Thelepus cincinnatus ( Fabricius, 1780 )

Nogueira, João Miguel De Matos, 2019, Redescriptions of Streblosoma bairdi (Malmgren, 1866) and Thelepus cincinnatus (Fabricius, 1780), based on types and material from type localities, Zootaxa 4544 (3), pp. 419-428 : 421-427

Thelepus cincinnatus ( Fabricius, 1780)

( Figs 3–4 )

Amphitrite cincinnata Fabricius 1780: 286. Thelepus cincinnatus . Malmgren 1866: 387–388, Pl. XXII, Fig. 58A–D.? Holthe 1986a: 140–142, Fig. 63a–c.? Jirkov 2001: 526, Fig. Thelepus cincinnatus 1–6.? Jirkov 2018: 38–43, Figs 2 , 3 A–D, 11B.

Material examined. Syntypes ( NRM 1270 ): 3 specs, coll. W Greenland, Pröven , 36.4 m, solid rock, 72 o 23’N 56 o 00’W, by O. Torell . Syntype 1: 100 mm long, 7 mm wide, 71 segs, complete, missing all buccal tentacles and most branchial filaments, with notopodia until segment 49, apparently regenerating thereafter, with shorter, thinner and densely packed segments, bearing neuropodia and lacking notopodia. Syntype 2: 95 mm long, 6 mm wide, incomplete, 40 segs, all with notopodia. Syntype 3: 67 mm long, 6 mm wide, incomplete, 35 segs, all with notopodia.

Additional material examined. NRM 103980: 4 specs. (originally 5, but one belongs into the Terebellidae sensu stricto, not Thelepodidae ), coll. Sweden, Bohuslän, Gullmarsfjord, Löken, by L. Orrhage, 21 Jul 1980; all incomplete, with notopodia until last segment and in excellent state of preservation, longest spec with 20 segments, ~ 27 mm long, 4 mm wide. NRM 103981: 4 incomplete specs and 2 mid-body fragments, coll. Sweden, Bohuslän, Gullmarsfjord, Löken, by L. Orrhage, 21 Jul 1980; all with notopodia until end of fragments and in excellent state of preservation; longest spec. with 36 segments, 24 mm long, 3 mm wide. NRM 103982: 1 complete spec. and 4 midbody fragments without an anterior end, coll. Sweden, Bohuslän, Gullmarsfjord, Löken, by L. Orrhage, 21 Jul 1980; complete spec. in excellent state of preservation, 60 segs, notopodia until seg. 32; not clear if mid-body pieces belong to the same species or even to a single animal. NRM 103983: 2 specs and a midbody piece, coll. Sweden, Bohuslän, Gullmarsfjord, Löken, by L. Orrhage, 21 Jul 1980; both specs incomplete, in good state of preservation.

Description. Prostomium at base of upper lip; eyespots in thin and continuous dark band, with 3–4 rows across posterior margin of basal part of prostomium; buccal tentacles long, deeply grooved ( Fig. 3 A–B). Peristomium continuing dorsally as narrow annulation; upper lip short, hood-like, distinctly wider than high, convoluted; lower lip in two parts, inner part rectangular, outer part with marginal lobe covering inner part ( Fig. 3 C–D, F–I). Segment 1 short, more developed ventrally, with low lobe below mouth ( Fig. 3 C–I); lobes on anterior segments absent, but segment 2 with thickened anterior margin all around, as protruding crest ( Fig. 3 C–I). Two pairs of branchiae, on segments 2–3, each with many distally blunt cylindrical filaments, originating from glandular cushion-like structures, with very narrow, almost inconspicuous medial gap, dorsalmost filaments about as long as body width at corresponding segments; on segment 2, branchial filaments arranged in 2 irregular rows, origin of filaments extending laterally well beyond level of notopodia ( Fig. 3 A–F, H); number of branchial filaments and width of mid-dorsal gap size-dependent, shorter specimens with fewer filaments and wider gap. Dorsal surface of body wall very distinctive throughout, quadriculated ( Fig. 3A , C–F, H, J); ventral surface of anterior segments strongly glandular, corrugated on anteriormost segments, discrete ventral shields absent ( Fig. 3 C–I); mid-ventral stripe beginning from ~segment 10. Notopodia starting from segment 3, extending for large number of segments, syntype 1, only complete, with notopodia until segment 49 (total of 71 segments), all other specimens with notopodia until end of fragments; all notopodia at mid-length of segments; notopodia roughly rectangular, distally rounded, bilobed, chaetae emerging between lobes, notopodia progressively smaller from around segments 20–25 to last with those structures ( Fig. 3 A–J). Narrowly-winged notochaetae in both rows, those of anterior row less than 1/3 of length of those from posterior row, wings bulbous basally, falcate, chaetae of posterior row with wings only at tips ( Fig. 4A ). Neuropodia starting from segment 5, as fleshy ridges, rounded at tips; tori closer to base than tip of neuropodia. Uncini typical of thelepodids, sub-distal dorsal button, conspicuous but distinctly short prow, much shorter than button, crest with 2 rows of secondary teeth, basal row with 2 teeth, second row with larger tooth inbetween teeth of basal row and tiny denticles laterally to them ( Fig. 4 B–F). Short, spherical nephridial and genital papillae on segments 4–7, between parapodial lobes (or corresponding position, on segment 4) and slightly posterior to them ( Fig. 3E, H ). Pygidium crenulated ( Fig. 3J ).

