Ogmaster capella ( Müller and Troschel, 1842 )

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 : 15-17

Ogmaster capella ( Müller and Troschel, 1842)

[Japanese name: Hadaka-akasuji-hitode]

( Figs 2D , 6 )

Goniodiscus capella Müller and Troschel, 1842: 61 .

Goniaster (Ogmaster) capella Martens, 1865: 359–360 .

Ogmaster capella: Sladen 1889: 261 ; Koehler 1910: 79; Fisher 1919: 262, 305; Döderlein 1935: 101–102; Guille and Jangoux 1978: 53; A. M. Clark 1993: 267; Liao and A. M. Clark 1995: 94; A. M. Clark and Mah 2001: 337; Arai et al. 2018: 194, 196.

Dorigona reevesii Gray, 1866: 7 .

Goniaster muelleri: Lütken 1871: 248–250 .

Material examined. NSMT E-9312, 1 individual, dry, KY-08-21, East of Chichi-jima Island , 95– 98 m .

Description. R= 16.5 mm, r= 6.9 mm, R/r=2.4, width of arm is 8.5 mm at base, 4.8 mm at half of R, and 1.9 mm at 1/10 R from the tip. Abactinal and actinal surface are flat. Arms are five, tapering toward a pointed tip ( Fig. 6A ).

Abactinal plates are polygonal, mostly pentagonal to hexagonal, and arranged in a regular tessellate manner leaving only small spaces around the corners. The plates on the interradial area are larger than those on the median of arms, and those neighboring the superomarginal plates are smaller ( Fig. 6B ). The plates are covered with a very thin skin without any granules, thus giving a naked appearance. The surface of the plates is rough with numerous glassy bosses. Papulae are confined to the proximal portion of the abactinal radial area. They are isolated, and occur at the corners of abactinal plates. Madreporite is circular, domed, and about 1.0 mm in diameter; gyri radiate from the center. Anal aperture lies at the center of the disc, and is surrounded by five granular ossicles.

Terminal plates are bell-shaped ( Fig. 6C ). They have a smooth surface and lack any appendages except two plates bearing a single cylindrical, blunt spine at either side of the tip, suggesting that each terminal plate should bear a pair of such spines.

Marginal plates are block-like, and regularly decreasing in size from the interradius toward the arm tip. There are nine superomarginal plates on each side of arms. The distalmost four plates are in contact with their counterparts on the other side of the arm, though there are small rhomboidal abactinal plates inserted at the median of arms between sixth and seventh superomarginal plates (also between seventh and eighth on two arms). The inferomarginal plates correspond to the superomarginal plates in shape, size and number.

Actinal plates are polygonal, mostly quadrangular or pentagonal, and arranged in a regular tessellate manner leaving no space in between. The surface of the plates is smooth with a skin lacking granules. A few low, faint glassy bosses occur on some of the plates.

Adambulacral plates bear four to five furrow spines and two to five subambulacral tubercles ( Fig. 6D ). Furrow spines are cylindrical or conical with a blunt tip, and arranged on the curved adradial margin of adambulacral plates. The most distal spine on each plate is about two times wider at the base than the others, and tapering more rapidly toward the tip with the same width as the others. The subambulacral spines are hemispherical, irregular in size, and mostly arranged in a straight line at the abradial side of the plate. On the distal plates corresponding to the final two to three inferomarginal plates, the number of furrow spines decreases to 1–3 including the larger distal spine, and the subambulacral ossicles disappear. Tubefeet are biserial and with a terminal disc. No pedicellariae occur on adambulacral plates or other plates.

Oral plates have a semicircular and domed abactinal surface. Each oral plate bears six oral spines at the margin and six to eight suboral tubercles on the abactinal surface. Oral spines are conical, slightly depressed with flanking spines. The most adradial one is the largest and about two times larger than the most abradial one which is almost identical with the neighboring furrow spines.

Color in life is dull pink with red lines between plates on the abactinal surface and white on the actinal surface ( Fig. 2D ).

DNA sequence. A partial sequence of COI (655 bp) was obtained from NSMT E-9312 and deposited in DDBJ (Acc. No. LC427075 ).

Remarks. The specimen from the Ogasawara Islands is small but generally agrees with all the past descriptions. Pedicellariae are lacking on adambulacral plates in the present specimen unlike larger specimens with R around 30 mm ( Döderlein 1935; Guille and Jangoux 1978). Additionally, the present specimen differs from the original description since none of its plates covered with granules. According to Döderlein (1935), who examined the type specimen, the original description by Müller and Troschel (1842) misinterpreted the crystal bodies (glassy bosses) as granules. In the present specimen, the surface of all the abactinal plates and some of the actinal plates shows numerous glassy bosses.

Liao and A. M. Clark (1995) suggested that Stellaster septemtrionalis Oguro, 1991 , which was described from 105 m deep in the East China Sea ( Oguro 1991), may be a junior synonym of O. capella although they did not give a detailed description or comparison of the two species. The two species are indeed very similar in having 5–6 furrow spines, 2–3 short subambulacral spines, and in lacking tubercles or spines on abactinal and marginal plates. However, O. capella can be distinguished from S. septemtrionalis by the absence of coarse granules on abactinal, marginal, and actinal plates and conical tubercles on actinal plates. We consider that these differences are sufficient to separate the two species and S. septemtrionalis should be maintained as valid.

Distribution. Southern coast of China, 60–129 m (Liao and A. M. Clark 1995). Kai Islands, Indonesia, 90 m; Timor Island, 112 m; Small Sunda Islands, depth unknown ( Döderlein 1935). Seram Island, depth unknown ( Guille and Jangoux 1978). Ogasawara Islands , Japan, 95–98 m (this study).

Japanese name. Hadaka-akasuji-hitode means a bare red-lined sea star, referring to the body hardly covered with granular skin and conspicuous red lines bordering abactinal plates in life.

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. 6. Ogmaster capella, NSMT E-9312. A, actinal side; B, abactinal surface of interradial area; C, arm tip, abactinal view; D, actinal, adambulacral and oral plates. Scale bars: 10mm (A), 1mm (B–D).