Spirodiscus groenlandicus McIntosh 1877, comb. nov.

Spirodiscus groenlandicus (McIntosh, 1877) comb. nov.

Figures 1 D, 6, 7

Ditrypa [SiC!] groenlandica McIntosh, 1877: 219.

Ditrypa groenlandica.— McIntosh 1879: 509, fig. 1A–B.— Hartman 1959: 573.—1971: 1426.— Zibrowius 1977: 292. Ditrupa grönlandica.— Ditlevsen 1914: 732.

? Filogranula spp.— Hartman & Fauchald 1971: 182 [in part, only in R/V CHAIN St. 85].

Material examined. R/V DISCOVERY, cruise 92, North-East Atlantic, south-west of Ireland: St. 9756 #14, 15.4.1978, 50°04.0'N, 13°55.6'W, 3680–3697 m (neotype NHMUK ANEA 2015.972, 1 spec. prepared for SEM AM W.46398).

R/V CRYOS, cruise ABYPLAINE: St. 10-DS10, 11.6.1981, 42°51.2'N, 15°55.3'W, 4270–4360 m (4 spec. MNHN PNT 26, 3 spec. SMF 23973, 1 spec. prepared X-ray diffraction analysis sample #1).

R/V LE SUROIT, cruise BENTHEDI, North Madagascar: St. 87 CH, 3.4.1977, 11°44'S, 47°35'E, 3716 m (5 spec. NBCL ZMA V. P o l 5542, 3 spec. LACM-AHF Poly 7015, 3 spec. NHMUK ANEA 2015. 973–975); St. 90 CH, 4.4.1977, 11°44'S, 47°30'E, 3700 m (3 spec. in tubes plus 1 removed, AM W.46399, tube fragments prepared for SEM PIN 5485/12 and X-ray diffraction analysis sample #2, 2 spec. USNM 1283053).

R/V MARION DUFRESNE, cruise MD 20, South-Eastern Madagascar: St. 15-CP8, 2.9.1979, 31°52.0'S, 48°28.8'E, 3825 m (1 spec. SMF 23972).

R/V CHAIN, St. 85, 37°59.2’N, 69°26.2’W, 3834 m (53 spec. and tubes LACM-AHF).

R/V VITYAZ-2, North Atlantic Ocean, St. 79, 34° 54.3N, 45°39.0’W, 4440 m (1 dry spec. in tube SIO).

Description. Tube: white opaque, free-lying, tusk-shaped, slowly expanding, octagonal in cross-section, with 8 smooth keels (longitudinal ridges) arranged all around the tube and grouped by pairs (Fig. 6 F, 7B). In spaces between two neighbouring keels (forming one pair) walls slightly thicker than in spaces separating different pairs. Sides between keels concave. Short growth stops resembling tiny irregularly displaced transverse constrictions present.

Tube ultrastructure: wall (Fig. 6 A) with two layers consisting of elongated rice grain-shaped crystals. Inner layer of irregularly oriented prismatic structure (IOP), crystal length up to 1.5–2.5 µm, diameter 0.5 µm. Innermost part of inner layer (Fig. 6 D) with isometric or slightly elongated crystals less than 0.5 µm long, with long axes oriented more or less along growth lines of parabolic lamellae. Middle and outer parts of inner layer (Fig. 6 C) consisting of elongated crystals of slightly larger size, oriented more or less along the growth lines with their long axes. Transition from inner to middle part of wall gradual, size of crystals gradually increasing towards outer tube surface. Outer layer (Fig. 6 B, E) very distinct, consisting of largest crystals, 4–5 µm long, 0.75–1 µm in diameter. Crystals of outer layer tending to form “bundles” of uniform orientation, lying more or less longitudinally along the surface (SOIOP structure). In transverse sections this layer usually looking like a pile of isometric crystals (Fig. 6 H) and hardly recognizable. Thickness of outer layer 5–12 µm (corresponding wall thickness 50 µm), no distinct boundary with inner layer. Parabolic lamellae often clearly visible (Fig. 6 A), marked by orientation of elongated crystals.

Tube mineralogy: sample #1: 100% aragonite (I arag=71); sample #2: 6% calcite (I calc=2), 94% aragonite (I arag=30).

Radiolar crown: 3–5 pairs of radioles not joined by inter-radiolar membrane. Radiolar eyes not visible in preserved material.

Peduncle: inserted as 2nd dorsal radiole, about twice as thick as radioles, with pinnules (Fig. 7 C, F).

Operculum: funnel-shaped, covered with concave brown chitinous endplate (Fig. 7 C, F); opercular bulb continuing smoothly into peduncle, constriction absent.

Collar and thoracic membranes: collar four-lobed, short, continuing into thoracic membranes reaching up to 2nd chaetiger (Fig. 7 E).