Comparison with Swedish specimens. The description above is based exclusively on the syntypes. Swedish specimens are much smaller and it is not certain they belong to the same species. Among the Swedish individuals, the mid-dorsal gap between filaments within pairs of branchiae is of variable width; most specimens have a narrow gap, but there is one individual with wider gap and another, which is the longest among all the specimens, apparently lacking any gap between sides of pairs; such variation may be due to muscular contraction at fixation. The ventral glandular area extends for large number of anterior segments, the proportion between corrugated and smooth segments varies between specimens, which may also be due to fixation.

Remarks. Jirkov redescribed this species twice ( Jirkov 2001, 2018). The first redescription was based on specimens from the Arctic ( Jirkov 2001) and it clearly does not match the one provided above, based on specimens from the type locality. Jirkov said the specimens from the Arctic have notopodia extending until the end of the body and neuropodia beginning on segment 4 (‘chaetigerous segment 2’), although this latter may be due to a miscounting by the author. The material from Greenland, in contrast, has neuropodia beginning on segment 5, as occurs in members of most if not all species of Thelepus , and notopodia are present on more than half of the body segments, but terminate far from the posterior end; the only complete syntype has 71 segments and notopodia are present until segment 49 (although that worm was regenerating the posterior end, with much thinner and densely packed segments, see Fig. 3C, J ).

For the second redescription, Jirkov examined a large number of specimens, almost 2,0 0 0, from more than 100 stations, from the Arctic to the Mediterranean, and depths from 2– 2,000 m ( Jirkov 2018). However, it seems likely that the author dealt with material of several species, given the amount of variation he observed in his material [for instance, size of the specimens, number of branchial filaments and morphology of the uncini, as shown in his Fig. 3 A–D ( Jirkov 2018: 41)]. It is worth to mention that in the same paper Jirkov described three new species from material previously identified as T. cincinnatus and from within the geographical range recorded for that species.

In spite of the variation noticed by Jirkov among his specimens of ‘ T. cincinnatus ’, there are some important differences from the material of the type locality of this species, after the redescription above. The material studied by Jirkov presents a wide mid-dorsal gap between left and right sides of branchial filaments within pairs, while among the specimens from the type locality such gap is narrow to almost absent. In addition, although Jirkov did not provide enough details of the notochaetae of his specimens, either on the description, or in the photo he showed ( Jirkov 2018: 52, Fig. 11B), that could allow the readers to understand their morphology, it is possible to see that the difference in length between chaetae of anterior and posterior rows is far less pronounced in his material than among the specimens from the type locality, and also the notochaetae from the anterior row of his specimens do not have the characteristic falcate shape of those of the specimens from Greenland, as shown herein (see Fig. 4A ).

Holthe (1986a) also provided a redescription for T. cincinnatus , but, except for the presence of more than 30 pairs of notopodia, most of the information provided are non-informative and could apply to many species of Thelepus . However, the drawings given by Holthe (1986a) for the uncini show a much longer prow than observed in the syntypes (compare Holthe’s Fig. 63d with Fig. 4 B–F in this paper).

Distribution. As in the case of S. bairdi , animals from many localities around the world were identified as T. cincinnatus , but it is recommended that material from outside the type locality region, western Greenland, is carefully reviewed. Currently, members of this species have been reported from the northern western and eastern Atlantic, from Newfoundland to the Arctic, and northern Pacific, from Japan to the state of Washington, USA ( Uschakov 1955; Imajima & Hartman 1964; Hobson & Banse 1981; Holthe 1986a, b; Jirkov 2018).

Figures

FIGURE 2. Streblosoma bairdi (Syntype, NRM 6491). (A). notochaetae, mid-body parapodium, arrows point to tips of chaetae of anterior row; (B–C). Tips of notochaetae of posterior row; (D–F). Uncini, mid-body parapodium.

FIGURE 3. Thelepus cincinnatus (Syntypes, NRM 1270). (A–C). Entire worms in left (A–B) and right lateral (C) views; (D–E). Anterior end in left lateral view, under progressively higher magnifications; black arrows in (E) point to nephridial/ genital papillae; (F–G). Anterior end in right lateral (F) and ventral (G) views; (H–I). Close ups of the anterior end in right lateral (H) and ventral (I) views; black arrows in (H) point to nephridial/genital papillae, white arrow points to first notopodium; arrow in (I) points to lobe of segment 1; (J). Posterior end. Numbers refer to segments; ll = lower lip; P = basal part of prostomium; ul = upper lip; * = distal part of prostomium.

FIGURE 4. Thelepus cincinnatus (Syntypes, NRM 1270). (A). Notochaetae, segment 9; (B–F). Uncini, segments 9 (B, C, E), 39 (F) and 51 (D).