Terebellides bigeniculatus Parapar, Moreira & Helgason, 2011, sp. nov.

Terebellides bigeniculatus sp. nov.

Figures 1 b, 4−7, 13b

Material examined. A total of 181 specimens (5.66% of total) were obtained in 50 BIOICE samples.

Type material. Icelandic Museum of Natural History: BIOICE sample 2591 (Holotype IMNH 24923 and six paratypes IMNH 24924; 67º12'53''N; 22º25'46''W; 333 m depth); BIOICE sample 2047 (five paratypes IMNH 24925; 65º42'09''N; 12º52'60'' 272 m); BIOICE sample 2619 (three paratypes in two SEM stubs IMNH 24929 and IMNH 24930; 67º16'86''N; 16º37'77''W, 600 m). BIOICE sample 2868 (three paratypes IMNH 24926; 64º40'94''N; 25º35'82''W, 212 m).

Museo Nacional de Ciencias Naturales de Madrid: BIOICE sample 2591 (26 paratypes; MNCN 16.01/13606) Non-type material. Icelandic Museum of Natural History: BIOICE sample 2021 (1 specimen); 2046 (3); 2047 (11); 2049 (4); 2087 (1); 2119 (3); 2124 (2); 2136 (2); 2143 (1); 2229 (1); 2241 (1); 2268 (2); 2273 (1); 2282 (1); 2303 (3); 2330 (1); 2362 (2); 2371 (2); 2427 (1); 2469 (1); 2528 (1); 2573 (4); 2583 (1); 2585 (1); 2612 (5); 2613 (16); 2618 (1); 2619 (9); 2627 (1); 2666 (3); 2673 (4); 2717 (1); 2741 (4); 2789 (1); 2887 (2); 2901 (5); 2979 (2); 2983 (1); 3023 (3); 3026 (5); 3099 (1); 3110 (1); 3247 (1); 3252 (7); 3550 (1); 3558 (1); 3588 (14).

Additional material examined. Zoologisches Institut und Zoologisches Museum, Universität Hamburg. Type material of Terebellides biaciculata P–20709.

Type locality. Off North-West Iceland (333 m deep).

Occurrence. Terebellides bigeniculatus sp. nov. is present at both sides of the GIF Ridge, but shows the narrowest depth range when compared to the other species, not reaching depths greater than 1000 m (Fig. 1 b). Depth range: 179-968 m; temperature range: -0.6 to 5.6ºC.

Description based on holotype. Complete specimens range from 10 to 24 mm in length (22 mm in holotype) and 1.0 to 2.0 mm in width (2.0 mm in holotype) (Fig. 4); body tapering posteriorly with segments increasingly shorter and crowded towards pygidium. Prostomium compact; tentacular membrane surrounding the mouth and provided with buccal tentacles with expanded tips. First segment forming an expanded structure below tentacular membrane (Fig. 4). Lateral lappets on segments 3–7 (chaetigers 1–5) (Fig. 4, 5 a–c). Some specimens with conspicuous dorsal rounded projection in chaetigers 3 (Fig. 5 c). Branchiae arising as single structure from segment 3, consisting of a single mid-dorsal stalked structure (Fig. 4, 5 a,c) made up of two pairs of lobes fused for about one third of length; inner pair shorter. Anterior branchial projection (fifth lobe) and pointed projection of posterior region of lobes present (Fig. 5 a). Both sides of branchial lamellae provided with several concentric rows of cilia (Fig. 5 d–e).

Eighteen pairs of notopodia (segments 3–20), shorter in chaetigers 1 to 6 becoming larger in subsequent chaetigers (Fig. 4, 5 f). Notopodia and notochaetae of first chaetiger similar in size to subsequent notopodia (Fig. 4, 5 a– b). Neuropodia present as sessile pinnules from chaetiger 5 (segment 7) to pygidium and provided with uncini in single rows throughout (Fig. 5 f). First and second thoracic neuropodia (chaetigers 5 and 6) with sharply bent, acute tipped, geniculate acicular hooks (Fig. 6 a–b, 7a); 4–5 hooks in both chaetigers, numbers showing some degree of variability (Fig. 6 a–b). Upper part of geniculate chaetae provided with minute teeth forming a capitium (Fig. 7 b). Third and all subsequent thoracic neuropodia with up to 8–10 uncini per torus (Fig. 5 f). Uncini provided with longshafted denticulate hooks with main fang large and surmounted by 4 large teeth and crest of several shorter denticles (Fig. 7 c), dental formula MF:4:∞. Twenty-two abdominal neuropodia as erect pinnules (Fig. 7 d) provided with about 20 uncini per torus (Fig. 7 e); uncini with six teeth above main fang surmounted by an upper crest of 3–4 shorter teeth and a variable number of smaller teeth (Fig. 7 f), dental formula MF:6:4–5:∞.

Small thoracic papillae arising dorsally to thoracic notopodia (Fig. 6 c). One large nephridial papilla on each notopodium of segments 5 and 6 (chaetigers 3 and 4) with the appearance of a short, distally truncated cone (Fig.

6d). Pygidium blunt, funnel-like depression with crenulated edge. MG staining pattern (Fig. 13 b): compact green coloration in first 11 segments, after turning into striped pattern in segments 12–14 and fading in the following segments. Colour in alcohol pale brown.

Etymology. The species name refers to the presence of two thoracic neuropodia provided with geniculate chaetae.

Remarks. Terebellides bigeniculatus sp. nov. differs from most Terebellides species by having two thoracic chaetigers provided with geniculate chaetae. The only currently known species sharing this feature are Terebellides intoshi Caullery, 1915, from Indonesia and Terebellides biaciculata Hartmann-Schröder, 1992, from the Rangiroa island lagoon (French Polynesia). Three paratypes of T. biaciculata were examined and the presence of geniculate setae on chaetigers 5 and 6 was confirmed. However, the presence of extremely long thoracic notochaetae in the latter (the author only illustrates the geniculate chaetae and thoracic and abdominal uncini), clearly separates it from T. bigeniculatus sp. nov. Hartmann-Schröder (1992) named this species as T. biaciculata in the text and T. biaciculatus in the figure plates; we follow Garraffoni et al. (2005) and adopt biaciculata as the species epithet. Hartmann-Schröder (1992) compares this species with Terebellides brevis Imajima and Williams, 1985 from Japan, which she considers similar in several characteristics but admitting that it has only acicular hooks on the sixth chaetiger.

Caullery (1915; 1944) characterizes T. intoshi by the possession of free branchial lobes, the great size of the notopodia of the sixth thoracic chaetiger and the shape of thoracic and abdominal uncini and states the presence of geniculate chaetae in 6th thoracic neuropodia. Later, Imajima and Williams (1985) redescribe T. intoshi from material collected in the coast of Japan and characterize the species by having special chaetae in chaetigers 6 and 7. Imajima and Williams (1985) recognize that although Caullery (1915; 1944) did not explicitly mention the presence of acicular chaetae in chaetiger 7, the specimens were identified as T. intoshi because they fit the original description according to other relevant characters. The nominal species of T. intoshi from Indonesia differs from T. bigeniculatus sp. nov. in having acicular chaetae only in chaetiger 6 and in having very large notopodia and notochaetae from chaetiger 6 and backwards. The Japanese species differs from T. bigeniculatus sp. nov. in bearing the geniculate chaetae in chaetigers 6 and 7 (segments 8 and 9) instead of 5 and 6. Caullery (1944) considers that T. intoshi resembles T. ehlersi McIntosh, 1885 from Fiji, but Imajima and Williams (1985), who examined the holotype, do not support this opinion.