Tharyx obtusus Blake 2018, new combination

Tharyx obtusus (Hartmann-Schröder & Rosenfeldt, 1990) new combination

Figures 58–59

Caulleriella obtusa Hartmann-Schröder & Rosenfeldt, 1990: 114, figs. 31–34.

Tharyx sp. 1: Hilbig et al. 2006: 724.

Material Examined. Antarctica, off Elephant Island, R / V Walther Herwig, Cr. 68/1, Sta. 148, 23 Feb 1985, 61°12,7ʹS, 55°56.4ʹW, grab, 134 m, holotype (ZMH P-19952).— West Antarctic Peninsula, Bransfield Strait, off Low Island, R / V Hero Sta. 691-26, 119– 124 m (1, USNM 187622).— South Shetland Islands, off King George Island, R / V Polarstern, EASIZ II (ANT-XV/3), Sta. 325, 17 Mar 1998, 62°21.9ʹS, 58°42.6ʹW, SM, 805 m (1, SMF 24953).— East Antarctic Peninsula, Prince Gustav Channel, RVIB Nathaniel B. Palmer Cr. 2000-3, coll. J.A. Blake, SM grab, Sta. 01, 14 May 2000, 64°17.625ʹS, 058°34.678ʹW, 768 m (35, LACM AHF-Poly 10231); Sta. 34, 24 May 2000, 64°10.995ʹS, 058°34.140ʹW, 865 m (1, JAB); near Cape Longing, Sta. 02, 15 May 2000, 64°18.387ʹS, 058°37.911ʹW, 504 m, (12, MCZ 149873); Sta. 02A, 15 May 2000, 64°18.387ʹS, 058°37.911ʹW megacore, 504 m (1, JAB); East Antarctic Peninsula, former Larsen Ice Shelf A area, border with Larsen B, RVIB Nathaniel B. Palmer, Cr. 2000-3, coll. J.A. Blake, SM grab, Sta. 11, 18 May 2000, 64°56.669ʹS, 060°19.281ʹW, 350 m (20, USNM 1490754); Sta. 12, 19 May 2000, 64°55.101ʹS, 060°24.459ʹW (30, USNM 1490755); Sta. 13, 19 May 2000, 64°53.517ʹS, 060°28.836ʹW, 323 m (20, MCZ 149871); Sta. 14, 19 May 2000, 64°51.818ʹS, 060°33.438ʹW, 419 m (2, JAB).— East Antarctic Peninsula, Former Larsen Ice shelf A area, Greenpeace Trough, RVIB Nathaniel B. Palmer, Cr. 2000-3, coll. J.A. Blake, SM grab, Sta. 19, 20 May 2000, 64°42.778ʹS, 060°20.846ʹW, 879 m (3, JAB); Sta. 23, 21 May 2000, 64°47.144ʹS, 060°21.566ʹW, 901 m (4, MCZ 149875).— Weddell Sea, East of Larsen Ice Shelf A, RVIB Nathaniel B. Palmer, Cr. 2000-3, coll. J.A. Blake, SM grab, Sta. 25, 22 May 2000, 64°43.314ʹS, 059°38.459ʹW, 628 m (5, JAB); Sta. 26, 23 May 2000, 64°39.564ʹS, 059°13.226ʹW, 564 m (34, MCZ 149872); off Lindenberg Island, Sta. 03, 15 May 2000, 64°53.533ʹS, 059°30.694ʹW, 385 m (3, MCZ 149874).— Weddell Sea, East of Antarctic Peninsula, USCG Glacier, Sta. 68-5, 09 Feb 1968, 76°00ʹS, 55°00ʹW, 400 m (1, USNM 1490756)

Description. A moderately sized species, holotype 11 mm long, 0.5 mm wide across thoracic segments for about 85 setigers. Two specimens from NBP Sta. 01 (LACM AHF-Poly 10231), 9.0– 9.45 mm long and 365–435 µm wide for 76–85 setigers. Anterior end through first 15–20 segments dorsally flattened, expanded laterally, each segment relatively short, 9.5 times wider than long (Figs. 58A, 59A); following segments longer, about 2.5 times as wide as long, rounded in cross section, but not moniliform; posteriormost segments narrowest, somewhat flattened ventrally. Weak ventral groove present or absent. Color in alcohol light tan; a few specimens with black pigment spots on posterior lateral margin of peristomium.

