Fromia eusticha Fisher, 1913

Arai, Mikihito & Fujita, Toshihiko, 2021, Sea Stars of Families Ophidiasteridae and Goniasteridae (Echinodermata: Asteroidea) from the Mesophotic Zone of the Ogasawara Islands, Including Two New Species, Species Diversity 26 (1), pp. 7-21 : 11-13

Fromia eusticha Fisher, 1913

[Japanese name: Nameraka-juzuberi-hitode] ( Figs 2B , 4 )

Fromia eusticha Fisher, 1913: 213–214 ; Fisher 1919: 375– 377; Domantay and Roxas 1938: 220; A. H. Clark 1952: 286; Jangoux 1978: 294–295; A. M. Clark 1993: 331; Arai et al. 2018: 194, 196.

Material examined. NSMT E-9298, 1 individual, KY-10- 31, West of Hutami Port , Chichi-jima Island, 96.5–96.8 m . NSMT E-10595, 1 individual, KY-17-24, West of Minamijima Island , 147– 149 m .

Description. R= 34.1 mm, r= 7.8 mm, R/r=4.4, width of arm is 8.8 mm at base, 5.5 mm at half of R, and 3.0 mm at 1/10 R from the tip in NSMT E-9298. R= 26.1 mm, r= 5.8 mm, R/r=4.5, width of arm is 7.1 mm at base, 4.6 mm at half of R, and 2.6 mm at 1/10 R from the tip in NSMT E-10595. Abactinal surface is flat and actinal surface is arched. Arms are five, slender, and tapering to blunt tips ( Fig. 4A, B ).

Abactinal plates are polygonal with rounded corners and weakly developed lobes. The plates are arranged in five regular, staggered longitudinal rows at the base of arms ( Fig. 4C ). The median row extends toward the tip of arms. The two adjacent rows terminate at 2/3R from the center of the disc, and the outermost two rows at 1/4R. The entire surface of abactinal plates is rough with numerous hemispherical glassy bosses (crystal bodies). Madreporite is circular, not conspicuously elevated from the abactinal surface, and about 1.2 mm in diameter ( Fig. 4E ). Gyri partly radiate from, but absent at, the center. Anal aperture is surrounded by four enlarged ossicles in NSMT E-10595; none are present in NSMT E-9298. Terminal plates are conical and truncated at the tip. The surfaces of the plates lack granules, and are smooth, except the basal portions where glassy bosses occur ( Fig. 4D ).

Superomarginal plates are block-like, rounded, and larger in diameter than neighboring abactinal plates. There are fifteen superomarginal plates on each side of the arm with R= 32.7 mm in NSMT E-9298. They are not alternating in size but diminishing toward the tip of arms. Inferomarginal plates also decrease in size toward the tip of arms, but they are smaller than the adjoining superomarginal plate.

Actinal plates are polygonal, mostly quadrangular, lacking lobes, and leaving no space in between. They are arranged in three longitudinal series at one side of the arm base in NSMT E-9298 ( Fig. 4F ). The row adjacent to adambulacral plates extends 2/3R from the center of the disc, the outer row 1/2R, and the outermost one 1/6R. There are only two rows in NSMT E-10595. Most actinal plates lack glassy bosses except some interradial plates.

Each adambulacral plate bears three furrow spines and two subambulacral spines followed by smaller granules on the abradial side ( Fig. 4H ). The number of furrow spines is two or four on a relatively small number of plates, and more plates are likely to bear only two furrow spines near the tip of arms (ca. 1/5R from the tip). Most furrow spines are truncated at and widening toward the tip, flattened, and constricted at the base. The proximal furrow spines on some plates are slightly tapering toward the tip. Subambulacral spines are tapering toward the blunt tip, conical or pyramidal, and flattened but thicker than furrow spines. The granules abradial to the subambulacral spines are identical in size and shape to those on actinal plates, thus these granules and the subambulacral spines are discontinuous in size. Pedicellariae occur among the ossicles on a small number of plates. Tubefeet are biserial and with a sucking disc.

