Therochaeta flabellata

Therochaeta flabellata (Sars in Sars, 1872)

(Fig. 4)

Trophonia flabellata Sars, 1869: 253 (species list, nomen nudum); Sars in Sars 1872: 409; 1873: 249-252, pl. 17, figs 1-12.

Stylarioides flabellata – McIntosh 1915: 100-102, pl. 94, fig. 1, pl. 96, fig. 4, pl. 104, figs 3-3e. — Fauvel 1927: 117, figs 41h-m.

Pherusa flabellata – StØp-Bowitz 1948a: 18 -20, fig. 3; 1948b: 36. — Hartmann-Schröder 1971: 372; 1996: 420, fig. 204. — Gillandt 1979: 57, fig. 21. — Jirkov & Filippova 2001: 362, figs 1-4

Therochaeta flabellata – Fauchald 1972: 416 (n. comb.). — Castelli 1990: 15

MATERIAL EXAMINED. — Northeastern Atlantic Ocean. Six specimens (ZMUB 18602), damaged, Osterfjord, Norway, 20.I.1930 (10-13.5 mm long, 1.5-2.0 mm wide, cephalic cage 1.2-2, 16-21 chaetigers; ova 125 µm). Four specimens (ZMUB 2285), damaged, only one complete, Hardangerfjord, Norway, Danielsen coll. (complete 12 mm long, 2 mm wide, cephalic cage 4 mm long, 20 chaetigers). Two specimens (ZMUB 41770), both with anterior end everted, Bergensfjord Strait, Norway, 17.III.1924, AppellØf coll. Two specimens (ZMUB 41771), R/V Michael Sars 1906 Expedition, Stat. 300, 7.VII.1906 (9.0- 10.5 mm long, 1.8 mm wide, cephalic cage 5.5 mm long, 17-18 chaetigers). Three specimens (LACM-AHF), Hardangerfjorden, Stat. 13-52, 144- 133 m, 23.IX.1958, K. Fauchald coll. (12.0- 21.5 mm long, 1.3-2.5 mm wide, cephalic cage 6 mm long, 19-30 chaetigers). One specimen (HDMSU unnumb.), R/V Sevastopol, Stat. 5.1092, 250 m, 12.VII.1957 (10 mm long, 1 mm wide, cephalic cage 5 mm long, 20 chaetigers; pseudocompound hooks in chaetigers 3-8).

DISTRIBUTION. — Northeastern Atlantic Ocean, Norwegian Sea, Arctic Ocean.

DESCRIPTION

Better specimens (ZMUB 41770) with body cylindrical, tapered posteriorly, with marked constrictions in chaetiger 2 and in some posterior chaetigers, looking beaded. Body completely covered by fine sediment grains, mostly whitish or grayish, with some black particles (Fig. 4A); 13.5-16 mm long, 1.5-2.0 mm wide, cephalic cage 3.5 (broken) – 5.0 mm long, 24-25 chaetigers. Tunic thick, with fine sediment particles. Body papillae long, digitate, arranged into two transverse rows, better defined at the level of chaetal lobes, becoming three irregular rows in posterior chaetigers.

Cephalic hood tube short (Fig. 4B), margin with tiny rounded papillae. Prostomium low, without eyes. Caruncle well developed, running to the posterior margin of the branchial plate, not separating branchiae into lateral groups. Palps pale, long, rugose (ZMUB 18602), about three times thicker than branchiae. Palp keels low, rounded. Dorsal and ventral lips reduced; lateral lips thick, pale.

Branchiae cirriform, arranged into two irregular, symmetrical, discontinuous lateral rows (Fig. 4C); each row with two middorsal larger filaments, and three thinner, inferior filaments; inner row with one large filament placed between the middorsal and inferior filaments and another medium-sized inferior filament, making 12 total branchial filaments, only scars left. Banchiae longer and much thinner than palps (ZMUB 18602). Nephridial lobes 2, minute, digitate, placed between branchial groups, externally to the most middorsal filament in each side.

