Plinthaster untiedtae, Mah, 2018

Mah, Christopher L., 2018, New genera, species and occurrence records of Goniasteridae (Asteroidea; Echinodermata) from the Indian Ocean, Zootaxa 4539 (1), pp. 1-116 : 79-81

Plinthaster untiedtae n. sp.

Figure 30 A–E

Etymology. The species name honors colleague Dr. Candice Untiedt, at the University of Tasmania, formerly of the Iziko Museum in Cape Town South Africa.

Diagnosis. Body weakly stellate (R/r=1.34–1.4) with curved to straight interradial arcs ( Fig. 30A,D ). Marginal plates six per interradius, superomarginals strongly convex ( Fig. 30A, C ). Distalmost superomarginal plates enlarged, irregularly polygonal in shape ( Fig. 30C ). Furrow spines six or seven, triangular to quadrate in crosssection, with angular edges, closely arranged with adjacent first row of subambulacrals, which are approximately twice as thick as furrow spines ( Fig. 30E ).

Comments. This species bears an immediate resemblance to Sphaeriodiscus mirabilis as described by A. M. Clark (1976) from St. Paul and Amsterdam Islands. It possess a comparable number of marginal plates in addition to abactinal plates that are similar in shape and arrangement. It is immediately distinguished by the complete absence of granules from the abactinal and marginal plate surface. The presence of crystalline tubercles embedded in the abactinal plates in addition to the complete absence of surficial granules on the abactinal plate was a primary identifying character of Plinthaster .

This species shows a close similarity to Plinthaster lenaigae n. sp., also collected from the Madagascar region. Specimens of known species are comparable in R, which is approximately 3.0–3.2 for specimens examined, making size-based explanations for these differences unlikely. Plinthaster untiedtae n. sp. is superficially similar in terms of marginal plate number (six per interradius) and in the presence of enlarged penultimate superomarginals. However, the superomarginals in P. untiedtae n. sp. are more strongly convex and the penultimate superomarginals are irregularly polygonal in shape, as opposed to those in P. lenaigae n. sp., which are more triangular.

The most significant difference between the two species, however, lies in the morphology of the furrow spines and adambulacral spination. Furrow spines in P. untiedtae are triangular or quadrate with strong angular corners. Subambulacrals number two or three per plate and are enlarged, and approximately twice as thick as the furrow spines. Pedicellariae are few to absent. In contrast, Plinthaster lenaigae n. sp. shows compressed furrow spines with rounded edges, which are quadrate in cross-section. Subambulacrals adjacent to the furrow spines number one to three and are similar to the furrow spines in thickness being separated widely on the adambulacral plate. Pedicellariae are present on each adambulacral plate and are present, one or two, on nearly every actinal plate adjacent to the oral region adjacent to the mouth

Plinthaster lenaigae n. sp. and Plinthaster untiedtae n. sp. share several characters that suggest they are closely related. Phenotypic variation is poorly understood in goniasterids and some characters such as pedicellariae can show a wide range of variation (e.g. Mah et al. 2014). Mortensen (1933) interpreted enlarged superomarginal plates as variable phentoypes in South African goniasterids, including Calliaster acanthodes and Toraster tuberculatus .

Occurrence. Southern Madagascar, 618– 624 m.

Description. Body stout, pentagonal to weakly stellate (R/r=1.39–1.4) in shape arms short, arm tips blunt, rounded. Interradial arcs wide, weakly curved to straight ( Figs 30A, D ).

Abactinal surface composed of abutted plates. Individual plates round to polygonal in outline, flat to weakly convex plate surface. Plates becoming more irregular, smaller in size distally, adjacent to superomarginal plates. Surficial accessories, such as granules, absent, surface smooth and bare. Crystalline bodies, 15–200, mostly 100– 150, embedded in plate surface of each plate in sunburst like patterns with linear series, each composed of approximately eight to 10 crystalline bodies, radiating out from center of plate ( Fig. 30B ). A single granular series, 20–40 present around each plate, each granule round to quadrate in shape. Madreporite quadrate in shape, level with other abactinal plates, flanked by four abactinal plates. Pedicellariae, small (<0.5 mm) tong-like in shape, present on a minority of plates on abactinal surface, sitting in small pits when present. Papulae present along radial disk regions, absent interradially.

