Terebellides ectopium Zhang & Hutchings 2018, n. sp.

Terebellides ectopium n. sp.

Figures 13–16

Material examined. MBM 286026 in IOCAS, Zhanjiang, Guangdong, Baibu Gulf, north South China Sea, 20.4099° N 109.7840° E, 17.5 m, mud, May 2016.

Paratype. MBM 286027 (1 specimen, on SEM stub) in IOCAS, collected from same site as holotype. (Catalog numbers in IOCAS are tentative, which will be changed after the specimens are deposited in IOCAS before this manuscript is published in its final form.)

Description (Based on both holotype and paratypes). Holotype complete, 22.4 mm in length and 0.3–2.2 mm in width (from posterior end to anterior chaetigers (excluding chaetae) respectively), with distinct demarcation between thorax and abdomen; with 27 abdominal segments; body tapering posteriorly with segments (Fig. 16A– B). 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 13A–B & 14B–D). Prostomium compact which is almost completely hidden by expanded 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 13A–B; 14C–D & 16A). Segments 1–2 with small lateral lappets and inconspicuous ventral collar; segments 2 with a dorsal lobe and pair of dorsal cirri at base of branchia (Figs 13A & 14B–D). Segments 3–7 with distinct lateral lappets, largest on segment four; forming thickened membranous ventral collars on anterior margins of segments 3–7, and lobes progressively shorter from segment 4 onwards; ventral collars of segment 3 with an inverted “n” shaped invagination in centre, other segments with smooth ventral collars (Figs 13A–B & 14C). Lateral lappets without conspicuous projection (Figs 13A & 14C–D). Ventral glandular bands and glandular areas around parapodia absent.

Branchia as single elongate structure dorsally inserted on segments 2–4, consisting of two pairs of posterior lobes and pair of anterior lobes (Figs 13A; 14D–E & 16A). Anterior lobes (BL5–6) fused completely, and prolonged about 1/4 posterior lobes. Posterior lobes fused for about 2/3 their length. BL1–2 much longer and larger than BL3–4. BL1–2 and BL5–6 with large lamellae, without distal tip; lamella of BL1–2 and BL5–6 with longitudinal ridges, without a row of papilla; BL3–4 with small lamellae and short distal tip (Fig. 14D–F).

Thorax with 20 segments without dorsal hump. Notopodia 18 pairs, present on segments 3–20 (thoracic chaetigers 1–18). First two pairs smaller and situated more dorsally than following ones; first one strongly reduced with shorter and finer chaetae, notochaetae almost appearing to arise from body wall (Figs 13A, H; 14B–D, G & 16A). 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 2/3 length of those of upper tier (Fig. 15D–H). Neuropodia beginning from segment 7 (chaetiger 5), and present on all subsequent segments. Thoracic neuropodia sessile pinnules. Neuropodia of chaetigers 5–6 with geniculate hooks arranged in single row; geniculate hooks with sharply constricted mucronate tips; ventral geniculate hooks smaller than dorsal ones; neuropodia of chaetiger 5 situated more ventrally than subsequent ones, and with 5 weakly bent geniculate hooks; neuropodia of chaetiger 6 with 5 sharply bent geniculate hooks (Figs 13C–E; 14C; 15A–C & 16C). Neuropodia of thoracic chaetigers 7–13 with one row of 16–18 long-handled uncini per torus; following thoracic neuropodia with partial double rows of 16–19 longhandled uncini per torus; uncini with several rows of secondary teeth above main fang, and 5–7 major teeth of first row (Figs 13F & 15I –K).

Abdomen with 27 segments with length gradually decreasing posteriorly. Abdominal neuropodia as foliaceous pinnules with about 26 avicular uncini; arranged in a single row (Fig. 15L). Abdominal uncini with strongly crested head, covered with numerous small and scale-like teeth (Figs 13G & 15M). Pygidium blunt without appendages (Fig. 16D).

The methyl green staining similar to pattern 4 of Schüller & Hutchings (2010), compact green colour of the first ten segment, and gradually fading (Fig. 11A–B). Margin of lower lip without white stripe. Following segment 10, segments without green stripes. Posterior abdominal neuropodia weakly stained. Thoracic notopodia, branchial tips and lamellae without staining.

Variability. Paratype complete, 19.2 mm in length, 0.3–1.99 mm in width and 25 abdominal segments. Neuropodia with 5 weakly bent geniculate hooks on segment 5; neuropodia with 4 sharply bent geniculate hooks on segment 6. Neuropodia of TC7–18 with one or partial double rows of 16–19 long-handled uncini per torus. As we only have two complete specimens of similar size, we cannot comment on variation of morphological characters.

Remarks. Terebellides ectopium n. sp. is easily distinguished from other species by TC5–6 with geniculate hooks with conspicuously constricted mucronate tips; neuropodia of TC5 situated more ventrally than following ones; first two pairs of notopodia are strongly reduced, and have shorter and finer chaetae than subsequent ones; a single branchia with three pairs of branchial lobes, the two pairs of posterior lobes are partially fused along their length, and BL1–2 are much longer and larger than BL3–4, BL5–6 distinctly prolonged for about 1/2 posterior lobes.

There are four other species of Terebellides having geniculate hooks in TC5–6, Terebellides akares Hutchings, Nogueira & Carrerette, 2015 (type locality Lizard Island, Australia), Terebellides biaciculata Hartmann-Schröder, 1992 (type locality French Polynesia), Terebellides bigeniculatus Parapar, Moreira & Helgason, 2011 (type locality Iceland) and Terebellides crux Schüller & Hutchings, 2013 (type locality Southern Ocean). Terebellides ectopium n. sp. clearly differs from these species by the geniculate acicular hooks with conspicuously constricted mucronate tips and neuropodia of TC5 situated more ventrally than following ones. The new species can also be distinguished from T. akares which has BL5–6 not prolonged evidently; from T. crux which has free posterior branchial lobes and equal length; from T. biaciculata which has lateral lappets until TC4 and anterior branchial lobes are distinctly larger than posterior lobes; from T. bigeniculatus of which the notopodia of TC1 is similarly developed to subsequent ones.

Terebellides ectopium n. sp. is similar to a specimen of Terebellides from the coast of Myanmar in the North Andaman Sea, which Parapar et al. (2016) believe is a new species for the following reasons: BL5–6 is large and about half the length of posterior lobes; TC5 and TC6 are both provided with geniculate acicular chaetae, and neuropodia of chaetiger 5 situated more ventrally than following ones. These characters are the same as Terebellides ectopium n. sp. but as Parapar et al. (2016) only had a single specimen they did not formally describe their specimen. We suggest that this specimen may be synonymous with T. ectopium but until more material becomes available this cannot be confirmed.

Distribution. Known only from Beibu Gulf (Fig. 1).

Habitat. Found in muddy bottom in shallow water (17.5 m depth).

Etymology. The specific name ectopium is derived from English word ectopia, which refers to the different position of geniculate hooks in thoracic chaetigers 5 and 6.