(Figs. 2, 3)
Cheiloporidion pulvinatum Cairns 1983a: 83, figs. 11a, 12a–f, map 3; Cairns 1983b: 442, figs. 4a–g; Cairns & Macintryre 1992: 98, table 1; Río Iglesias et al. 2012: 191, 224; Bax & Cairns 2014: 107, 109, 110, table 1, map 5
Distribution. Off Bahía Blanca, 38° 58’ S; 55° 17’ W, 595–642 m; Cape Horn, 642–1137 m; Patagonia, 42° S to 48° S. New record off Mar del Plata, 852–1289 m.
Material examined. USNM 52649 (holotype) off Bahía Blanca, Vema St. 17-RD14 (38° 58’ S, 55° 17’ W); MACN-In 40641 off Mar del Plata, Argentina, St. 36 (37° 57.508’ S, 54° 23.989’ W), 1289 m, May 2013; MACN- In 42493 off Mar del Plata, Argentina, St. 38 (37° 59.308’ S, 54° 25.207’ W), 1099 m, May 2013; MACN-In 42494 off Mar del Plata, Argentina, St. 11 (37° 59.258’ S, 54° 41.436’ W), 854 m, August 2012; MACN-In 42495 off Mar del Plata, Argentina, St. 10 (37° 59.706’ S, 54° 41.854’ W), 852 m, August 2012.
Description. Uniplanar growth with non-expansive base. Dead specimens of the scleractinian Bathelia candida or octocorals frequently used as substrate for attachment (Fig. 2); several points of attachment with substrate, so primary basal branch is not recognizable. Dichotomous branching (Fig. 3a) with frequent anastomosis. Largest specimen examined (MACN-In 42493) 9.1 cm wide and 7.5 cm tall. Thickest basal branches 6.8 x 6.6 mm and 5.2 x 5.6 mm in diameter. Thinnest branches down to 1.0 x 2.0 mm in diameter. Anterior and posterior face well defined. Branches rectangular to elliptical in cross section, with larger axis perpendicular to plane of fan. Thickest branches usually smooth on posterior side whereas thinner ones frequently bear a thin ridge, which can be discontinuous, with its elements placed in postero-lateral position (Fig. 3d). Ridge may also be present on anterior side of branches, being coarser and less defined in thickest ones. Intermediate-diameter branches often bear vestigial ridges on posterior face, which are present as low, short longitudinal or diagonal coenosteum rims. Incipient branchlets have a more rounded tip and more dactylopores than elements of a discontinuous ridge, which are more sharply edged and bear one or no dactylopores (Fig. 3c, d). Polychaete tubes rectangular/ovoid in cross section, with larger axis parallel to plane of fan, present on anterior or posterior face, along or transverse to branches. Those on anterior face have a coarse texture, with granules and longitudinal coenosteum rims, whereas those in posterior face are usually smoother. Both anterior and posterior tubes may bear a few rounded perforations on top. Sides of tubes partially sealed, allowing worm to be seen through ovoid openings.
Coenosteum porcelaneous, pink to light orange in best preserved specimens. Texture reticulate in thickest branches, with convex coenosteal strips 41–68 µm wide. Texture of coenosteal strips smooth, although some sections may be granular. Granules irregularly shaped, possibly originated from a former reticular-imbricate texture (Fig. 3f). Incipient branchlets and ridge elements bear straight parallel coenosteal strips placed perpendicular to surface of branch (Fig. 3c, d).
Gastropores round, arranged in a row on lateral or antero-lateral sides of branches. Diameter 0.16–0.33 mm (average 0.24 mm, n=32, σ=0.05). Abcauline margin of gastropores often bears a bulbous prominence that corresponds to a developing branchlet (Fig. 3b, c). Gastropore tube short and peripheral. Basal section of tube spherical, following gastrostyle shape closely. At about 2/3 of gastrostyle total heigth, gastropore tube constricts taking on a conical shape which widens towards the surface. Illustrated gastrostyle (Fig. 3g) robust and short (Height: Width ratio=2.1). Base cylindrical, without spines and occupies about 1/3 of total gastrostyle length. Central third of gastrostyle widened in a crown of short, thick spines that fuse together and are arranged uniformly. At tube constriction gastrostyle narrows abruptly into a thin tip that almost reaches coenosteal surface and bears a few spines arranged in diagonal rows.
Dactylopores round, with an elevated perimeter (Fig. 3e) and scattered uniformly on all surfaces, although less abundant on posterior face. External and internal diameter 71–113 µm and 37–65 µm respectively. Dactylostyles absent. Ampullae ovoid, placed just below surface with larger axis parallel to it, up to 0.68 mm in larger internal diameter. Sex unidentified.
Discussion. Cheiloporidion pulvinatum was originally described by Cairns (1983a) based on specimens collected off Bahía Blanca and Cape Horn. That same year Cairns (1983b) redescribed the genus based on the same specimens and including more details. Bax and Cairns (2014) mentioned the occurrence of Cheiloporidion pulvinatum at two more locations, Burdwood Bank and Shag Rocks in the Scotia Arc, but specific information about the geographic location and depth of these stations, as well as number of specimens, has hitherto not been published. Río Iglesias et al. (2012) identified C. pulvinatum among stylasterids collected in Patagonia from 42° S to 48° S within a depth range of 200–1500 m (specific depth and position not given).
Specimens collected in the study area agree with Cairns’ (1983a, 1983b) description of Cheiloporidion pulvinatum in all characters described. Regarding coenosteal texture, Cairns (1983a, 1983b) described it as reticulate and smooth, without granules. Some of the specimens here described have coenosteal strips with a granular texture, so it is possible that the texture of C. pulvinatum is initially granulate and subsequently wears down. The bathymetric range and location of the collected specimens coincides with those previously reported by Cairns (1983a, 1983b).