Terebellides rigel, Schüller, Myriam & Hutchings, Pat, 2013

Schüller, Myriam & Hutchings, Pat, 2013, New species of Terebellides (Polychaeta: Trichobranchidae) from the deep Southern Ocean, with a key to all described species, Zootaxa 3619, pp. 1-45 : 19-22

Terebellides rigel sp.n.

Figs 12, 13 , 14

Holotype: ANDEEP III, St. 78-9, EBS (ZMH-26023)

Paratypes: ANDEEP III, St. 121-7, AGT [drawing, MG photo] (AM W39660); ANDEEP III, St. 78-9, EBS (3 specimens, ZMH-26021, ZMH-26022, ZMH-26024); ANDEEP III, St. 80-9, EBS (ZMH-26025); ANDEEP III, St. 142- 5, EBS (2 specimens, AM W39661, AM W39662); ANDEEP III, St. 142-5, EBS [SEM, stub MI495] (AM W38715) Description: (Based on both holotype and paratypes)

Holotype 6 mm in length, 1 mm in width, posteriorly incomplete with 18 chaetigers, gravid, body cavity with oocytes. Paratypes ranging in length from 4–6 mm and 0.2–1.0 mm in width, all posteriorly incomplete, all with thorax complete but with only 0–5 abdominal segments.

Head region: Tentacular membrane compact, with little expansion. Lower lip rectangular, only laterally slightly expanded ( Figs 12, 13 A). Tentacles only short simple ones present on outer margins of tentacular membrane ( Fig. 13 A, B).

Branchiae: Anterior branchial lobes free from each other, lamellae broad, loose, only few present, with filamentous tips ( Figs 12 A, 13B, D). Dense ciliary fields between each of lamellae ( Fig. 13 C, D). Posterior branchial lobes distinctly smaller than anterior lobes. Fifth branchial lobe absent. Annulation of branchial stem not visible.

Anterior chaetigers: Notopodia from segments 3–20, 18 pairs. Notopodia of first and second chaetiger reduced and not horizontally aligned, those of chaetiger 2 more dorsal, chaetae appearing to arise directly from body wall ( Fig. 13 A). Subsequent chaetigers with well developed notopodia increasing in size posteriorly and inserted more ventrally with pre- and post chaetal lobes similar in length. Notochaetae, fine capillaries with long extended tips ( Fig. 13 E, F), shaft with expanded thecae ( Fig. 13 F), arranged in two tiers, long tier with about five chaetae and short tier with four ( Fig. 13 F). Neuropodia from segment 8 (chaetiger 6) present on all subsequent segments (all material posteriorly incomplete). Neurochaetae of chaetiger 6 sharply curved geniculate hooks with long, extended tips, five present ( Fig. 13 G) although one represented by stub, surface of hooks appears structured. 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. 13 H).

Lateral lappets: Present on thoracic chaetigers 1—4, possibly to TC-6, with TC-1>2>3>4>5=6.

Ventral pads: Ventrum of thoracic segments glandular with anterior margins of segments elevated, especially marked on chaetigers 1–7, continue laterally to form lateral lappets on chaetigers 1–4 ( Fig. 13 E).

Nephridial papillae: Present on segments 3, 4, 6, 7.

MG staining pattern 1 ( Figs 2 , 14 ): With distinctly solid anterior part and broad stripes in median and posterior segments. White band, pronounced anterior margins and neuropodial staining missing, notopodial staining only weak if present ( Fig. 14 ).

Pygidium: Unknown.

Remarks: Terebellides rigel sp.n., is characterised by having branchial lobes free from each other and each with few lamellae, distinct lateral lappets from chaetigers 1–4 decreasing in size posteriorly to chaetiger 6, first neuropodia with geniculate hooks only and all subsequent neuropodia with a single row of long handled hooks with strongly denticulate heads. This species can easily be separated from two widely distributed species in the region T. kerguelensis McIntosh, 1885 and T. longicaudatus Hessle, 1917 which are both characterised in having five branchial lobes and each with numerous lamellae in contrast to T. rigel sp.n. This new species belongs to a group of species with branchial lobes with few loosely packed lamellae ( Terebellides banalis Schüller & Hutchings, 2012 , Terebellides sepultura Garraffoni & Lana, 2003 , Terebellides distincta Williams, 1984 , and Terebellides irinae, Gagaev, 2009 ) but it can be distinguished from these by the presence of lateral lappets until thoracic chaetiger 6. For more details see the key.

Habitat: Weddell Sea, lower continental slope to abyss in 2182–3403 m. Known only from type locality.

Etymology: The specific name rigel refers to one of the brightest stars of Orion, the most famous seasonal constellation.

Figures

FIGURE 2. MG staining patterns of observed species, adapted and extended from Schüller & Hutchings (2010).

FIGURE 13. Terebellides rigel sp. n. (AM W 38715), A—anterior thorax, lateral, B—anterior thorax, dorsal, C—branchiae, D—branchial lamellae, detail, E—anterior thorax, ventral, F—median thoracic notopodia, G—geniculate neuropodial hooks on TC- 6, H—thoracic uncini of median neuropodium.

FIGURE 14. Terebellides rigel sp. n. (AM W 39660), MG-staining, A—thorax, ventro-lateral, B—anterior thorax, lateral, C—anterior thorax, ventro-lateral.