Prostomium triangular, as long as wide; bluntly pointed on anterior margin (Figs. 58 A–B, 59A); a few specimens with prostomium shorter, more rounded anteriorly; eyespots absent; nuchal organs barely visible as notch at posterior margin. Peristomium elongate, slightly longer than wide, with three annular rings of similar length (Figs. 58 A–B, 59A), best seen laterally and when stained with Shirlastain A; with low dorsal crest. Paired dorsal tentacles arising from medial position at posterior margin of peristomium (Figs. 58 A–B, 59A); first pair of branchiae arising lateral and posterior to tentacles at border of peristomium and setiger 1 (Fig. 58 A–B).

Notosetae of first 60–65 setigers all capillaries, with those of first few setigers numbering 6–8 per fascicle and vaguely arranged in two rows with those of posterior row longer than those of first row; subsequent setae over most of body occurring in a single row, numbering 4–5 in a fascicle. Notosetae relatively long throughout body with those of posterior segments becoming curved and thicker, with some in far posterior segments becoming spinous rather than finely tipped (Fig. 58D); a few developing a flattened, knobby tip (Fig. 58E). Neurosetae of middle segments becoming curved, geniculate and shorter but with capillary tips; in far posterior segments 1–4 ventral most setae becoming hooked, with tips bearing two sub-bidentate knobs and with short barbs along concave margin producing a weakly serrated edge (Fig. 58 F–H); serrations not visible in all angles and otherwise difficult to detect on some specimens.

Pygidium with large ventral lobe, anal cirri absent (Figs. 58C, 59 B–C).

Variability. The greatest variability is in the nature of the posterior noto- and neuropodial spines on small specimens or juveniles with fewer than 25–30 setigers. In many cases, the spines are not curved or geniculate but appear straight and pointed except for those in the last 2–3 segments. While this likely means that with growth, the larger specimens will have more posterior segments with curved neuroacicular spines, it does mean that smaller specimens cannot always be reliably identified as a species of Tharyx.

Another point of variation is that the peristomial annulations are not apparent in juveniles, making the presetigerous region appear long, narrow, and smooth.

Methyl Green stain. No staining pattern apparent on either the type specimens or the new materials.

Remarks. Caulleriella obtusus agrees well with the definition of Tharyx as originally established by Blake (1991) and is here transferred to that genus. Tharyx obtusus has noto- and neuropodia set close to one another and has the classic knob or sub-bidentate tipped acicular spines first described by Blake (1991) for the type species, T. acutus. In addition, the presence of serrations along the concave margin of these same spines is similar to those of T. kirkegaardi from deep-water off North America (Blake 1991, 1996) and T. moniliformis n. sp. (see above). Tharyx obtusus is superficially similar to Chaetozone homosetosa in the same samples. However, that species has posterior acicular spines that are short, curved, and blunt-tipped instead of curved with knobby tips and a serrated edge along the shaft. In addition, T. homosetosa has a prominent MG staining pattern that is absent in T. obtusus.

Habitat. Tharyx obtusus was one of several bitentaculate cirratulids that were common and often abundant along the East Antarctic Peninsula in the Prince Gustav Channel and vicinity of the Larsen Ice Shelf A collected in May 2000. The surficial sediments in these locations consisted of 20–40% sand in the upper 0–5 cm (Gilbert & Domack 2003).

Distribution. Known only from off Elephant Island, Bransfield Straits, and off the East Antarctic Peninsula in depths of 119– 768 m.