Figs 15, 16, 17
Holotype: ANDEEP II, St. 132 - 6, GKG (ZMH- 26005) Paratypes: ANDEEP II, St. 132 - 6, GKG [drawing, MG photo] (ZMH- 26004); ANDEEP II, St. 132 - 6, GKG (ZMH- 26006); ANDEEP II, St. 131 - 8, GKG (AM W 39663); ANDEEP II, St. 131 - 8, GKG [MG photo, SEM, stub MI 541 & MI 542] (AM W 38716); ANDEEP II, St. 132 - 4, GKG (2 specimens, AM W 39664, AM W 39665); DZMB-HH 6309 - 1, ANDEEP III, St. 121 - 12, GKG (ZMH- 26007)
Description: (Based on both holotype and paratypes)
Holotype 6 mm in length, 1 mm width, complete with 35 abdominal chaetigers. Paratypes range from 6 mm to 1.2 cm in length to 1 to 1.2 mm in width with 30 to 35 abdominal chaetigers.
Head region: Tentacular membrane greatly expanded. Lower lip rectangular, distinctly folded upwards, also expanded (Fig. 15). Tentacles short simple ones plus long tentacles with expanded tips present on outer margins of tentacular membrane (Fig. 16 A, D).
Branchiae: Branchial lobes fused about 20–40 % with each other, lamellae broad, loose, rather few present, filamentous tips absent (Fig. 16 A, B). Posterior branchial lobes shorter (about 70–80 %) than anterior ones. Fifth branchial lobe absent. Annulation of branchial stem present. Only one specimen with fully developed branchiae, remaining specimens with anterior branchial lobes somewhat drop-like in shape, arranged at an angle of about 120 ° to each other (Fig. 17). Ciliary fields between branchial lamellae not apparent (Fig. 16 B, C).
Anterior chaetigers: Notopodia from segments 3–20, 18 pairs. First notopodia reduced in size with chaetae more or less originating from body wall (Fig. 16 D) and second notopodia smaller than subsequent ones (Fig. 16 D). Notochaetae, capillaries with fine tips and arranged in two tiers (Fig. 16 E).
Neuropodia present from segment 8 (chaetiger 6), those of first neuropodia geniculate hooks, about 5–7 per side, sharply bent with extended fine tips and inflated bases (Fig. 16 F). Subsequent neuropodia with long shafted denticulate hooks with main fang and several multidentate rows of teeth above, not vertically aligned, so providing a dental formulae not possible (Fig. 16 G), neuropodia become increasingly erect pinnules posteriorly. Abdominal uncini few within a torus and with elongate hooks, main fang completely covered by numerous elongate accessory teeth (Fig. 16 H).
Lateral lappets: Present from TC- 1–6, with TC- 1> 2 = 3> 4 = 5 <6.
Ventral pads: Ventrum of thoracic segments slightly glandular with anterior margins of segments elevated, especially marked on chaetigers 1–4.
Nephridial papillae: Present on segments 3, 6, 7, possibly also 5, globular.
MG staining pattern 1 (Figs 2, 17): Solid on anterior and striped in median thorax. No apparent white bands or pronounced anterior margins present. Both noto- and neuropodia stain.
Pygidium: With two low lateral papillae.
Remarks: Terebellides sirius sp.n., is characterised by four branchial lobes being partially fused along their length, lobes made up of a few loosely packed lamellae with no terminal filamentous tips, lateral lappets on chaetigers 1–6 with the first three the largest and similar in size and with a greatly expanded lower lip. This new species T. sirius sp.n., belongs to a group of species with branchial lobes at least partially fused along their length (Terebellides kowinka Hutchings & Peart, 2000, Terebellides californica Williams, 1984, Terebellides horikoshii Imajima & Williams, 1985, Terebellides japonica Moore, 1903). Terebellides sirius sp.n., can be separated from T. kowinka by the branchiae lacking filamentous tips, and the other species in this group have branchiae with lamellae densely packed (T. toliman sp.n., this paper and T. canopus sp.n., this paper) whereas T. sirius sp.n., like T. kowinka has branchiae with only a few lamellae. For more details see the key.
Habitat: Western Weddell Sea in 2085–3068 m. Known only from type locality.
Etymology: The specific name sirius is one of the brightest stars in the constellation Carina.