Terebellides atlantis Williams, 1984

Parapar, Julio, Moreira, Juan & Helgason, Gudmundur V., 2011, Taxonomy and distribution of Terebellides (Polychaeta, Trichobranchidae) in Icelandic waters, with the description of a new species, Zootaxa 2983, pp. 1-20 : 2-6

Terebellides atlantis Williams, 1984

Figures 2−3 , 13 a

Terebellides atlantis Williams 1984: 121 –123. Holthe 1986b: 115. Solís-Weiss et al. 1991: 156. Bremec and Elías 1999: 177. Hutchings and Peart 2000: 244, Table 3a, b. Garraffoni and Lana 2003: 356. Garraffoni and Lana 2004: 973. Garraffoni et al. 2005: 8.

Terebellides stroemii Hartman 1965: 227 , in part; not Sars, 1835.

Material examined. A total of 1684 specimens (54.15% of total) were obtained in 150 BIOICE samples. BIOICE sample 2317 (one specimen in one SEM stub IMNH 24927; 64º07'00''N; 09º03'00''W, 996 m), BIOICE sample 2741 (two specimens in one SEM stub IMNH 24928; 67º38'90''N; 20º14'28''W, 514 m).

Occurrence. The species is present in a wide range of depths and preferably in warmer waters at both sides of the GIF Ridge. Depth range: 173−3000 m; temperature range: -0.9ºC to 7.5ºC.

Description. Complete specimens range from 8 to 18 mm in length and 0.5 to 1.0 mm in width; body tapering posteriorly with segments increasingly shorter towards pygidium. Prostomium compact; tentacular membrane surrounding the mouth and usually devoid of buccal tentacles ( Fig. 2 a). First segment forming an expanded structure below tentacular membrane. Lateral lappets on segments 3–7 (chaetigers 1–5) ( Fig. 2 a). Branchiae arising as a single structure from segment 3, consisting of a single mid-dorsal stalked structure ( Fig. 2 a) made up of two pairs of lobes not fused and provided with pointed projection of posterior region; posterior pair of lobes slightly shorter and thinner. Sometimes only anterior pair of lobes present; in small specimens usually only stalk persisting. No anterior projection (fifth lobe) present. Both sides of branchial lamellae provided with several concentric rows of cilia ( Fig. 2 b).

Eighteen pairs of notopodia (segments 3–20), compact, rectangular and of increasing size from first chaetiger backwards. Notochaetae of first three anterior chaetigers shorter and less numerous than notochaetae of subsequent notopodia ( Fig. 2 a). Neuropodia present as sessile pinnules from chaetiger 6 (segment 8) to pygidium. Neuropodial uncini in single rows throughout ( Fig. 2 c; 3a). First thoracic neuropodia provided with about 5 sharply bent, acute tipped geniculate acicular hooks ( Fig. 2 c–d). Upper part of geniculate chaetae provided with minute teeth forming a capitium ( Fig. 2 e–f). Second and all subsequent thoracic neuropodia with up to 15 uncini per torus ( Fig. 2 c; 3a). Uncini long-shafted denticulate hooks with main fang large, surmounted by 4–5 teeth and an upper crest of numerous teeth progressively shorter ( Fig. 3 b–c), dental formula: MF:4–5:∞. About 25 abdominal neuropodia as erect pinnules, with near 25 uncini per torus ( Fig. 3 d–e); uncini provided with 4 teeth above main fang surmounted by 5– 6 teeth and an upper crest of a variable number of smaller teeth ( Fig. 3 f), dental formula MF:4:4–5:∞.

No nephridial papilla observed. Pygidium blunt, funnel-like depression with crenulated edge. MG staining pattern ( Fig. 13 a): compact green coloration in first 13 segments, turning into a stripped pattern in segments 14–20 and fading in the following segments. Colour in alcohol pale brown.

Distribution. Terebellides atlantis was only known from the type locality in the New England slope ( USA) ( Williams 1984; Hutchings & Peart 2000). The large number of specimens found among BIOICE material suggests that this species might have been overlooked in later studies in the North Atlantic, probably being confused with small specimens of T. stroemii . Williams (1984) reports the species in a narrow depth range (466 to 508 meters); our findings in the coast of Iceland increase this range significantly, from shallower depths (173 m) of the continental shelf to the deep slope near the bathyal boundary (3000 m).

