Terebellides gracilis Malm 1874

Terebellides gracilis Malm, 1874

Figures 8−10, 13 c

Terebellides gracilis Malm 1874: 100.

Terebellides williamsae – Jirkov 1989: 124. Jirkov 2001: 529, figs. 1–4.

Material examined. A total of 357 specimens (11.48% of total) were obtained in 95 BIOICE samples. BIOICE sample 2619 (three specimens in two SEM stubs IMNH 24931 and IMNH 24932; 67º16'86''N; 16º37'77''W, 600 m).

Additional material examined. Type material of T. gracilis. Göteborgs Naturhistoriska Museum (Holotype, GNM Polych 641), Hågarnsskären, Bohuslän.

Occurrence. Present in a wide range of depths and temperatures at both sides of Iceland. Depth range: 68 to 2076 m; temperature range: -0.6 to 7.0ºC.

Redescription based on holotype. Complete specimen of 32 mm in length and 2.5 mm in width in its widest part; 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. 8, 9 a). Branchiae arising as a single structure from segment 3, consisting of a single mid-dorsal stalked structure made up of two pairs of lobes of same length and fused in about one third of length; anterior projection (fifth lobe) present. Posterior region of lobes with pointed projection. Both sides of branchial lamellae provided with several tufts of cilia (Fig. 9 b–c). Lateral lappets on segments 3–7 (chaetigers 1–5) (Fig. 9 a). One large nephridial papilla on each notopodium of chaetigers 4–5 (Fig. 9 d), difficult to see in holotype, with the appearance of a buttonhole (Fig. 9 e–f). Eighteen pairs of notopodia (segments 3–20), compact, rectangular and increasing in size from first to sixth chaetiger. Thoracic neuropodia as sessile pinnules; present from chaetiger 6 (segment 8) to pygidium. Notochaetae of first chaetiger less numerous than notochaetae of subsequent notopodia. All notochaetae long capillaries. Neuropodial uncini in single rows throughout. First thoracic neuropodia started, as erect pinnules, in chaetiger 6 (segment 8), the latter slightly inflated (Fig. 9 d) and provided with 6–9 (9 in holotype) sharply bent, acute tipped, geniculate acicular hooks (Fig. 9 d, 10a). Upper part of aciculae chaetae provided with minute teeth forming a capitium (Fig. 10 b). Second and all subsequent thoracic neuropodia with up to 16–22 uncini per torus. Thoracic uncini long-shafted denticulate hooks with main fang large and surmounted by two big teeth and a crest of five shorter denticles (Fig. 10 c); dental formula MF:2–5:∞. About 43 abdominal neuropodia with near 45 uncini per torus with 3 teeth above main fang surmounted by 5 teeth and an upper crest of a variable number of smaller teeth (Fig. 10 d), dental formula MF:3:5:∞. Pygidium blunt, funnel-like depression with crenulated edge. Colour in alcohol pale brown; four anterior thoracic chaetigers ventrally white. MG staining pattern (Fig. 13 c): compact green coloration of first 13 segments, abruptly fading in following segments.

Variation. The BIOICE specimens are usually much smaller than the type specimen (10–25 mm vs. 32 mm) and sometimes the whitish coloration of anterior segments is only evident in the fourth chaetiger or in most of the ventral part of the first three chaetigers, with the fourth one much less developed than the previous three, as was illustrated by Jirkov (1998, fig. 23.7.) for T. williamsae.

Distribution. The type locality of T. gracilis is Gullmar Fjord, Koster area, Spitsbergen, Norwegian Sea, Barents Sea (74°30'N, 28°00'E, 385–390 m). Jirkov (1989) reports T. williamsae in the Barents and Norwegian seas (92–450 m depth) and off Iceland.

Remarks. Terebellides gracilis has been considered a synonym of T. stroemii (e.g. Hartman, 1959; Holthe 1986a; 1986b). Nevertheless, Hansson (1998), after providing the translation of Malm’s original description of T. gracilis, showed that this species mostly differs from T. stroemii in having ventral parts of the first 4 thoracic chaetigers with white coloration. In addition, Jirkov (1989) characterised Terebellides williamsae Jirkov, 1989 against T. stroemii according to the same feature. Examination of the type specimen of T. gracilis corroborated Hansson’s suggestion and therefore we consider T. gracilis as a valid species. In addition, we did not find any significant differences between T. gracilis and T. williamsae; the latter should be regarded as senior synonym of T. williamsae. The study of several specimens under the SEM revealed a number of features not previously reported and which seem to be characteristic of T. gracilis. These are the presence of lateral lappets in chaetigers 1–5, the buttonhole shape of the nephridial pores, and the ciliature of the branchial lamellae. This ciliature is characteristically arranged in tufts instead of rows, which is the only disposition so far reported for the ciliature in the branchial lamellae of the genus Terebellides (Jouin-Toulmond & Hourdez 2006; Parapar & Moreira 2008a; 2008b). The geniculate chaetae are also endowed with denticles on their apical surface, the thoracic uncini have two main teeth surmounting the very large tooth (rostrum).