Circeaster abyssalis, Mah, 2024

Mah, Christopher L., 2024, New genera and species of deep-sea Goniasteridae (Asteroidea) from the North Pacific, Zootaxa 5543 (4), pp. 451-500 : 465-468

Circeaster abyssalis n. sp.

FIGURE 5A–G

Etymology

The species epithet, abyssalis alludes to the depth occurrence of this species in the abyssal zone, between 3000– 6000 m.

Diagnosis

Body stellate, R/r=3.38, arms elongate, quadrate in cross-section with blunt tips. Interradial arcs weakly curved. Abactinal plates numerous, polygonal to irregularly round, surrounded by large peripheral granules, 4–15, each of the peripheral granules consisting of 20–30% of total plate+granule diameter. Only a small number of disk plates encroaching on proximal arm region, only slightly larger than those on disk. Marginal plates abutted over midline, approximately 15 pairs. Marginal plates interradially lateral facing, not showing a dorsal face save on arms where they show a prominent part of the surface. Actinal intermediate plate surface covered by large, coarse granules. Furrow spines 5–6 per plate, subambulacral spine single large and prominent with a pointed, roughened tip, approximately 2 to 3X the thickness of furrow spines. Pedicellariae present. Alveolar opening of large pedicellariae proximally located on the adambulacral plate adjacent to the subambulacral spines, variably bivalve to wide, paddle-shaped valves with 5 to 6 teeth on each, present on nearly all ambulacral plates.

Comments

Circeaster abyssalis n. sp. is the first species in this genus to be recorded from the abyssal zone (3188–4179 m) and the first to be recorded from this area in the central Pacific. It is the deepest known Circeaster from the Pacific, which occurs between 320 to 2305 m.

This species invites comparison with other South Pacific Circeaster , including Circeaster pullus and Circeaster sandrae which are similar in overall appearance, especially the adambulacral spination and accessories. However, Circeaster abyssali s lacks large bivalve pedicellariae present in species such as C. pullus and displays numerous granules covering the actinal intermediate plate surfaces where those in C. sandrae are bare save for a single pedicellaria. Furrow spine number in C. sandrae tends to be higher, 12–14, but can range between 5 and 15 versus C. abyssalis n. sp. with only 5 to 6 furrow spines. Circeaster abyssalis displays fewer abactinal plates on the arm surface and superomarginal plates are abutted across the midline, and lacks granules on the abactinal plates surface.

Sibogaster nieseni , which has similar enlarged coarse grained granules present around the abactinal plates. However, C. abyssalis n. sp. shows distinctly different adambulacral spination and pedicellariae, notably the presence of the enlarged subambulacral spine as well as enlarged tubercles on the superomarginal and inferomarginal plates versus the superomarginal plates on Sibogaster which are bare and smooth. Superomarginal plates on C. abyssalis n. sp. are abutted along most of their length whereas those on S. nieseni are not.

Occurrence

Central Pacific, between Utiroa, Buriki and east of Rongorongo, Nauru, 4179– 3188 m.

Description

Disk thick. Body stellate, R /r=3.38, arms elongate, quadrate in cross-section with blunt tips. Interradial arcs weakly curved ( Fig. 5A, E ).

Abactinal plates numerous, approximately 12–15 across arm base, polygonal to irregularly round, arranged irregularly, mostly flat to weakly convex, sizes and shapes heterogeneous distributed across disk surface. Plates present on disk and proximal arm region, ending where superomarginals have abutted distally. Abactinal plates on arms large, strongly convex relative to adjacent plates on disk, surface smooth and convex. Most disk plates proximally, especially on disk, with peripheral granules, large polygonal 4 to 15, occupying approximately 20–30% of total plate surface plus granule diameter. Granular cover proximally, especially centrally on disk especially abundant, densely arranged ( Fig. 5B ). Pedicellariae small, comparable in size to adjacent granules, paddle-shaped with mostly 2, in minority of instances 3 valves with blunt tips, mostly clustered around arm base. Madreporite sunken, surrounded by granules, 10–12, polygonal in shape.

Marginal plates, 15 superomarginals per arm side, 30 per interradius, arm tip to arm tip, 18 inferomarginals, 36 per interradius, arm tip to arm tip. Superomarginals facing laterally on disk, but larger, broader dorsal-facing on arms where they gradually abut over midline at approximately 50% of the distance ( Fig. 5C ). Approximately 10–12 distalmost superomarginals abutted along arm ( Fig. 5A, C ). Superomarginals and inferomarginals on disk with large granules, 2 to 10, most are widely spaced. A minority of superomarginals at arm base with a single tubercle raised on a central tumescence. Superomarginals and inferomarginals on arms with no surficial features, no granules, no spinelets or other features. Inferomarginal plates with small pedicellariae, 1 or 2, paddle-like, when present. Marginal plates with large peripheral granules, polygonal to quadrate in shape, 10–15 per side.

Actinal intermediate regions with 2 to 3 complete series, remaining 1 or 2 incomplete on disk. No series extending onto arm. Individual plates quadrate arranged in chevron formation, plate surface covered by granules, large, approximately 2 per 1.0 mm line count, obscuring plate boundaries.

Furrow spines 5 to 6 in straight series, evenly spaced, quadrate in cross-section, blunt tipped. Subambulacral spine single, large, prominent, with pointed, roughened tips, approximately 2 to 3X the thickness of furrow spines ( Fig. 5G ). Alveolar opening of large pedicellariae proximally located on the adambulacral plate adjacent to the subambulacral spines; valves wide, paddle-shaped with 5 to 6 teeth on each, present on nearly all adambulacral plates.

Oral plates with furrow spines, 10, widely spaced, blunt tipped, widely spaced with one plate directed into mouth, 2 large spines similar to those on the adambulacral plates per interradius, adjacent to the oral plates. Oral plates with short pointed, angular spines, 10 to 15 covering oral plate surface ( Fig. 5F ).

Material Examined

Holotype. SIO E3465 Central Pacific, between Utiroa, Buriki and east of Rongorongo , Nauru. 1.3468° S, 175.3433° E, 4179– 3188 m. Coll. E. Pillard, 7 Feb 1999, R/V Melville. 1 wet spec. R=4.4 r= 1.3 cm .

Figures

FIGURE 5. Circeaster abyssalis n. sp. SIO E3465. Holotype A. Abactinal. B. Closeup of abactinal surface. C. Side view showing lateral surface, marginal plates along arm. D. Side view showing lateral surface, interradial marginal plates. E.Actinal. F. Closeup oral region, actinal surface. G. Adambulacral plates showing furrow spines, subambulacral spines and pedicellariae. Scale bar=5.0 mm: A, D, E.; Scale bar=2.0 mm: C; Scale bar=1.0 mm: B, F, G.