Sibogaster digitatus Doderlein 1924

Sibogaster digitatus Döderlein, 1924

Figure 10 A–E

Döderlein, 1924: 63; A.M. Clark, 1993: 283 (as Sibogaster) Macan 1938: 383 (as Eugoniaster ephemeralis- new synonymy) A.M. Clark 1993: 277 (as Plinthaster ephemeralis)

Comments. The two additional records described below from the Philippines and Solomon Islands are the first two records of this species since its original description by Döderlein (1924) from Indonesia.

Comparison of this species with Eugoniaster ephemeralis Macan 1938 from original description suggests that it should be included in Sibogaster as a synonym of S. digitatus and not Plinthaster as implied by Halpern (1970a, b). This is based on the smooth, flat abactinal plates with “minute bosses”(interpreted elsewhere as the glassy tubercles), arrangement and shape of abactinal plates, the comparably elongate superomarginals (L>W), the mostly bare marginal plate surface showing only a few, widely spaced granules (Fig. 10 C), the consistent number of furrow spines (nine) and the overall similarity in body shape. The number of superomarginals per interradius is fewer than what has been documented below (22 per interradius at R=5.0 cm) versus 36–42 at R= 9.1 to 10.6 cm, but this could be attributed to the difference in size between individuals.

Smaller individual of a goniasterid which were identified as this species (R=3.2–3.3, CASIZ 186295) have been collected from the Philippines at 2107–2350 m. These showed a more stellate body form (R/r=2.3 vs. 1.7–1.8 in larger individuals) with more elongate arms and display inset crystalline bosses on abactinal and actinal plate surfaces.

Occurrence. Philippines, Solomon Islands, S. of Flores, Indonesia. 721–960 m

Description. Disk broad, arms triangular (R:r=1.75–1.86). Interradial arcs broad, linear (Fig. 10 A).

Abactinal disk plates round to polygonal, closely abutting becoming more irregular and forming a closer pavement on arms. Plate outlines becoming more irregular and smaller, distally adjacent to superomarginal plates. Abactinal plates with no granules (a plate separate from surface) on center of plate, surface are bare with no granules or spines (Fig. 10 B). However, small crystalline (i.e., translucent) bosses, 30–50 are embedded on the surface of each plate (Fig. 10 B). Each abactinal disk plate with large, quadrate peripheral granules 20–50 forming a distinct border proximally, sometimes flush with plate surface. Peripheral granules become smaller and more inconspicuous distally on abactinal arm plates. Small bosses also absent on distal abactinal arm plates. Papulae, three to six, are large and present on disk, at each “corner” of plate (Fig. 10 B, C). Papulae absent distally on arms and interradially on disk. Paddle-like pedicellariae, one per plate present primarily on disk, each about 0.75mm in length.

Marginal plate, 36–42 from armtip to armtip for each series. Superomarginals and inferomarginals mostly bare except for coarse widely spaced deciduous granules which are absent from arm plates (Fig. 10 C,D). Both superomarginal and inferomarginals with weakly concave to flattened central region. Granules on superomarginal surface vary from none to approximately 20. Granules on inferomarginals more numerous, ranging up to 30 interradially but absent on arms. Superomarginals elongate (L>W) and interradially becoming relatively wider (W>L) and more angular in cross-section distally (Fig. 10 D). Where granules absent in examined specimen, only concavities on marginal plate surface. Central granules densest proximally fading out distally becoming absent near terminus. Slight superomarginal/inferomarginal plate offset-primarily distally. Quadrate to rounded granules, ~60–100 surrounding each plate with approximately 15–25 per side. No fasciolar grooves. Superomarginals abutted over midline on three to five distalmost plates, penultimate superomarginals enlarged to twice the size as other superomarginals on arm. One arm with irregular plate arrangement, creating maze of abactinal/marginal plates. Terminal plate triangular, lacking granules.

Actinal intermediate regions large, with plates in four to six series adjacent to adambulacrals in chevron arrangement with other plates irregularly positioned (Fig. 10 E). Each plate polygonal to irregular in shape, covered by 50–125 coarse, hemispherical granules. Plates largest proximally becoming smaller, more numerous and more irregular adjacent to inferomarginals. Fasciolar grooves absent. Pedicellariae not common but present in one of five interradii. Inferomarginals form straight line contact with actinal plates. Adambulacral plates rectangular forming discrete dark contact lines adjacent to one another becoming more elongate distally.

Furrow spines eight to ten, quadrate to triangular in cross section, angular (Fig. 10 E). Discrete space separates spines from 3–5 short subambulacrals and 1–3 pedicellariae often in same series. Furrow spines tallest. Subambulacrals approximately 50% shorter than furrow intergrading to dense aggregate of granules identical to those of actinal plates. Oral plate furrow spines 12–15. Subambulacrals identical to those on adambulacrals 7–10 similar/adjacent to bivalve or trivalve pedicellariae. One to two spines projecting into mouth. Granules 10–16 flanking each gap produced by oral plates.

Color in life is bright orange.

Material examined. CASIZ 186315. Verde Island Passage, South China Sea, Philippines. 13.59767, 120.649667; 13.588167, 120.61533, 721–761 m, Coll. Mooi et al. California Academy of Sciences. 1 wet spec. R=10.6 r=5.7. CASIZ 186295, W of Simo Banks, NW of Lubang Island, Philippines, 14º07’N 119.97ºE, 2107–2350 m. Coll. R. Mooi, et al. aboard MV-DA-BF. 2 wet specs. R=3.2, r=0.5; R=3.3, r=1.4; IE- 2013-12582. Solomon Islands 7º58’S, 157º34’E, 836 m. Coll. Bouchet, Waren and Samedi aboard N/ O Alis, SALOMON- 2. 27 Oct 2004. 1 wet spec. R=9.1, r=5.2.