Thorax: with 5 thoracic chaetigers, 4 of which uncinigerous (Fig. 7 C). Collar chaetae simple limbate only (Fig. 7 F), of two sizes. Chaetae thick with distal limba, of two sizes, Apomatus chaetae absent (Fig. 7 G). Uncini saw-torasp- shaped, with 3–5 rows above wide peg divided into 2 (occasionally 3) lobes; with 14–15 teeth in profile view (Fig. 7 H). Dental formula slightly variable within a row of uncini, P:4:4:4:3:3:3:2:2:2:2:1:1:1:1, or similar (e.g. P:4:3:3:2:…, etc). Achaetigerous zone between thorax and abdomen absent.

Abdomen: with up to 40 segments. Chaetae short, with flat triangular denticulate blade (Fig. 7 I), slightly longer on posteriormost segments (Fig. 7 L); each torus with a single chaeta. Uncini rasp-shaped, with 9–12 teeth in profile view and 5–6 rows, dental formula P:6:5:5:5:5:4:4:3:3:3(Fig. 7 J).

Size: total body length up to 12 mm, including up to 1.5 mm long radioles, width of thorax up to 0.2 mm. Tube length up to 13 mm. Maximum external tube diameter in between angular margins up to 0.45 mm, corresponding lumen diameter 0.25 mm. Thickness of tube wall in between keels about 1/4–1/5th of the outer diameter, up to 1/2 when measured across keels.

Distribution. North Atlantic Ocean, 3404–4400 m; Indian Ocean, Madagascar, 3404–3716 m.

Remarks. This species is currently listed as nomen dubium in WORMS (ten Hove 2010). It was first found in 1875 as an empty tube from an abyssal location (3404 m or 2660 m) in the North-West Atlantic, Labrador Sea (R/ V VALOROUS, St. 12, 56°11'N, 57°41'W).The origin was incorrectly given as Davis Strait by Ditlevsen (1914) and Hartman (1959, 1971). In fact, it had been collected considerably further south in the North-West Atlantic on the return trip of an expedition to Davis Strait. The species was originally described as Ditrypa groenlandica by McIntosh (1877) who wrote: “ The tube is about half an inch in length, not much thicker than a thread, and curved from end to end like a bow (fig. 1). It tapers very gradually from the anterior to the posterior end, contrasting in this respect with the more decided diminution in D. arietina. The oval aperture (fig. 2) forms a smooth slightly constricted rim, which is narrower than the tube almost by the depth of the ridges. The latter are eight in number, and run from the anterior to the posterior end of the tube, though, it must, be added, none of the specimens were quite perfect. It was obtained from station ISTo. 12 (1450 fathoms). It differs from any other Ditrypa known to me in its slender form and the well-marked longitudinal ridges.”

Zibrowius (1977: 292) commented: “ looks strange for a serpulid, but C. P. Palmer (in litt.) suggested that it is not a scaphopod mollusc because of the unusually low expansion rate ”. The species remained enigmatic for decades, the type being lost. The problem is resolved now, as specimens that fit McIntosh's description and illustration are available from deep dredging in the North-East Atlantic. Moreover, material of Hartman & Fauchald (1971) from the North-Western Atlantic reported as? Filogranula spp. and re-examined by EKK contained numerous specimens of S. groenlandicus comb. nov. in typical free tusk-shaped octagonal tubes. An additional dried specimen in the tube collected from North-Western Atlantic was found in SIO collections (Fig. 1 D). Very similar free small-sized serpulid tubes with 8 longitudinal ridges from the SW Indian Ocean (depth about 3000 m) appear to be the same species.

The fact that tube keels are grouped by pairs (Fig. 6 F; Fig. 7 B) suggests that tetragonal cross-section probably was plesiomorphic, and subsequent bifurcation of keels resulting in octagonal cross-section is a secondary, apomorphic, modification. Fossil material provides examples of species with a similar morphology. Regenhardt (1961) established a taxon “Octogonae” of uncertain rank, but with the “generotype” Dentalium octocostatum Fraas, 1867, for octagonal tubes from the Late Cretaceous of Europe. Ziegler (2006) redescribed the same group of species as the genus Octogonella nov. gen., but designated another type species, Ditrupula faxensis Brünnich Nielsen, 1931. D. octocostatum has long large tubes (outer diameter up to 7–8 mm), curved somewhat irregularly, and clearly is not related to S. groenlandicus comb. nov. Instead, it is likely a member of the genus Pyrgopolon de Montfort, 1808 sensu Jäger 2005. Ditrupula faxensis has smaller tubes (outer diameter about 2 mm), but they are still extremely large relative to those of S. groenlandicus comb. nov. In conclusion, Octogonella Ziegler, 2006 is unlikely to be a junior synonym of Spirodiscus Fauvel, 1909, but this needs to be verified with examination of the type material.