Uokeaster ahi, Mah, 2021

Mah, Christopher L., 2021, The East Pacific / South Pacific Boundary: New taxa and occurrences from Rapa Nui (Easter Island), New Caledonia and adjacent regions, Zootaxa 4980 (3), pp. 401-450 : 414-416

Uokeaster ahi n. gen. n. sp.

Figure 5A–E

Mecho et al. (2019): 91 (as Family Asterodiscididae sp.).

Etymology. The genus is named for the Rapa Nui marine deity Uoke, who submerged Rapa Nui into the ocean using a large lever, and “ aster ” for star. The species epithet ahi is the Rapa Nui word for “fire” which alludes to the bright orange color of this species. Noun is held in apposition.

Diagnosis. This species is distinguished from other asterodiscidid forms based on the presence of the distinct continuous dermal layer present from the abactinal, marginal and actinal surface, and the marginal plates forming blunt, conical spines with similar types of spines covering the abactinal surface.

Comments. Amphiaster and Paulia have abactinal surfaces with a close-set, dense, granular covering. and have fewer, larger, spineless marginal plates which show a strongly convex surface. Amphiaster displays alternating bald marginal plates with large, conical projections.

This species shares the enlarged penultimate superomarginal plates and displays morphology intermediate form between the East Pacific Amphiaster and Paulia and the widely occurring Indo-Pacific Asterodiscides . Morphological assessments by Rowe (1977, 1985) and phylogenetic analysis by Blake & Portell (2011) all support the hypothesis that Amphiaster , Paulia and related asterodiscidid genera are more stemward relative to the more derived Asterodiscides . Uokeaster has not previously been included in a cladistic analysis but its marginal plate and body morphology are similar to those of Amphiaster , Paulia and Kionaster which suggest a more plesiomorphic morphology rather than that of Asterodiscides . However, Uokeaster does have disproportionately enlarged penultimate superomarginal plates which are present in Asterodiscides , but not present in either Amphiaster , Paulia or the extinct Kionaster .

Mecho et al. (2019) has observed this species in situ from Pukao and Apolo islands in Rapa Nui between 160–180 m ( Fig. 5A ).

Biogeographic Trends. Blake & Portell (2011) and especially Rowe (1985) touch upon the biogeographic significance of Amphiaster and Paulia occurring in the East Pacific relative to Asterodiscides occurring widely throughout the Indo-Pacific. Although Lane and Rowe (2009) have produced a cluster analysis for Asterodiscides , no phylogeographic analysis for species has been performed.

Occurrence data of the asterodiscidid taxa suggests interesting trends across the tropical Eastern Pacific. Amphiaster insignis occurs primarily along the coast of Mexico with observations south to Malpelo Island, off the coast of Columbia ( Maluf 1988). Paulia horrida has also been recorded from the coast, but also to the Galapagos and Tres Marinas Islands off Peru. Pauliella aenigma is recorded from Cocos Island and Rocas Alijos and finally, Uokeaster n. gen. is recorded, for now, only from Rapa Nui, the westernmost of the asterodiscidid genera relative to Asterodiscides . Shared characters between Uokeaster and Asterodiscides and the biogeographic proximity suggest the former could be the sister taxon to the latter.

Description. Body stout, stellate (R/r=2.3) in shape, arms triangular, disk arched with curved interradial arcs.

Abactinal plates obscured by soft dermal layer displaying a creased texture and perforated by one to ten small papular pores per dermal segment, which are triangular in shape. Primary structures are large, blunt conical spines, with bare, smooth surface. These spines are large, pointed plates rather than articulated structures as seen on other spiniferous goniasterids. Pointed conical plates present on disk and along radial and adradial regions and vary from pointed to semi-oval (egg shaped) in shape. Approximately four to eight large spines from arm tip to primary circlet on disk. Skin around base of spines and on remainder of surface with spine-free regions interradially adjacent to superomarginal contact. Smaller, spines, apparently incipient, present around periphery of other primary, larger spines. Madreporite triangular in shape, flanked by three large primary pointed plates. No pedicellariae. Papulae prominent, translucent emerging from three to five papular pores per papular region, present on disk and arms.