Superomarginals, inferomarginals six per interradius (arm tip to arm tip), each individual plate strongly convex, quadrate in shape especially interadially becoming wider distally with superomarginals and inferomarginals adjacent to terminal plate enlarged ( Fig. 30C ), attaining approximately twice the size of the adjacent marginal plate and becoming strongly convex with one edge abutted over the midline of each arm. Superomarginals with widely scattered, round, deciduous granules, 20–50 ( Fig. 30C ) leaving pitting on plate surface where lost. Superomarginal plate surface otherwise bare and smooth without surficial accessories (e.g., no granules). Pedicellariae, tong-like, <0.5 mm in length, one to four, mostly three, present on the superomarginal surface adjacent to the abactinal surface. Inferomarginals with a raised, bare surface, covered with widely scattered round granules, 100–400, identical to those on superomarginal surface but more closely arranged, more abundant. Dense arrays of pointed to blunt granules, 20–60, similar in size to those on scattered inferomarginal face are present on inferomarginal adjacent to actinal surface, arranged more closely than the others on the marginal plate surface. Marginal plate with widely spaced pointed granules present around the periphery of the plates, approximately 30 along each width, 60 along each length, totally approximately 180–200 forming periphery around each plate. Small tong-like pedicellariae identical to those present on the other body surfaces present on the inferomarginal plates, approximately one or two per plate. Terminal plate triangular in shape, smooth surface sitting between two enlarged superomarginal plates.

Actinal surface composed of three full series in chevron like formation, including one to three irregular series plates ( Fig. 30D ). Individual plates quadrate to polygonal in shape. Plate surface composed of round to polygonal granules, 20–60, mostly 30–50, with blunt tops, densely but evenly spaced. Peripheral granules arranged into approximately five to 10 per side of each plate. One to three plates with a single small tong-like pedicellariae, mostly adjacent to oral plates (proximally on actinal intermediate region).

Furrow spines six to seven (proximally to distally), blunt tipped, triangular to quadrate in cross-section, closely arranged with complimentary edges ( Fig. 30E ). Furrow spine series straight to weakly curved. Subambulacral spines, one or two, blunt, round tipped, each spine twice as thick as one of the furrow spines, sitting immediately behind furrow spines. Subambulacral spine tips with weak to well-developed notches. A discrete space present between the furrow and subambulacral spines. Proximal side of the adambulacral plate each with incipient pedicellariae composed of two or three short valves. Remainder of adambulacral plate with two or three rows of blocky angular granules, each row composed of four to seven, short angular granules, variably sized which gradually assume identical stature with granules on the actinal plate surface. Oral plate furrow spines fourteen, crowded, with one spine projecting into mouth from each plate (two total projecting into mouth), all spines triangular to angular in cross-section. Three subambulacral spines behind furrow spines on oral plate. Oral plate surface with eight or nine quadrate to triangular granules present in paired formation over oral plate sulcus bisecting center of oral plate. Remainder of oral plate with 15–25 angular granules widely spaced.

Color of specimens from deck shots is a rich, bright orange with a lighter orange on the oral surface.

Material Examined. Holotype: IE-2013-17281 . Madagascar 25°36–37’S 46°21’E, 618–624 m, Coll. ATIMO

VATAE, N/ O Nosy B e 11 CP 3566. 1 dry spec. R=3.1 r=2.2. Paratypes : IE-2007-3993 . Madagascar 25°36–37’S 46°21’E, 618–624 m, Coll . ATIMO VATAE, N/ O Nosy B e 11 CP 3566. 3 wet specs. R=3.1 r=2.1; R=3.2 r=2.3; R=3.1 r=2.1 .

Figures

FIGURE 30. Plinthaster unteidtae n. sp. Holotype IE-2013-17281. R=3.1, r=2.2 cm. A. Abactinal surface. Scale bar=1.0 cm. B. Closeup of abactinal plates showing crystalline nodules. Scale bar=0.2 cm. C. Closeup showing superomarginal surface details and enlarged penultimate plate. Scale bar=0.5 cm. D. Actinal view. Scale bar=1.0 cm. E. Closeup of furrow and adambulacral spination. Scale bar=0.5 cm.