Cephalic cage chaetae golden (coppery in ZMUB 41771), as long as three times body width. Chaetigers 1-2 involved in cephalic cage; chaetae arranged in short lateral series, chaetiger 2 slightly dorsally displaced. Chaetiger 1 with thicker, larger chaetae; six noto- and four neurochaetae; chaetiger 2 with thinner, shorter chaetae, four per bundle. Anterior dorsal margin of first chaetiger with median, truncate anterior lobe; margin papillated. Chaetigers 1-2 with very long papillae, arranged in a postchaetal fan, with up to seven papillae per bundle. Chaetiger 1 short, chaetiger 2 longer, posteriorly slightly constricted, chaetiger 3 shorter than chaetiger 2. No post-cephalic cage chaetigers elongated. Chaetal transition from cephalic cage to body chaetae abrupt, pseudocompound neurohooks start in chaetiger 3. Gonopodial lobes not seen.

Parapodia poorly developed; only chaetigers 1-4(5) more projected from the body wall; in others chae - tae emerging from the body wall. Parapodia lateral; median neuropodia ventrolateral. Noto- and neuropodia in chaetigers 1-2 with foliose chaetal lobe and five-seven elongate papillae; remaining notopodia less pronounced with one-three infrachaetal papillae per bundle; remaining neuropodia reduced with one-two papillae per bundle. Noto-and neuropodia not closely aligned.

Median notochaetae arranged in short longitudinal series; all notochaetae multiarticulate capillaries, articles medium-sized medially, elongating distally, three per bundle, as long as 1/5-1/6 body width. Neurochaetae multiarticulate capillaries in chaetigers 1-2. Chaetigers 3-8 with pseudocompound hooks, one (rarely two) per ramus (Fig. 4D), decreasing in size posteriorly; each with anchylosed, short articles basally, longer medially, blade smooth. Chaetiger 9 with yellowish, shorter, falcate simple hooks, arranged in a transverse series, two-three per ramus (Fig. 4E); each with anchylosed, short articles basally, longer medially, distally hyaline.

Posterior end slightly reddish or brownish, without sediment, tapered into a blunt cone; pygidium with anus terminal, no anal cirri.

REMARKS

Therochaeta flabellata was made a new informal combination by Fauchald (1972) and followed by Castelli (1990). However, the Mediterranean form differs in the relative size of sediment particles, being mostly fine in the Northwestern European populations whereas they are much larger in Mediterranean ones (see below).

Therochaeta flabellata and the similar species from the Mediterranean Sea are the only two species having sediment particles covering most of the body surface, and both have a reduced sediment cover along the last few chaetigers. The nominal forms differ from those present in the Mediterranean Sea especially in the relative size of the sediment particles on their tunics; the Norwegian form has fine sediment particles whereas the Mediterranean form has larger sediment particles and is informally described below.

Therochaeta cf. flabellata Mediterranean Sea (Fig. 5)

Stylarioides flabellata – Fauvel 1927: 117, figs 41h–m (partim, non Sars in Sars, 1w872).

Therochaeta collarifera – Amoureux 1982: 192, fig. 2Tc (non Ehlers, 1887).

Therochaeta flabellata – Castelli 1990: 15 (non Sars in Sars, 1872).

MATERIAL EXAMINED. — Mediterranean Sea. One specimen (USNM 299599), cephalic cage damaged, apparently made only with chaetae from chaetiger 1. One specimen (SMF 11284), Ionian Sea, Mediterranean Sea, 1008 m. Indian Ocean, Gulf of Oman. One specimen (LACM / AHF unnumb.), broken in two.

REMARKS

The Mediterranean form differs from the Norwegian one (McIntosh 1915, StØp-Bowitz 1948a, b) because its chaetigers 1-2 are not fused, it has large particles over the body (Fig. 5A), whereas the Northern form adheres fine sediment grains only. This was already stated in the original description (Sars 1872: 409): “Cutis arenulis minutis cinereis papillisque minimus conicis […]” (Transl.: “Skin with minute, grayish sand attached to minute papillae”).This species does not belong into Pherusa, as correctly indicated by Fauchald (1972) and Castelli (1990) because it has sediment particles covering the body, pseudocompound neurohooks in some anterior chaetigers (Fig. 5B), and falcate neurohooks in posterior chaetigers (Fig. 5C). This species has been recorded from the Mediterranean Sea by Castelli (1990); Indian Ocean specimen resembles the Mediterranean one, but the scarcity of specimens from the same locality has blocked its description for this contribution.