Asclerocheilus abyssalis, Blake, 2023

Blake, James A., 2023, New Species of Scalibregmatidae (Annelida) from Slope and Abyssal Depths off Eastern Australia, Records of the Australian Museum (Rec. Aust. Mus.) 75 (3), pp. 271-298 : 273-277

Asclerocheilus abyssalis sp. nov.

urn:lsid:zoobank.org:act:6FDBBB34-AEC2-48B2-A79D-3ED6B44B851C

Figs 1–2

Holotype: Eastern Australia, abyssal plain off Newcastle , New South Wales, RV Investigator, Sta. 065, coll. 30 May 2017, Benthic Trawl , distance 3.5 km, 33.441°S 151.469°E, 4280 m to 33.435°S 152.665°E, 4173 m (Australian Museum W.49504 ) . Paratypes (4): same data as (2, AM W.52704); abyssal plain off New South Wales, Jervis Marine Park, RV Investigator, Sta. 053, coll. 28 May 2017, Benthic Trawl, distance 4.2 km, 35.114°S 151.469°E, 3952 m to 35.084°S 151.441°E, 4011 m (2, AM W.52703) .

Description. A large species, all specimens complete, with a thick sausage-shaped body, narrowing only in far posterior segments ( Fig. 1A ). Parapodia reduced, only slightly enlarged on setigers 1–3; rest of body with no prominent podial lobes; parapodial cirri entirely absent. Holotype with 37 setigers, 4.6 cm long, about 1 cm wide (AM W.49504); largest paratype (AM W.52704) with 39 setigers, 4.9 cm long, and 0.8 cm wide. Body segments divided by a transverse groove, dividing each segment into two raised annuli extending across the body ( Fig. 1A, B ); these biannulate parts then subdivided by numerous crossing grooves consisting of rows of pads along body forming a basic biannulate pattern along body; some middle segments becoming triannulate. These pads best observed in far posterior segments; in anterior and middle body segments, annuli and pads becoming stretched and expanded by enlarged body, obscuring individual pads. Setigers 1–3 with grooves and pads not evident due to enlarged parapodia. Venter with shallow mid-ventral groove containing ridge along mid-line from posterior lip of mouth ( Fig. 1B ); ridge composed of raised rounded pads between segmental transverse grooves. Branchiae absent. Colour in alcohol light tan to grey; a few isolated areas of brown pigment associated parapodia of setigers 1–3 on holotype, otherwise without body pigment; paratypes without pigment.

Prostomium short, pear-shaped, bearing two short, forwardly directed lobes on anterior margin, each rounded on tips, appearing as short frontal horns ( Fig. 1D ); eyes absent; nuchal organs as a notch between prostomium and peristomium, swollen and everted on one paratype (AM W.52703). Peristomium a single lobed ring surrounding prostomium dorsally and surrounding mouth ventrally ( Fig. 1C ); oral opening surrounded by 9–10 elongate lobes; pharynx partially everted on one paratype (AM W.52703).

Parapodia reduced along entire body. Setigers 1–3 with noto- and neuropodia elongate rounded mounds separated by low ridge; large curved acicular spines arising from curved groove on posterior side of podia. Subsequent parapodia of middle segments reduced to minute, oval, barely visible mounds, dorsal and ventral cirri entirely absent.

Setae of setigers 1–3 each with a single row of large brasscoloured acicular spines in both noto- and neuropodia ( Figs 1B , 2A–D ), alternating in part with short spinous setae ( Fig. 2A, C, D ); these often observed anterior to larger spines in position occupied by furcate setae in more posterior setigers; spinous setae no more than 1/3 length of large acicular spines. Acicular spines of setigers 1–3 numbering 6–8 per notopodium and 6–7 per neuropodium; each spine curved, tapering to rounded tip ( Fig. 2A, B ).

Setae from setiger 4 through middle segments with 5–6 simple capillaries in both noto- and neuropodia. Furcate setae not observed in anterior and middle segments, but in prepared slide mounts, 3–4 minute capillaries, some sickleshaped, observed in a bundle at base of 1–2 of the larger noto- and neuropodial capillaries ( Fig. 2E ). Furcate setae of posterior setigers with unique morphology consisting of a short, narrow, sharply pointed tyne lacking denticles and a second tyne a long capillary-like blade with numerous denticles along inner margin ( Fig. 2F–G ). These unusual furcate setae numbering about 5–8 in posterior notopodia with 3–4 much larger simple capillaries; furcate setae also occur in posterior neuropodia but number no more than 2–4 with 2–3 capillaries. Typical short bifid furcate setae as found in other scalibregmatids not present.

Pygidium of holotype with about ten elongate lobes surrounding anal opening ( Fig. 1E ); cirri absent.

Remarks. Out of 15 known species of Asclerocheilus , only three including A. abyssalis sp. nov. are reported to have the three anteriormost setigers with acicular spines ( Table 1 ). The other two are A. intermedius ( Saint-Joseph, 1894) from the eastern North Atlantic in shelf depths, and A. shanei Hartmann-Schröder, 1994 from off Tasmania, Australia, also in shelf depths. Another Australian species, A. victoriensis Blake, 2000 from Victoria in shallow subtidal depths, has four anterior setigers with acicular spines. The other 11 known species have either only one or two anterior setigers with acicular spines.

Asclerocheilus abyssalis sp. nov. is the only abyssal species of the genus and differs from other species of Asclerocheilus and all known scalibregmatids in the nature of the unusual furcate setae where a simple short smooth tyne or spur extends from the base of a second branch consisting of a long capillary-like blade with numerous thin denticles along the inner side ( Fig. 2F–G ). These unusual furcate setae occur in fascicles with larger smooth capillaries in posterior setigers.

There is little DNA data for scalibregmatids of the genus Asclerocheilus . The DNA sequencing effort for A. abyssalis sp. nov. only produced results for COI and 18S rRNA. The successful COI sequence for A. abyssalis sp. nov. diverged as a long branch not clustering with any published sequences of Asclerocheilus or other genera. The 18S rRNA sequence has an uncorrected distance of 5–6% from various scalibregmatid genera.

GenBank Accession Number: mt COI: OP572115; 18S rRNA: OP583944.

Etymology. The epithet is from abyss, Latin for a bottomless pit or the deep sea, in reference to this species being collected from abyssal depths greater than 3000 m.

Distribution. Eastern Australia off New Wales, abyssal depths, 3952–4280 m.

Figures

Figure 1. Asclerocheilus abyssalis sp. nov. (A) Entire worm, right lateral view (from life); (B) anterior end, ventral view; (C) same, detail of pre-setiger region and setigers 1–3; (D) anterior end, dorsal view; (E) posterior end, dorsal view.A–C, E, holotype (AM W.49504); D, paratype (AM W.52704); B, C, stained with Shirlastain A; A, in life photo taken at sea by Ms Karen Gowlett-Holmes.

Figure 2. Asclerocheilus abyssalis sp. nov. (A) Setiger 2, anterior view; (B–D) acicular spines and short spinous setae from setiger 2; (E) posterior notopodium, anterior view; (F,G), Furcate setae.A–E paratype (AM W.52703); F–G, paratype (AM W.52704).