Mediaster setosus n. sp.

Fig. 23 A–G

Etymology. The species epithet setosus is Latin for “bristles” and alludes to the bristle-like actinal surface present in this species.

Diagnosis. This species is stellate (R/r=2.0) (* Fig. 22A) with stout body, angular marginal plates, abactinal plates showing eight to 22 round granules, granule-covered prominent marginal plates, with abutted superomarginals on distal arm tip and furrow spines, five or six flanked by a row of subambulacral spines, four to six and with actinal surface distinctively covered with blunt spinelets four to 20, present on each actinal plate (Fig. 22F).

Comments. Based on the tabulae, marginal shape and actinal spination, this species shows similarities with Pseudarchaster (Pseudarchasteridae) . However, dissection of the abactinal surface showed that the tabulae possess five radiating ossicles present at the base of each plate (Fig. 23B), linking it most strongly with Mediaster or Rosaster . This species lacks the extensive superomarginal arm development, nor could superambulacrals be identified, as seen in Rosaster spp. which would appear to identify this species as being placed within Mediaster .

Within Mediaster, M. setosus n. sp. is most distinctively recognized for the numerous bristly spinelets present on the actinal plate surface. Nearly all of the other Mediaster spp. display granules on their actinal surface, except for the Atlantic Mediaster pedicellaris (as summarized in Clark and Downey 1992; Halpern 1970), which has a greater R/r (3.5) with much longer arms and has abactinal tabulae and marginal plate surfaces covered with spinelets. Mediaster setosus n. sp. displays a more weakly stellate body (R/r=2.0) with tabulae and marginals covered by round, granules. A small individual of M. ornatus (R=1.9, USNM E53688) displays spinelet-like granules and Macan (1938) indicates that M. murrayi shows some variation in actinal granulation, bearing both round and pointed granules. Spinelets present in M. setosus n. sp. however are discrete and cylindrical and appear similar to those present in Pseudarchaster . Well-developed fascioles are also present among abactinal and actinal plates.

This species also displays some similarities with Mediaster arcuatus (sensu Clark & McKnight 2001) in that the abactinal and marginal plate are similar in formation and shape. Mediaster arcuatus, however, has a granulate actinal surface, which is different from M. setosus n. sp., which is covered by blunt spines (Fig. 22F).

Occurrence. Western Indian Ocean, 11°29’S 47°29’E, 614– 583m

Description. Body stellate (R/r=2.0), arms triangular, interradial arcs broadly round (Fig. 23A, F).

Abactinal plates tabulate, plates round to polygonal in outline (Fig. 23 C, D). Most plates circular, carinal plates more oblong in shape. Abactinal plates, lower, abutted, smallest distally adjacent to, flush with superomarginals. Fasciolar grooves well-developed, papular pores especially present in these areas, interradial and distalmost areas areas shallower, with more weakly developed, papulae less abundant. Abactinal plates extend along most of arm stopping short at distalmost two or three superomarginal plates, which are abutted along midline. Base of each abactinal plate each with six radiating cylindrical plates (Fig. 23B). Abactinal plate surfaces with differing granule morphology. Peripheral granules, 8–22, mostly 15–18, elongate, cylindrical, blunt-tipped, widely spaced from one another. Central granules two to 14, round, widely spaced slightly shorter than peripheral granules (Fig. 23D). Granules interradially, fewer much more crowded per plate. Pedicellariae small (approximately <0.5 mm wide), tong-like roughly the same size as an individual granule (Fig. 23C). Papulae six per plate, present at each angle.

Superomarginals and inferomarginals, 26–28, arm tip to arm, tip rectangular, wide in shape, forming distinct peripheral abactinal border. Superomarginals form 20% (0.3/1.5) of the total “r” distance, distalmost two to three superomarginals abutted on arm tip. Superomarginal surface covered by 150–300 widely spaced deciduous, coarse grained granules, consistent in size, shape with granules on abactinal surface (Fig. 23E). Fasciolar grooves present between marginal plates. Superomarginals and inferomarginals offset, forming zig-zag contact. Inferomarginals covered by 150–250 blunt, cylindrical granules, widely spaced. Differing peripheral granules, 60–80, around inferomarginal edges, with pointed tips, consistent with actinal granule morphology. No pedicellariae observed. Terminal plate triangle in outline, bare surface.

Actinal surface in seven or eight series, each plate quadrate in shape, homogeneous in size, shape. Each plate with pointed spinelets, four to 15, widely spaced (Fig. 23F). Spines fewer per plate distally but more densely arranged. Actinal plates limited to disk and basal arm region. Fasciolar grooves shallow.

Furrow spines five or six, spines blunt, quadrate in cross-section with round tips in straight to weakly palmate series. Subambulacral spine series, four to six, triangular in cross-section, similar in size to furrow spines, immediately adjacent to furrow spines, but set off by distinct space (Fig. 23G). Remainder of adambulacral plate surface with two to three widely spaced pointed spines. Oral plate furrow spines, four or five with one spine pointed into oral region, triangular in cross-section. Oral plate surface covered by widely spaced pointed spines.

Material Examined. Holotype: IE-2013-17169 Western Indian Ocean, 11°29’S 47°29’E, 614– 583m. Coll. Samadi et al. BIOMAGLO 25.01.17. DW 4813. 1 wet spec. R=3.0 r=1.5