Rhianastra isosceles, Mah, 2024

Mah, Christopher L., 2024, Two New Taxa of Goniasteridae (Asteroidea, Echinodermata) and Noteworthy Observations of Deep-Sea Asteroidea by the NOAA Ship Okeanos Explorer in the North and Tropical Atlantic, Zootaxa 5432 (4), pp. 461-508 : 491-494

Rhianastra isosceles n. gen. n. sp.

FIGURE 13A–F , 14A–B

Etymology

The genus is named for Dr. Rhian Waller, Biology Science Lead of EX2104 who collected the specimen. The epithet isosceles refers to the distinctive triangular arms present on this species.

Diagnosis

Strongly stellate body form with sharply triangular arms ending in strongly tapering tips ( Fig. 13A, E ). Arms five, but with one specimen showing six. Abactinal plates tabulate ( Fig. 13B, C ), papular region along six to seven series from disc to armtip along central radius along arm. Surface covered by dense covering of granules (3 count along a 1.0 mm line) ( Fig. 13C ). Marginal plates, approximately 64–65 per interradius, quadrate in shape, covered by granules ( Fig. 13D ). Furrow spines, 8 to 10, narrow, blunt with square tips in straight to weakly furrow formation ( Fig. 13E–F ). Subambulacral spines adjacent to furrow spines, with 6 thick spines, approximately twice as thick as each furrow spine. Third and distalmost subambulacral “spines”, identical in stature to granules on actinal surface.

Comments

Rhianastra isosceles n. gen. n. sp. possesses a generalized Ceramaster -like goniasterid set of characters, including tabulate abactinal plates along the radial region ( Fig. 13A–C ), the relatively well-developed radial fasciolar grooves and papular regions as well as the consistent, complete granule covering over the abactinal, marginal and actinal intermediate plate surface ( Fig. 13A–F ). However, the body form and abactinal/marginal plate arrangement are reminiscent of taxa in the Paxillosida , especially in the Astropectinidae , which are known for having triangularshaped arms with elongate, tapering arm tips with lateral-facing marginal plate series.

Rhianastra n. gen. invites comparison with taxa such as Pseudarchaster ( Pseudarchasteridae ), which also have tabulate abactinal plates, and in some species elongate, triangular arms. However the actinal plate pattern in Rhianastra is distinctly chevron-like, unlike those in pseudarchasterines, and the marginal plates, while wide, do not encroach onto the abactinal surface as they do in most Pseudarchaster species.

Other than the comparison of tabulate plate and granule characters, which are generalized for goniasterids, it was difficult to find an obviously close taxon for comparison.

Occurrence

Only known specimen from Gosnold “Shallow” Seamount with second observation from Retriever Seamount, both in North Atlantic, 1752–1878 m.

Description

Body strongly stellate (R/r=4.2), arms sharply triangular, arm tips tapering, upturned ( Fig. 13A, E , Fig. 14A, B ). Interradial arcs angular. Disc, arms thick. Specimen shows five arms but another imaged, but not collected specimen shows six ( Fig. 14B ). Marginal plate surfaces with strongly lateral facing, with strongly angular dorsolateral and actinolateral edges.

Abactinal surface with discrete radial papular regions ( Fig. 13A, C ) with fasciolar regions present amidst low tabulate plates ( Fig. 13A ) in 6 to 7 series beginning on disc, extending along arms, decreasing to a single series along distalmost arm tip ( Fig. 13A, C ). Abactinal plates on disc and on adradial regions on arms becoming flush with surface distally along arm. Radial fasciolar grooves shallow but with distinct fasciolar grooves, proximally eventually “shallowing out” out distally. Single row of plates extending to arm tip. Surface covered by coarse, round granules, approximately 3 granules counted along a 1.0 mm line. Abactinal plates on radial regions, round to hexagonal in outline ( Fig. 13B ). Each of these radial abactinal plates with 9 to 40 granules, widely spaced per plate. Peripheral granules, 7 to 30, more quadrate in shape, central granules round, 2 to 10. Granular cover on non-radial regions, round and homogenous in size and shape, more densely arranged than on radial regions, which are more widely spaced. Non-radial regions covered by granules that obscure plate boundaries. Madreporite irregularly round set within a deep impression below the granules with deep, well-defined sulci. Anus flanked by five round to elongate quadrate-shaped plates, each covered by 20–30 granules. No pedicellariae observed on abactinal surface.

Marginal plates approximately 65 per interradius, armtip to armtip (32 per arm with 1 interradially), Inferomarginals with an extra interradial plate in each interradius, triangular in shape, present in each interradius ( Fig. 13D ). Both series of marginal plates quadrate in shape with distinct corners with W>L, inferomarginals wider than superomarginals. Inferomarginal lower plate extending onto actinal surface ( Fig. 13E ). Marginal plate surfaces covered with uniformly distributed, round granules, approximately 150–400, identical to those on the abactinal surface ( Fig. 13D ). No pedicellariae on marginal plates.

Actinal plates diamond to round in shape, in 4 full series in chevron arrangement ( Figs. 13E–F ). Actinal plates covered by granules, 7 to 30 widely spaced, quadrate to polygonal. Simple two-piece pedicellariae present on proximal 3 to 6 actinal plates, 3 observed adjacent to the oral plates 1 to 2 pedicellariae on other actinal plates, tending to be present closer to the oral region.

Adambulacral plates with spines in three series. Furrow spines, 8 to 10, narrow, blunt with square tips in straight to weakly palmate formation ( Figs. 13E–F ). Subambulacral spines adjacent to furrow spines, with 6 thick spines, approximately twice as thick as each furrow spine. Third and distalmost subambulacral “spines”, identical to actinal granules in stature. Each row of spines on adambulacral plates widely spaced from one another. Oral plate furrow spines 11, oral plates with 15–20 short, thickened spinelets, evenly spaced from one another ( Fig. 13F ). Spinelets arranged on either half of the oral plates, separated by a wide central fossae.

Tube feet unpointed, in biserial series, decreasing to a single series distally. Each furrow wide ( Fig. 13E–F ).

Color in life pale orange, darkening on radial regions and along lateral edges ( Fig 14A–B ).

Material Examined

Holotype. USNM 1658841. Gosnold “shallow” Seamount. 38.2925 -62.5329, 1752 m. Coll. NOAA Ship Okeanos Explorer (R. Waller) 24 July 2021. 1 wet spec. R = 7.6 cm r= 1.8 cm.

EX2104_IMG_20210724T163944Z_ROVHD.jpg

Other images

North Atlantic Gosnold “shallow” seamount, 38.293253 -62.533149, 1778 m ( Fig. 14A ). EX2104_IMG_20210724T155922Z_ROVHD.jpg

Retriever Seamount, 39.837256 -66.230123, 1878m.(6rayed form) ( Fig.14B ).EX2104_IMG_20210727T173546Z_ ROVHD.jpg

Figures

FIGURE 13. Rhianastra isosceles n. gen, n. sp. A. Abactinal. B. Abactinal plates. C. Abactinal surface. D. Lateral surface showing marginal plates. E. Actinal view. F. Furrow and actinal surface. Scale:A,E=10.0 mm, B=1.0 mm, C, F=3.0 mm, D=2.0 mm

FIGURE 14. Rhianastra isoceles n. gen, n. sp. in situ. A. Holotype, Gosnold Seamount, 1752 m. B. Six rayed variant, Retreiver Seamount, 1878 m.