Tharyx moniliformis Blake 2018, new species

Tharyx moniliformis new species

Figures 56–57

Tharyx antarcticus: Blake & Narayanswamy 2004: 1806, 1813 (in part). Not Hartman 1978.

Material examined. East Antarctic Peninsula former Larsen Ice Shelf A area, RVIB Nathaniel B. Palmer, Cr. 2000-3, coll. J.A. Blake, SM grab, Sta. 05, 17 May 2000, 64°46.520ʹS, 060°10.720ʹW, 978 m, holotype (LACM- AHF Poly 10229) and 2 paratypes (LACM-AHF Poly 10230); Sta. 06, 17 May 2000, 64°45.518ʹS, 060°10.720ʹW, 733 m (2, MCZ 149862); Sta. 7, 18 May 2000, 64°43.523ʹS, 060°04.771ʹW, 839 m, 11 paratypes (MCZ 149863); Sta. NBP 14, 19 May 2000, 64°51.818ʹS, 060°33.438ʹW, 419 m (1, MCZ 149864); Sta. 21, 20 May 2000, 64°45.827ʹS, 060°19.450ʹW, 912 m, 3 paratypes (MCZ 149865); Sta. 23, 21 May 2000, 64°47.144ʹS, 060°21.566ʹW, 901 m, 3 paratypes (MCZ 149867 ). — Weddell Sea, East of Larsen Ice Shelf A area, 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 (6, MCZ 149868); Sta. 26, 23 May 2000, 64°39.564ʹS, 059°13.226ʹW, 564 m (8, MCZ 149869).— Weddell Sea, E of Antarctic Peninsula, R / V Polarstern, ANDEEP II (ANT-XIX/4), Sta. PS61/131-7, 06 Mar 2002, 65°19.45ʹS, 51°30.97ʹW, MUC, 3057 m (1, MCZ 149870); Sta. PS61/132-4, 06 Mar 2003, 65°17.75ʹS, 53°22.92ʹW, box corer, 2085 m, 2 paratypes (SMF 24947); Sta. PS61/132-5, 07 Mar 2002, 65°17.68ʹS, 53°23.00ʹW, MUC, 2084 m (1, SMF 24948); Sta. PS61/132-6, 07 Mar 2002, 65°17.77ʹS, 54°0.00ʹE, box corer, 2086 m (2, SMF 24949); Sta. PS61/133-6, 07 Mar 2002, 65°20.18ʹS, 54°143.6ʹW, MUC, 1120 m, 1 paratype (SMF 24950); 2, photographed alive (JAB); Sta. PS61/133-7, 07 Mar 2002, 65°20.10ʹS 54°14.87ʹW, box corer, 1110 m (1, SMF 24952); Sta. PS/139-10, 20 Mar 2002, 58°14.18ʹS, 24°20.47ʹW, box corer, 3935 m (2, SMF 24951).— Scotia Sea, Powell Basin, R / V Polarstern, ANDEEP III (ANT-XXII/3), Sta. PS67/121-10, 14 Mar 2002, 63°41.74ʹS, 50°42.99ʹW, box corer, 2621 m (2, SEM stubs JAB).

Description. A small, fragile, threadlike species; holotype complete, 7.12 mm long, 0.25 mm wide across middle segments, with 52 setigers; complete paratype from ANDEEP II Sta. 133-6 (SMF 24950), 8.5 mm long, 0.2 mm wide across anterior segments, with 68 setigerous segments. Body more or less cylindrical throughout with no obvious dorsal or ventral ridges or grooves. Anterior segments narrow, but no more than three times as wide as long (Figs. 56B, 57E); middle and posterior segments rounded, moniliform (Figs. 56C, 57C). Most specimens with some middle and posterior segments distended with eggs and/or developing embryos (Fig. 57 C–D, F–H). Far posterior segments tapering to simple rounded pygidium (Fig. 56C). Color in alcohol opaque white, no body pigments except 1–2 black spots laterally in middle of peristomium on some specimens (Fig. 56A).

Pre-setal region long, narrow, smooth, lacking any obvious annuli (Figs. 56 A–B, 57C–E). Prostomium triangular, tapering to narrow rounded apex (Figs. 56 A–B, 57C–E); eyespots absent; nuchal organs not observed. Peristomium narrow, elongate, about twice as long as wide (Fig. 56 A–B); dorsal tentacles arising from near posterior margin, with first pair of branchiae located directly posterior to tentacles (Fig. 56 A–B). Second pair of branchiae arising from posterior margin of setiger 1, dorsal to notosetae (Fig. 56 A–B); subsequent branchiae from a similar location in subsequent segments. Branchiae not observed after about 20 segments.

Anterior setae all capillaries, with 4–6 in notopodia and 4–5 in neuropodia. Some capillaries of middle noto- and neuropodia becoming shorter, thicker, transitioning to curved acicular spines in posterior segments from about setiger 40. Notoacicular spines numbering 1–2 per notopodium, accompanied by 1–2 capillaries; neuroacicular spines numbering 3–4 per neuropodium, with 1–2 capillaries. Noto-acicular spines short, weakly geniculate, with a swollen tip usually having a clear center (Fig. 56D). Neuroacicular spines with sub-bidentate apical knob on concave side longer than on convex side, these spines also with short barbs or serrations along concave side of shaft (Figs. 56E, 57 A–B).

Methyl Green stain. No pattern, body not retaining stain.

Etymology. The species name is derived from monile, Latin for a “string of beads” and forma, Latin for “shape”, referring to the bead-like segments that occur on this small species.

Remarks. Tharyx moniliformis n. sp. is a small threadlike species that can be distinguished from other species in Antarctica by the presence of moniliform middle and posterior segments that, when mature, bear dorsal eggs or brood chambers. In addition, the morphology of the two types of posterior acicular spines and the presence of a row of serrations along the concave side of the shaft of the sub-bidentate acicular spines are diagnostic. The subbidentate acicular spines are typical for other species, but to date, only T. kirkegaardi Blake, 1991 from deep-water off North America and T. obtusus from shallow depths off the Antarctic Peninsula have been recorded with subapical serrations along the shaft (Blake 1991; 1996; this study).

Habitat & biology. The specimens reported here were collected as part of the Larsen Ice shelf A cruise along the eastern Antarctic Peninsula in May 2000 and the ANDEEP II cruise off the Peninsula in the Weddell Sea in January and February 2002. The surficial sediments in the vicinity of the Larsen Ice Shelf area consisted of 20–40% sand in the upper 0–5 cm (Gilbert & Domack 2003). The surficial sediments along the Weddell Sea transect consisted of dark greenish-grey silt or clay with numerous pebble-sized drop stones observed on the surface. The fine sediments below the surface were observed in sediment profile images to have considerable bioturbation with feeding voids visible to a depth of 11.8 cm (Diaz 2004).

Most specimens are gravid females with long natatory notosetae and swollen segments bearing large rounded eggs and/or developing embryos (Fig. 57 C–D, F–E). The swollen upper half of these segments appear to be brood chambers. Eggs measured from 85–135 µm in diameter and there are no more than 1–2 per segment where they occur. There appears to be only one embryo developing per segment. At the stages observed, each embryo consists of numerous cells that form an elongate mass, some of which are curved into what might be anterior and posterior ends. Some of these were damaged by handling and preservation, but study of living specimens and thin sections of the gravid segments would help understand how these embryos are developing.

Distribution. Off the East Antarctic Peninsula and central Weddell Sea, 564–3935 m.