Terebellides guangdongensis Zhang & Hutchings 2018, n. sp.

Terebellides guangdongensis n. sp.

Figures 2–5

Material examined. Holotype. MBM 286009 in IOCAS, Guangdong, Nanao Island, South China Sea, 23.4982°N 117.2546°E, 15.0 m, mud, Sep 2015.

Paratypes. MBM286010 (2 specimens), MBM286024 (1 specimen mounted for SEM) in IOCAS: Guangdong, Nanao Island, South China Sea, 23.4982° N 117.2546° E, 15.0 m, mud, Sep 2015; MBM286011 (3 specimens) in IOCAS: Guangdong, Daya Bay, South China Sea, 22.6078° N 114.7286° E, 12.0 m, mud sand, Sep 2013; MBM286012 (2 specimens), MBM286021 (4 specimens) in IOCAS: Guangdong, Daya Bay, South China Sea, 22.5764° N 114.6839° E, 17.0 m, mud, Jun 2015; MBM286013 (1 specimen), MBM286015 (1 specimen) in IOCAS: Guangdong, Daya Bay, South China Sea, 22.7158° N 114.6644° E, 9.0 m, mud, Jun 2015; MBM286014 (1 specimen) in IOCAS: Zhanjiang, Guangdong, Beibu Gulf, South China Sea, 20.4099° N 109.7840° E, 17.5 m, mud, May 2016; MBM286016 (4 specimens) in IOCAS: Maoming, Guangdong, the coast of Yuexi, South China Sea, 21.2500° N 111.1500° E, 19.0 m, mud, Jun 2016; MBM286017 (1 specimen) in IOCAS: Guangdong, Daya Bay, South China Sea, 22.6083° N 114.7289° E, 7.5 m, silt, Jun 2015; MBM286018 (5 specimens) in IOCAS: Guangdong, Daya Bay, South China Sea, 22.6675° N 114.6642° E, 10 m, silt, Aug 2016; MBM286019 (1 specimen) in IOCAS: Guangdong, Daya Bay, South China Sea, 22.7050° N 114.7217° E, 9.0 m, mud, Aug 2016; MBM286020 (1 specimen) in IOCAS: Shanwei, Guangdong, the coast of Yuedong, South China Sea, 22.2888° N 115.1530° E, 40.0 m, mud, Aug 2015; MBM286022 (2 specimens) in IOCAS: Guangdong, Daya Bay, South China Sea, 22.6247° N 114.6719° E, 14 m, mud, Jun 2015; MBM286023 (1 specimen) in IOCAS, P20161225001 (2 specimens) in SCSEM: Guangdong, Daya Bay, South China Sea, 22.5750° N 114.5186° E, 14 m, mud, Jun 2015; MBM286025 (1 specimen) in IOCAS: Guangdong, Daya Bay, South China Sea, 22.6672° N 114.6639° E, 10.5 m, silt, Jun 2015.

Description (Based on both holotype and paratypes). Holotype complete, 29.3 mm in length and 0.3–2.6 mm in width (from posterior end to anterior chaetigers (excluding chaetae) respectively), with distinct demarcation between thorax and abdomen; with 39 abdominal segments; body tapering posteriorly with segments. Dorsum and ventrum smooth, colour pale to reddish in preserved specimens.

Buccal tentacles of two types on dorsal margin of large tentacular membrane, uniformly tapered and with expanded tips (Figs 2A–B & 3A–B, H). Prostomium compact almost completely hidden by tentacular membrane. Eyespots absent. Peristomium consisting of expanded lower lip which forms an elongate rectangular structure and upper lip often covered by lower lip and tentacular membrane (Figs 2A–B & 3A–B). Segments 1–2 with small lateral lappets and inconspicuous ventral collar (Figs 2A–B; 3B–C & 5A–C). Segments 3–7 with distinct lateral lappets, largest on segment 4; forming thickened ventral membranous collars (ventral lobes) on anterior margins of segments 3–7, lobes progressively shorter from segment 4 onwards; ventral lobes of segment 3 with undulating margins in the middle, others with smooth margins (Figs 2A–B; 3A–B & 5A–B). Thoracic chaetiger 2 (segment 4) with a nephridial papilla at base of notopodia (Figs 3B–C & 5C). Segment 2 with a dorsal lobe anterior to branchial stem (Figs 3B–C & 5C). Lateral lappets without projections (Figs 2A; 3B & 5C). Ventral glandular bands and glandular areas around parapodia absent.

Branchia as single elongate structure dorsally on segments 2–4, consisting of two pairs of posterior lobes (BL1–2 on the outside and BL 3–4 in the middle) and pair of anterior lobes, lobes composed of tightly packed lamellae (Figs 2C; 3D & 5B). Anterior lobes (BL5–6) fused completely, and very short about 1/12 posterior lobes.

Posterior lobes fused for about 3/5 their length. BL1–2 as long as BL3–4. BL1–2 and BL5–6 with large lamellae, broad almost 3 times more than BL3–4; lamella of BL1–2 and BL3–4 with longitudinal ridges; outer edges of lamellae with a row of small rounded papilla; inside edges of lamellae smooth; without cilia (Fig. 3D–G); BL3–4 with small lamellae, without distal tip.

