Leioastra triangularis, 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 : 48-51

Leioastra triangularis n. gen., n. sp.

Figure 18 A–D

Etymology. The genus name “ leios ” is Greek for “smooth” which alludes to the bald, smooth abactinal and marginal surfaces. “ astra ” is the female form for “star.” The species epithet “ triangularis ” for triangle refers to the distinctive triangular shape of the arms in this species.

Diagnosis. Abactinal and marginal plates smooth with no granules. No crystalline granules on plates. Marginal plates eight per interradius ( Fig.18A ), large and wide, forming distinct border, approximately 47% of total radius (r) on disk. Superomarginal plates abutted along arm radius along full arm distance. Actinal plates covered by granules ( Fig. 18C ).

Comments. Based on a comparison with concepts of similar goniasterids, character evidence does not support placement of this species within a known genus; a new genus and species therefore is proposed.

Leioastra triangularis n. sp. possesses smooth abactinal and marginal plate surfaces which are devoid of granules, although smaller ones are present around the periphery. These characters are also observed in several other goniasterids, including Iconaster, Glyphodiscus , and Lithosoma ( Mah 2005) that differ in that their actinal plate surfaces are devoid of granules and other accessories, whereas in Leioastra triangularis n. gen. n. sp. granules are present. Pontioceramus and Diplasiaster share similarities with L. triangularis n. gen. n. sp. in that they possess smooth, bare abactinal and marginal plate surfaces, in conjunction with granules covering the actinal surface; however, there are significant differences in overall R/r, body, arm, and plate shape. Plinthaster differs in that it possesses fine, crystalline bosses on the abactinal plate surfaces, granules on the marginal plates, and exhibits a different R/r.

Lithosoma novaezelandiae McKnight 1973 , as outlined in Clark and McKnight 2001 bears a superficial resemblance to Leioastra triangularis n. gen, n. sp. in terms of abactinal and marginal plate shape and morphology. However, Leioastra triangularis n. gen. n. sp. differs in that crystalline bosses such as are present on the abactinal plates of are absent. Leioastra triangularis n. gen, n. sp. also possesses granular covering and distinct subambulacral spination on the actinal and adambulacral surface, respectively, both characters absent from Lithosoma novazelandiae .

Occurrence. Madagascar, 549 m. Description. Body stellate, R/r=1.9. Body stout, arms short, interradial arcs weakly curved to straight ( Fig. 18A, C ) .

Abactinal surface planar, abactinal plate region pentagonal and sits below the plane of the superomarginals. Plates hexagonal to irregularly polygonal ( Fig. 18B ). Plate surfaces bare and smooth, granules absent. Surface opaque, crystalline inset granules not observed. Plates larger proximally becoming smaller distally adjacent to contact with superomarginal plate boundary. Individual plate boundaries distinct. Individual plates bearing 25–30 small, round granules with approximately five to seven per side. Primary circlet distinct, with seven carinals present from disk to arm. Madreporite triangular, flanked by three plates.

Marginal plates large, swollen, strongly convex in outline. Superomarginal and inferomarginal plates ten in each series (arm tip to arm tip) ( Fig. 18A ). Superomarginals and inferomarginal plates correspond 1:1 but are offset creating a angular contact at edges. Marginal plates smooth bare, granules absent from surfaces of both superomarginals and inferomarginals ( Fig. 18B ). Plates with ~25–30 tiny granules per edge forming a distinct but very thin boundary around each plate. Superomarginal plates abutted over midline, three to four. Distal superomarginals becoming more triangular in shape ( Fig. 18A, B ). Proximal superomarginal and inferomarginal plates more trapezoid in outline. Pre-terminal superomarginals sometimes very small, indistinct. Terminal triangular, smooth surface.

Actinal surface composed of approximately three series of plates (one series, adjacent to inferomarginal contact is incomplete) ( Fig. 18C ). Individual plates quadrate, angular to irregularly quadrate in outline. Actinal plates covered by 40–60 evenly spaced granules, with approximately seven to ten forming peripheral border on each side.

Furrow spines five to six, short blunt. Subambulacrals three, half as long with further series behind them becoming more consistent with granular morphology on actinal surfaces ( Fig. 18D ). Each oral plate bisected by distinct furrow flanked on either side by four to five paired series of granules ( Fig. 18C ). Oral plate furrow spines five to six, with one spine projecting into mouth.

Two marginal plates present interradially show damage with distinct chunks absent from the distal edge ( Fig. 18A, C ). It is unclear if this was caused during collection or perhaps from natural causes, such as predation (e.g. Neumann 2000).

Material Examined. Holotype: IE 2007 -3946 Madagascar 26°9’S 45°40’E, 549 m, Coll. Atimo Vatae expedition, N/ O Nosy Be 11, 5 May 2010. 1 wet spec. R=1.9, r=1.0.

Figures

FIGURE 18. Leioastra triangularis n. sp. Holotype IE-2007-3946. r=1.9, r=1.0 cm. A. Abactinal surface. Scale bar=0.5 cm. B. Closeup of arm and superomarginal plates. C. Actinal surface. Scale bar=0.5 cm. D. Closeup of furrow region. Scale bar=0.5 cm.