Each oral plate bears five oral spines and four to five suboral spines in NSMT E-9298; there are five to six oral and three to four suboral spines in NSMT E-10595. The remaining abradial portion of the plate is covered with granules. Oral spines are conical, tapering toward the pointed tip, slightly curved, and longer than furrow spines. Those near the center of the mouth are more slender and longer than the distal ones. Suboral spines are similar to oral spines in size and shape near the mouth. However, they are shorter in the abradial portion of plates, and a few of them are intermediate in size between oral spines and granules. The granules are slightly larger and coarser than those on actinal plates.

The abactinal, marginal, and actinal plates are covered with a skin on which granules occur. There are 125 granules per square millimeter on abactinal plates at the base of arms, and 79 granules on actinal interradial plates in NSMT E-9298. In NSMT E-10595, there are 124 and 75 per square millimeter, respectively. The granules on abactinal and superomarginal plates are fine and even in size. On the other hand, those on actinal plates are larger at the center than the periphery of plates. A few at the center of the distal nine to ten inferomarginal plates are enlarged and tubercular ( Fig. 4D ). Papulae are isolated and confined to the corners of abactinal plates except only a few occurring between actinal plates. They are absent in abactinal interradial areas.

Pedicellariae occur on some actinal plates and adambulacral plates at the disc and basal part of arms ( Fig. 4G ). They are the most abundant on the proximal most actinal interradial plates. The valves of pedicellariae are similar to the surrounding granules, but two to three times larger. They are gathered in pairs or triplets except one pedicellaria with four valves, and meeting with each other at the smooth surface. There is no alveolus under these pedicellariae, so they leave no trace on the underlying plates when detached. The longer axis or diameter measured 0.19–0.27 mm with the average of 0.24 mm.

Color in life is orange to vermilion on the disc, arm tip, and margins of skeletal plates, and pale yellow on other part of arms on the abactinal side in NSMT E-10595 ( Fig. 2B ). The actinal side is white, except margins of skeletal plates which are orange to vermilion, just as on the abactinal side. No color remained in both specimens after preservation in ethanol.

DNA sequence. A partial sequence of COI (655 bp) was obtained from NSMT E-9298 and deposited in DDBJ (Acc. No. LC427073 ).

Remarks. The present specimens differ from the description of the holotype of F. eusticha by Fisher (1919) in the absence of intermarginal papulae, only a few actinal papulae, and less abundant pedicellariae.

Distribution. Sulu Archipelago, 44 m ( Fisher 1913). Puerto Galera, Mindoro Island, Philippines, depth unknown ( Domantay and Roxas 1938). Bikini Atoll, 55 m (A. H. Clark 1952). Obi Islands, Indonesia, on the beach and reef ( Jangoux 1978). Ogasawara Islands , Japan, 96.5–149 m (this study).

Japanese name. Nameraka means smooth, referring to its fine granules and graduating superomarginal plates. Juzuberi-hitode is a Japanese name for the genus Fromia .

Figures

Fig. 2. Abactinal view of the live specimens (A–D) and ethanol-preserved specimen (E). A, Bathyferdina caelator sp. nov., NSMT E-8265, holotype; B, Fromia eusticha, NSMT E-10595; C, Fromia labeosa sp. nov., NSMT E-9297, holotype; D, Ogmaster capella, NSMT E-9312; E, Tamaria tenella, NSMT E-9277. Scale bars: 10mm.

Fig. 4. Fromia eusticha, NSMT E-9298. A, abactinal side; B, actinal side; C, proximal portion of a partly denuded arm, abactinal view; D, arm tip, lateral view. Arrow indicates terminal plate; E, madreporite, F, proximal portion of a partly denuded arm, actinal view; G, pedicellariae on actinal plates (arrow); H, adambulacral spines. Scale bars: 10mm (A, B); 5mm (C, D, F), 1mm (E), 500µm (G, H). Abbreviations: sm, superomarginal plates; im, inferomarginal plates; fs, furrow spines; ss, subambulacral spines.