Superomarginals 8 to 10, inferomarginals 16 to 20 per interradius (arm-tip to arm-tip) with dermal covering from abactinal surface extending intermarginally to actinal surface. Superomarginal plates pointed, strongly convex but with smooth bare surface, round in cross-section, widely spaced along the lateral surface. Distalmost superomarginals round, strongly convex and enlarged, approximately two to three times the size of adjacent superomarginal plates, and abutted over midline. Larger individuals (R>3.5) with a single spine emerging from central superomarginal surface. Inferomarginals substantially smaller than superomarginals but similar in shape, surfaces bare, smooth, round to quadrate in cross-section. Smaller specimen with short, nipple-like spines present on the tips of primarily interradial inferomarginal spines continuing to arm tips. Inferomarginals become lower and less pointed distally along series adjacent to the arm tip. Both superomarginal and inferomarginals in continuous, mostly uninterrupted series, but one interradius with absent inferomarginal, covered by dermal tissue. Terminal plate, smooth, triangular in shape with pointed tip.

Actinal surface with approximately three complete plate series and one or two incomplete plate series adjacent to inferomarginals. All actinal plate boundaries obscured by continuous dermal layer as present on abactinal and marginal surfaces. Each plate with a large, cylindrical blunt spine sitting on the center of each plate. Proximal actinal plates with one to four, mostly three smaller spines approximately <20% the size of larger, more abundant actinal spines. The larger actinal spines sit in linear series tracking the more fully developed actinal series along the arm until they end adjacent to the enlarged distalmost convex superomarginal plates. A spine is present on nearly every actinal plate extending out to the contact with the inferomarginals, and are widely spaced from one another across the surface.

Furrow spines two, blunt, cylindrical in cross-section, with blunt, round to quadrate tips. Each adambulacral plate with a single, large cylindrical blunt subambulacral spine identical to the others present on the actinal plate surfaces. Subambulacral spines absent on first adambulacral plate. Each oral plate with two furrow spines and a third blunt, spine, triangular in cross-section projecting into mouth. Remainder of oral plate surface bare, smooth, covered by dermal layer.

Color in life a bright to dull orange.

Occurrence. Rapa Nui, 33–110 m, 160–180 m (in situ)

Material described. Holotype: CASIZ 222370 . Moto Nui , Isla de Pascua (Rapa Nui), 33 m. Coll. Bart Shepherd, Tyler Phelps and Luis Rocha, 5 March 2017, 1 wet spec. R =3.5, r=1.8.

Paratypes: CASIZ 222372 . Moto Nui, Isla de Pascua (Rapa Nui), ~ 82 m. Coll. Bart Shepherd, Tyler Phelps and Luis Rocha, 6 March 2017, 1 wet spec. R =3.8, r=1.6. CASIZ 222375 . Hanga Piko , Isla de Pascua (Rapa Nui), 27°9′11.67″S, 109°26′52.42″W, 110 m. Coll. Bart Shepherd, Tyler Phelps and Luis Rocha, 11 March 2017, 2 wet spec. R =2.3, r=1.0; R =1.4, r=0.6 .

Figures

FIGURE 5. Uokeaster ahi n. gen. n. sp. Holotype CASIZ 222370 (Rapa Nui): A. In situ observation (not the holotype). Courtesy A. Mecho. B. Abactinal-lateral view. Scale bar equals 1.0 cm. C. Abactinal surface. Scale bar equals 2.0 cm. D. Actinolateral view showing marginal plate series. IM = intermarginal plates. Scale bar equals 0.75 cm. E. Actinal surface. Scale bar equals 2.0 cm. F. Close-up of actinal surface showing furrow spines. Scale bar equals 0.5 cm.