Terebellides crux, Schüller, Myriam & Hutchings, Pat, 2013

Schüller, Myriam & Hutchings, Pat, 2013, New species of Terebellides (Polychaeta: Trichobranchidae) from the deep Southern Ocean, with a key to all described species, Zootaxa 3619, pp. 1-45 : 12-16

Terebellides crux sp.n.

Figs 6, 7 , 8

Holotype: ANDEEP III, St. 121-11, EBS (ZMH-26027)

Paratypes: ANDEEP II, St. 132-6, GKG [drawing, MG photo] (ZMH-26008); ANDEEP III, 110-8, EBS (ZMH-26026); ANDEEP III, St. 121-11, EBS (5 specimens, AM W39656, AM W39996, AM W39657, AM W39658, AM W39659); ANDEEP III, St. 121-11, EBS [SEM, stub MI496] (AM W38714); ANDEEP III, St. 16- 10, EBS (ZMH-26028); ANDEEP III, St. 78-9, EBS (2 specimens, ZMH-26029, ZMH-26030)

Description: (Based on both holotype and paratypes)

Holotype, 7 mm in length, 0.5 mm in width, posteriorly incomplete. Paratypes ranging from 6–8 mm in length, 0.5–1.0 mm in width, majority posteriorly incomplete. Complete specimens with up to 35 abdominal chaetigers.

Head region: Tentacular membrane compact, not greatly expanded. Lower lip rectangular, distinctly folded upwards with smooth rim ( Fig. 6). Tentacles only short, thick, wrinkled, present on outer margins of tentacular membrane in some specimens ( Fig. 6), others with long numerous tentacles of two types (simple and with expanded tips) ( Fig. 7 A–C, E).

Branchiae: Lobes lost with only stem present on holotype, but part of lobes present in paratypes ( Fig. 7 A). Thick wrinkled stem with lobes with few short compact lamellae ( Fig. 7 A–C), ciliary fields present between lamellae ( Fig. 7 D). Lobes free from each other, all of similar size, with distinct filamentous tip ( Fig. 7 B, C).

Anterior chaetigers: Notopodia from segment 3, 18 pairs. Chaetiger 1 reduced in size with chaetae more or less originating from body wall and more dorsally aligned than subsequent ones ( Fig. 7 A, F). Following chaetigers with elongate rectangular notopodia with elongate capillary notochaetae, numbering 6–8 per podium arranged in two tiers ( Fig. 7 A, F); capillaries with fine pointed tips, thecae with flared tips ( Fig. 7 F). Neuropodia from segment 7 (chaetiger 5). First neuropodia with 3 curved hooks with extended pointed tips ( Fig. 7 G). Segment 8 (chaetiger 6) with 4 curved hooks ( Fig. 7 G). Subsequent neuropodia with 3 or more long shafted denticulate hooks with main fang and several multidentate rows of teeth above, not vertically aligned ( Fig. 7 H) so providing a dental formulae not possible.

Lateral lappets: Present from thoracic segments 1–6, with thoracic chaetigers TC-1=2<3>4=5=6 ( Figs 6, 7 E).

Ventral pads: Ventrum glandular, segments 2 and 3 narrow, width of ventrum increasing on segments 4-6, with anterior margins expanded forming thickened membraneous collars ( Fig. 7 E). Subsequent segments narrow ventrally with anterior margins forming thickened collars ( Fig. 7 E).

Nephridial papillae: Present on segments 3, 6, 7 ( Fig. 7 A).

MG staining pattern 1 ( Figs 2 , 8 ): With anterior segments solidly stained and striped median thoracic segments. Distinct white bands or pronounced anterior margins of segments lacking; noto- and neuropodia stain. Pygidium: Rounded blunt cone without appendages.

Remarks: This new species is characterised by having the first two rows of neuropodia with geniculate hooks starting on chaetiger 5, a character only known for three other species of the genus: T. biaciculata Hartmann- Schröder, 1992, T. bigeniculatus Parapar, Moreira & Helgalson, 2011 and T. intoshi Caullery, 1915 . According to Imajima and Williams (1985) in T. intoshi geniculate hooks are present in chaetigers 6 and 7 as opposed to chaetigers 5 and 6 in the other three species, this trait was however not mentioned in the original description by Caullery (1915). Terebellides crux sp.n., clearly differs from T. biaciculata and T. bigeniculatus by its branchial lobes, which are free and all of equal length in T. crux sp.n., and partially fused with distinctly larger anterior lobes than posterior lobes in T. biaciculata and T. bigeniculatus .

All other described species of Terebellides have neuropodia starting in chaetiger 6 and only the first neuropodia with geniculate hooks, the remaining with long shafted denticulate hooks. This has necessitated the modification of the generic diagnosis which we have preferred to do rather than erecting a new genus at this stage.

Habitat: South eastern South Atlantic and Weddell Sea deep waters in 2086–4720 m.

Etymology: The species name crux is an alternative name for the Southern Cross.

Figures

FIGURE 2. MG staining patterns of observed species, adapted and extended from Schüller & Hutchings (2010).

FIGURE 7. Terebellides crux sp. n. (AM W 38714), A—anterior thorax, lateral, B—anterior thorax, dorsal, C—branchiae, D—branchial lamellae, detail, E—anterior thorax, ventral, F—median thoracic notopodium, G—geniculate neuropodial hooks on TC- 5 and TC- 6, H—thoracic uncini of median neuropodium (Arrows indicate filamentous tips).

FIGURE 8. Terebellides crux sp. n. (ZMH- 26008), MG-staining, A—thorax, dorsal, B—anterior thorax, dorsal, C—thorax, ventral, D—geniculate hooks on TC- 5 and TC- 6.