Thorax with 20 segments without dorsal hump. Notopodia 18 pairs, present on segments 3–20 (chaetigers 1– 18). First two pairs smaller and inserted more dorsally to subsequent ones; first one markedly reduced with shorter and finer chaetae, notochaetae appearing to arise from body wall (Figs 2A; 3A–C; 4A–B & 5C). Chaetae arranged in 2 tiers, lower tier with finely pointed chaetae and upper tier with narrowly–winged chaetae; chaetae of lower tier finer and about 1/2 length of those upper tier (Figs 2G–H & 4A–B, E–G). Neuropodia beginning from segment 8 (chaetiger 6), and present on all subsequent segments. Thoracic neuropodia as sessile pinnules. First neuropodia with geniculate acicular hooks (holotype right 6 and left 7); geniculate hooks arranged in single row throughout with pointed tips and weakly bent, and gradually decreasing in size from dorsal to ventral; the smaller geniculate hooks being sharply bent (Figs 2E & 4C–D). Subsequent thoracic neuropodia with one row or partial double rows of 23–25 long-handled uncini per torus; uncini with main fang and several rows of secondary teeth, with 3–4 large teeth in the first row (Figs 2F & 4H–I).

Abdomen with 39 segments with length gradually decreasing posteriorly. Abdominal neuropodia as foliaceous pinnules with about 30 avicular uncini; arranged in single row (Fig. 4J). Abdominal uncini with strongly crested head, covered with numerous small and scale-like teeth (Figs 2I & 4K–L). Pygidium blunt without appendages (Figs 2D & 5E).

The methyl green staining similar to pattern 1 of Schüller & Hutchings (2010), compact green colour on the first five segments, then developing into striped pattern (Fig. 5A, D). Margin of lower lip with white stripe on peristomium. Thoracic stripes gradually fade from segment 6, and disappearing by about segment 15. Green bands present on lateral thorax and abdominal ventrum. Additionally, staining on thoracic notopodia, base of posterior abdominal neuropodia and ventrum of tips of posterior branchial lobes. Lamellae of branchial lobes without staining.

Variability. Complete individuals ranging from 2.3–29.2 mm in length, 0.2–2.6 mm in width of anterior chaetigers excluding the chaetae and 18–40 abdominal segments. The large specimens with greater width and more abdominal segments than smaller ones (Fig. 6A). The length ratio of thorax and abdomen (LRTA) ranging from 1.2–6.2, showing large specimens possibly with a smaller LRTA and relatively long abdomen (Fig. 6B). Geniculate hooks ranging from 5–7 in number, with numbers appearing to increase with body size. Thoracic neuropodia with less than 15 (smaller specimens) to 25 uncini per torus on larger specimens, suggesting that the number increases with size of individual and presumably age.

Remarks. Terebellides guangdongensis n. sp. is characterised by the following morphological characters: one branchia has two pairs of posterior lobes and pair of anterior lobes not distinctly prolonged; all branchial lobes have densely packed lamellae; BL1–2 are as long as BL3–4, and broader than BL3–4; thoracic chaetigers of which the first five have lateral lappets, without conspicuous projections; first notopodia are strongly reduced, and have shorter and finer chaetae compared to subsequent ones; only chaetiger 6 has geniculate hooks.

Terebellides guangdongensis n. sp. is very similar to Terebellides californica Williams, 1984, Terebellides horikoshii Imajima & Williams, 1985 and Terebellides japonica Moore, 1903 in that they all have one branchia with six lobes which include a pair of anterior lobes completely fused lacking marked elongation and two pairs of posterior lobes partially fused along their length; all branchial lobes with densely packed lamellae; lateral lappets present on chaetigers 1–5 and chaetiger 6 with geniculate hooks only. Terebellides guangdongensis n. sp. can be distinguished from Terebellides horikoshii and Terebellides japonica by thoracic neuropodia having single or partial double rows of few (less than 25) uncini per torus, but the latter two species have double rows of 40 or more uncini per torus. Terebellides horikoshii and Terebellides californica both have well developed notopodia 1 and 2, compared to Terebellides guangdongensis n. sp. which has poorly developed notopodia 1 and 2, especially the first one. The abdomen of Terebellides japonica is longer than the thorax with an LRTA of almost 0.5, but the LRTA of Terebellides guangdongensis n. sp. range is from 1.2–6.2. Terebellides guangdongensis n. sp. is also very similar to Terebellides hutchingsae Parapar, Moreira & Martin, 2016. These two species can be distinguished by the characters of branchiae and chaetae of thoracic neuropodia. BL1–2 and BL3–4 of Terebellides guangdongensis n. sp. are of equal length and partially fused lobes, whereas BL1–2 and BL3–4 of Terebellides hutchingsae are free, and BL1–2 longer than BL3–4. Neuropodia of Terebellides guangdongensis n. sp. have weakly bent geniculate chaetae on TC6 and 23–25 long-handled uncini per torus in subsequent thoracic chaetigers. In contrast neuropodia of Terebellides hutchingsae have sharply bent geniculate chaetae on TC6 and relative few (8–10) uncini per torus in subsequent ones.

Distribution. Known only from Shantou waters, Guangdong of the northern South China Sea (Fig. 1).

Habitat. Found in muddy substrates in shallow water (7.5–40 m depths).

Etymology. The specific name guangdongensis is derived from guang dong, latinized Chinese characters for the locality, which refers to the type locality in the Guangdong Province waters.