(Figs. 20–22)
Stylaster erubescens: Moseley 1876: 94
Styalaster densicaulis Moseley 1879: 449–454, pl. 34, fig. 5, pl. 35, fig. 3, pl. 40; Moseley 1881: 57–62, 81, pl. 1, fig. 5, pl. 2, fig. 3, pl. 7; Boschma 1957: 4, 8; Cairns 1983a: 136–142, figs. 41a, 42a–i, 43a–b, map 13; Cairns & Macintryre 1992: 102, table 1; Pettibone 1993: 11, 14; Zamponi 2008: 188, 198, fig. 9; Roberts et al. 2009: 43, table 2.6; Bax & Cairns 2014: 108–111, table 1, map 4; Molodtsova et al. 2016: 392, table 25.1
Not Stylaster densicaulis: Hickson & England 1905: 7, 8, 12
Distribution. SWA off Argentina from Río de la Plata to Tierra del Fuego and Scotia Arc to South Georgia, 357– 1244 m. New record off Mar del Plata, 819–1289 m.
Material examined. USNM 60016 off Burdwood Bank, Eltanin St. 1593 (54° 43–42’ S, 56° 37–39’ W); MACN-In 40639 off Mar del Plata, Argentina, St. 36 (37° 57.508’ S, 54° 23.989’ W), 1289 m, May 2013; MACN- In 40640 off Mar del Plata, Argentina, St. 35 (37° 54.045’ S, 54° 24.091’ W), 1245 m, May 2013; MACN-In 42485 off Mar del Plata, Argentina, St. 31 (38° 1.499’ S, 54° 44.171’ W), 819 m, August 2012; MACN-In 42486 off Mar del Plata, Argentina, St. 15 (38° 0.500’ S, 54° 25.069’ W), 1200 m, August 2012; MACN-In 42487 off Mar del Plata, Argentina, St. 12 (37° 57.907’ S, 54° 31.921’ W), 1144 m, August 2012; MACN-In 42488 off Mar del Plata, Argentina, St. 42 (37° 59.110’ S, 54° 41.136’ W), 877 m, May 2013; MACN-In 42489 off Mar del Plata, Argentina, St. 37 (37° 59.848’ S, 54° 24.206’ W), 1275 m, May 2013.
Description. Specimens belong to Group C within Stylaster, given arrangement of cyclosystems exclusively on sides of branches (Cairns 1983b). Growth uniplanar, although some branches may deviate from plane. Branching dichotomous. Branches round to slightly elliptical in cross section, with larger axis perpendicular to fan. Largest specimen examined has a basal branch 20 x 15 mm wide and a non-expansive base. Branchlets rounder in cross section, may be down to 2 mm thick and loaded with ampullae and sympodial cyclosystems. Anastomosis frequent, as well as presence of polychaete tube along anterior or posterior face of branches (Fig. 20). External wall of polychaete tubes smooth and porcellaneous like rest of branches and bears lateral openings through which the worm is visible.
Coenosteum white, with reticulate-granular microtexture that may be reticular-imbricate (or a transition between both) in some sections (Fig. 21e). Coenosteal strips 63–95 µm wide. Slits curved, continuous and deep. In eroded sections granules may be rounder, smaller, sparser, and strips poorly delimited, probably because slits become shallower and only the deeper pores that pierced them remain (Fig. 21f). In other eroded fragments strips may be completely smooth (Fig. 21e). One or two large pores, possibly vestigial dactylotomes, are frequently present near cyclosystems (Fig. 21b).
Thinnest branches bear cyclosystems in sympodial arrangement whereas in thicker ones they are aligned along laterals. Cyclosystems round to elliptical shaped, 0.7–1.9 mm wide (average 1.4 mm, n=45, σ= 0.3), either flush with branch surface or raised, possibly originating a new branch. Gastropores 0.53–1.24 mm wide (average 0.82 mm, n=45, σ=0.18). Number of dactylotomes per cyclosystem 2–13 (mode 11, n=45). They are 74–146 µm wide, although there may be a dactylotome 50 µm wide. Dactylotomes extend downwards until around 1/5 of total gastropore tube height. Pseudosepta variable in width. Frequently one or two considerably wider than the rest are present in a cyclosystem (Fig. 21a, b). Cyclosystems from basal zones of colony bear less dactylotomes due to presence of diastema in adcauline margin.
Dactylostyle rudimentary, bearing two or three adjacent rows of rod-like elements up to 40 µm tall and 10–14 µm in diameter (Fig. 22b). Dactylostyle around 45 µm wide and extends downwards into depths of dactylotome.
Gastropore tube peripheral, mainly straight (Fig. 21c). Gastrostyle robust (H:W=1.8), conical in upper section and cylindrical at base, occupying 1/4 of gastropore tube height. Its conical section is covered uniformly in smooth spines with 2–4 tips that resemble deer horns (Fig. 21d). At about gastrostyle tip height a diffuse ring palisade is present.
Ampullae ovoid and conspicuous on surface of branches. Female ampullae up to 1.6 mm in external maximum diameter, bear a large lateral efferent pore and are undercut, which makes them protrude significantly (Fig. 22c, d). Male ampullae up to 0.9 mm in external maximum diameter, bear apical efferent pores which make them more conical in shape and are not undercut (Fig. 22a). Two or more ampullae often cluster, making it hard to quantify them. Male ampullae tend to align along branches whereas female ampullae tend to cover branch surfaces uniformly.
Small Gorgonocephalidae ophiuroids were clinging to one of the examined S. densicaulis specimens.
Discussion. Stylaster densicaulis was originally described by Moseley (1879) based on specimens from off Mar del Plata, collected in station 320 of the Challenger expedition. In his preliminary notes, Moseley (1876) mistook the specimens for S. erubescens Pourtalès, 1868. In 1879 he corrected this, naming the specimens S. densicaulis. Cairns (1983a) examined the holotype and specimens from New Guinea and Seram identified by Hickson and England (1905) as S. densicaulis and concluded that this identification was incorrect. He also described specimens from Scotia Arc, south of Tierra del Fuego and off Península Valdés, establishing the second record of this species. Río Iglesias et al. (2012) placed the third record of this species based on the identification of specimens from off Patagonia, from 42° S to 48° S within a depth range of 200–1500 m (specific depth not stated). Bax and Cairns (2014) show in Map 4 the occurence of S. densicaulis in Drake Passage and Antarctic Peninsula, but specific location of these stations and description of the specimens have not been published hitherto.
The examined specimens agree with S. densicaulis in growth form and cyclosystem, coenosteum and ampullar characteristics. The conical shape male ampullae adopt is here added to the description, since hitherto ampullae of this species have only been described as hemispherical or rounded (Moseley 1879, Cairns 1983a). The rectangular ridge Cairns (1983a) described was not identified in these specimens.