Remarks. Two of the most characteristic features of this species are its small size, much smaller than the other species found in Iceland (mature specimens were of 13 mm in length) and the presence of branchiae, consisting of 1–4 basally fused lobes provided with loosely fused lamellae. This feature has been previously reported for other Terebellides species such as T. intoshi Caullery, 1915 ( Indonesia) , T. lobatus Hartman and Fauchald, 1971 (New England), T. mundora Hutchings and Peart, 2000 ( Australia) and T. sepultura Garraffoni and Lana, 2003 ( Brazil) . Terebellides intoshi differs from T. atlantis in its larger body size (50–60 mm long and 4 mm wide), number of abdominal chaetigers (35–40 vs. 25 in T. atlantis ) and, following Imajima and Williams (1985) description of this species, by the presence of two chaetigers with geniculate chaetae instead of one (but see below Discussion of T. bigeniculatus sp. nov.). Terebellides lobatus differs from T. atlantis in its larger body size (up to 30 mm long and 4 mm wide), and the degree of curvature of geniculate chaetae (90º in T. atlantis vs. 135º in T. lobatus ) and thoracic uncini (the front teeth of capitium are much larger than the following) (see plate 20 in Hartman & Fauchald 1971). Terebellides mundora has sharply bent geniculate chaetae such as in T. atlantis but the rostrum is much shorter and with a blunt tip (although authors qualify it as pointed). Terebellides sepultura differs in having a very large branchial stalk and a posterior pair of branchial lobes much shorter than in T. atlantis . Recently, Gagaev (2009) described T. irinae from the Canadian basin; this small-sized (6.6 to 14.6 mm long and 0.3 to 0.6 mm wide) species has also free branchial lobes. This species, characterized by Gagaev (2009) by the shorter size of the ventral pair of branchial lobes, very much resembles T. atlantis . Unfortunately, the author does not provide any comparison between the new species and T. atlantis .

The study of several specimens under the SEM shows the presence of denticles on the upper part of the geniculate setae. In addition, the capitium of the thoracic uncini is endowed with teeth progressively shorter, which differs greatly from the appearance of that of the other three species of Terebellides found in Iceland (see below).

The MG staining pattern observed in Icelandic specimens agrees well with pattern 1 proposed by Schüller and Hutchings (2010) (i.e., anterior solid, subsequently striped and fading towards posterior thorax); nevertheless, the number of anterior completely stained chaetigers (solid chaetigers) is 11 as is described in Williams (1984) (“ventral bands stain on 11 chaetigers”) and not three.

The specimens from the BIOICE cruises previously identified as Terebellides cf. stroemii by Parapar and Moreira (2008a) correspond to T. atlantis .

Figures

FIGURE 2. Terebellides atlantis (IMNH 24927 – 24928) (a) anterior end, latero-ventral view, (b) branchial ciliation, (c) thoracic chaetigers 5 – 7, (d) geniculate chaetae from chaetiger 6, (e – f) detail of teeth of geniculate chaetae capitium. Scales: (a) 900 Μm, (b) 40 Μm, (c) 200 Μm, (d) 40 Μm, (e) 8 Μm, (f) 6 m.

FIGURE 3. Terebellides atlantis (IMNH 24927 – 24928) (a) thoracic chaetigers 6 – 11, latero-ventral view, (b – c) thoracic uncini, (d) abdominal uncinigers, (e) anterior abdominal unciniger, detail, (f) abdominal uncini. Scales: (a) 600 Μm, (b) 10 Μm, (c) 7 Μm, (d) 300 Μm, (e) 60 Μm, (f) 10 Μm.

FIGURE 13. Methyl green staining of all Terebellides species found in BIOICE samples, anterior region in ventral view. (a) T. atlantis, (b) T. bigeniculatus sp. nov., (c) T. gracilis, (d) T. stroemii sensu Holthe (1986 a).