Sphaeriodiscus ganae, 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 : 84-85

Sphaeriodiscus ganae n. sp

Figure 32 A–E

Etymology. This species is named for friend and colleague, Ms. Christina Gan, professor at Highline Community College in Seattle, Washington.

Diagnosis. This species lacks the secondary plates present on the abactinal surface ( Fig. 32B ), has fewer superomarginal plates (ten instead of 12 in S. mirabilis ), lacks enlarged distalmost inferomarginal plates, and has marginal plates which are wide rather than elongate. Actinal accessories and spination are similar however.

Comments. Sphaeriodiscus ganae n. sp. shares the very large convex superomarginal plates with the Atlantic Sphaeriodiscus bourgeti (Perrier 1885) . However, Sphaeriodiscus ganae n. sp. has ten superomarginals per interradius (R=~3.0 cm) versus only six to eight in comparably sized specimens of S. ganae n. sp.. Sphaeriodiscus ganae n. sp. also has three or four furrow spines per plate versus four or five in S. bourgeti . Superomarginal plates, especially those enlarged ones in S. ganae n. sp., display significantly wider rather than more elongate superomarginals in S. bourgeti . Otherwise, both species are similar, lacking smaller, secondary plates with similar granulation patterns on both abactinal and marginal plates surfaces.

Occurrence. Walters Shoal, western Indian Ocean, 647– 712 m.

Description. Body, stout, pentagonal (R/r=1.45–1.5), interradial arcs weakly curved ( Fig. 32A, D ). Abactinal plates polygonal, most hexagonal with some interradial plates more irregular in shape ( Fig. 32B ). Secondary plates absent. Surficial plates covered by homogeneous, covering of round granules, approximately four granules present along a 1.0 mm line. Granules approximately 15–90 per plate, homogeneous overall, weakly differentiated into central and peripheral series. Peripheral granules in single row, approximately seven to 25–30, central region, approximately eight to-50 with granules in some regions densely distributed becoming so abundant that boundary between plates is obscured. Radial regions displaying papular regions in conjunction with shallow fasicolar grooves. No pedicellariae observed. Madreporite quadrate to triangular, flanked by four or five plates.

Superomarginals, inferomarginals ten per interradius (arm tip to arm tip) ( Fig. 32A, C ). Superomarginals elongate throughout, relatively narrow interradially becoming wider and significantly more swollen distally, especially penultimate to terminal plate ( Fig. 32C ). Central pair of superomarginals in holotype appear significantly smaller relative to distalmost three plates on holotype (IE-2013-17139) ( Fig. 32A, C ). Central pair on paratype are more proportional to other superomarginals but are noticeably small relative to distalmost plates. Superomarginals and inferomarginals slightly offset with zigzag contact between them. Superomarginals have strong abactinal facing, occupying approximately (0.5/2.4) 20% of the total “r” distance interradially with distalmost two pairs of superomarginals directly abutting over midline. Penultimate superomarginal plates with strongly convex surface followed by triangular to triangular plates adjacent to the terminal plate ( Fig. 32C ). Superomarginal plate surface covered in a dense field of granules identical to those on the abactinal surface save for a large prominent bald patch on the abactinal surface of each plate bearing scattered, round and widely-spaced granules, ranging from seven to 30. These bald regions on some plates can be 25% of the size of the largest ones observed on the plates elsewhere. Largest penultimate superomarginals with strongly convex bald areas with up to 30 widely spaced granules. Absent granules leave concave pitting on surface. Where bald areas are reduced or absent, most of superomarginal plates covered by approximately 200–500 densely arranged, round granules, consistent in size with those on abactinal plates. On those superomarginal surfaces with larger bald regions, only 50–100 granules are present surrounding bald regions which are raised above superomarginal surface. Peripheral granules, approximately 120 total, approximately 30 on the width, 40 on the length. Inferomarginal plate series mostly covered by 200–500 coarse, round granules identical to those on superomarginal plates. Bald areas irregularly present ranging from smaller patches with three widely spaced granules to large bald patches with up to 30–40 widely spaced round granules. Penultimate inferomarginals also strongly swollen with triangular plates adjacent to terminal plate. Terminal plate small, triangular.

Actinal surface with four full actinal plate series in chevron formation ( Fig. 32D ). Individual plates ranging from quadrate to irregularly polygonal in shape. Surface almost completely covered by coarse, round granules, six to 40 per plate, consistent with those observed on the abactinal and marginal surfaces, becoming so dense that boundaries between plates are obscured. No pedicellariae observed.

Furrow spines three or four per plate, thick, blunt, round-tipped, quadrate or triangular in cross-section in a straight or weakly palmate series ( Fig. 32E ). but with alternating furrow spine series, three on lower series and four on upper series. These variably shown in different series or in single series depending on state of specimen preservation. Three well-defined subambulacral spine rows. First subambulacral row adjacent to furrow spine composed of primarily two spines, one smaller proximal spine and one larger and thicker distal spine. Second subambulacral spine row composed of two or three angular granules (quadrate to triangular in cross section), set off from third series composed of three granules, which eventually become continuous with granules on actinal surface or “pinch out” distally along arm. Furrow spines on oral plate four, twice as thick as furrow spines on adambulacral plates, blunt tipped, quadrate in cross-section. One spine on apex of each half of oral plate, projecting into mouth, total of two on each interradial angle. Oral plates with five short, angular granules, quadrate to triangular in cross section present along the edge of sulcus dividing each oral plate and paired on each side.

Material Examined. Holotype . IE-2013-17139 Southwestern Indian Ocean , Walters Shoal. 33°8’S 44° 1’E to 33°8’S 44° 2’E, 700–712 m. Coll. MD 208 (Walters Shoal) CP 4902, 7.5.17. 1 wet spec. R=3.3 r=2.2. Paratype. IE-2013-17141 Southwestern Indian Ocean , Walters Shoal. 33°8’S 44° 0’E to 33°9’S 44° 1’E, 647–672 m. Coll . MD 208 (Walters Shoal) CP 4901, 7.5.17. 1 wet spec. R=3.5 r=2.4

Figures

FIGURE 32. Sphaeriodiscus ganae n. sp. Holotype IE-2013-17139. R=3.3, r=2.2 cm. A. Abactinal surface. Scale bar=1.0 cm. B. Closeup showing abactinal plates and granulation. Scale=0.5 cm. C. Lateral view showing marginal plates and surface details. Scale bar=1.0 cm. D. Actinal plate surface. Scale bar=1.0 cm. E. Closeup of furrow and adambulacral spination. Scale bar=0.5 cm.