateral margins rounded, smooth, expanded medially. Fan slightly ribbed, slightly expanded beyond posterolateral corners; fan margin crenulated or smooth, with a shallow and wide median notch ( Fig. 3 E–H).

Marginal chaetae including 10 lateral fascicles and 5 posterior ones ( Fig. 3 E–H); lateral fascicles successively longer from anterior to posterior, last two bundles closer to each other, chaetae arranged in oval arrangement; posterior fascicles similar in length, chaetae in linear arrangement. Peg chaetae tapering, with stout base in cross section. A small fascicle of delicate capillary chaetae, 2–3 times longer than posterior fascicles, between peg chaetae and outermost lateral fascicle.

Branchiae abundant ( Figs 3 A; 4A), inserted on two plates. Branchial filaments long, slender, curled or spiraled; inter-branchial papillae curled, often with fine sediment particles. Branchial plates short, slightly divergent, distal portion blunt ( Figs 3 D; 4C).

Variation. There are three variations in the pigmentation and shape of the ventro-caudal shield. First, the shields are mainly brick red ( Fig. 3 F–H) but sometimes are yellowish-brown ( Figs 3 E; 4B, E). Second, the concentric bands are absent in most individuals but found in the marginal and sub-marginal areas in some specimens ( Figs 3 H; 4B). Third, the fan slightly expands posteriorly forming a shallow median notch, but in rare cases it can be at the same level as the posterolateral corners without a distinct median notch ( Fig. 3 G).

Juvenile shields. Immature specimens illustrated in Figure 4 A were separated out from the same sample with the holotype. They have similar characters of the anterior part, ventro-caudal shield and branchial plates, and are herein identified as juveniles of S. radiata sp. nov. There are well defined clusters of papillae arranged in rows on the dorsum of the abdomen, which are absent in the adults ( Fig. 4 C). Their shields show a reddish-orange or yellowish-brown pigmentation with distinct radial stripes on the lateral plates ( Fig. 4 B–E) and sometimes a brown concentric band on the shield margin ( Fig. 4 B). Width-length ratio of the shield ranges from 1.69 to 2.15. The concentric lines are not well defined, but the ribs are present in bigger specimens ( Fig. 4 D). The anterior keel is well developed, more than half as long as the anterior depression ( Fig. 4 B, D). The suture extends throughout the shield. The fan margin expands beyond the posterolateral corners with a shallow median notch ( Fig. 4 B, D, E).

Etymology. The species name is derived from the Latin adjective radiatus (radiate), referring to its distinct radiate stripes on the ventro-caudal shield.

Distribution. Continental shelf of the northern South China Sea (water depth 16–105.5 m) ( Fig. 1 ).

Remarks. Sternaspis radiata sp. nov. can be distinguished from its congeners by a combination of characters as follows: the shield width is two times larger than its length; the lateral plates have well-defined radial stripes; both concentric lines and radial ribs are present; and the fan margin is slightly expanded beyond the posterolateral corners, with a shallow median notch.

Sternaspis radiata sp. nov. resembles S. affinis Stimpson, 1864 , S. chinensis Wu et al., 2015 and S. scutata ( Ranzani, 1817) because their ventro-caudal shields have concentric lines and radial ribs, and the posterior margins are expanded beyond the posterolateral corners with a shallow median notch. However, S. radiata sp. nov. can be separated from these similar species by the characters that the shield is much wider than long with conspicuous radial stripes on the lateral plates. On the other hand, Sternaspis chinensis is unique in having remarkable concentric bands from the margin of the shield to the center, and S. scutata has a different shield with posterior margin markedly expanded beyond the posterolateral corners. These species are also different in their geographic distribution: S. affinis is found from the NE Pacific Ocean, S. chinensis from the Bohai Sea to the northern East China Sea, and S. scutata from the Mediterranean Sea to the English Channel.

Sternaspis radiata sp. nov., also resembles S. britayevi Zhadan et al., 2017 discovered in the Gulf of Tonkin of Vietnam in the shape of the shield. However, S. radiata sp. nov. has a slightly ribbed shield, and small and divergent branchial plates, whereas in S. britayevi the shield is distinctly ribbed and the branchial plates are expanded and nearly parallel.

It is worthy of note that the geographic distribution of S. radiata sp. nov. overlaps with that of S. spinosa Sluiter, 1882 , as described below ( Fig. 1 ). The two species occasionally co-existed in the same samples collected from the northern South China Sea. Both species have relatively wide shields (width-length ratio> 2), but S. spinosa differs in having a shield with a truncate fan margin and no radial stripes.

Figures

FIGURE 1. Distribution of four sternaspid species (except Sternaspis sunae sp. nov.) in the northern South China Sea based on the examined material. Note: The specimens of S. sunae were collected from the sea area of the Nansha Islands without information of exact co-ordinates.

FIGURE 3. Sternaspis radiata sp. nov., holotype (A – C, E: MBM 020950) and paratypes (D, F: MBM 020952; G: MBM 020957; H: MBM 283395). A. Complete specimen in ventral view; B. Anterior end in frontal view; C. Introvert hooks of anterior three chaetigers; D. Branchial plates in rear end view, branchiae removed; E – H. Shield morphology. Scale bars: 2 mm (A), 0.5 mm (B – D) and 1 mm (E – H).

FIGURE 4. Juveniles of Sternaspis radiata sp. nov. A. Three specimens (A 1 – A 3: MBM 283397); B. Shield morphology of specimen A 1; C. Branchial plates of specimen A 2 in rear end view, branchiae removed; D. Shield morphology of another specimen (MBM 020953); E. Shield morphology of specimen A 2; F. Anterior end of specimen A 2. Scale bars: 2 mm (A), 1 mm (C) and 0.5 mm (B, D – F).