Sea Stars of Families Ophidiasteridae and Goniasteridae (Echinodermata: Asteroidea) from the Mesophotic Zone of the Ogasawara Islands, Including Two New Species

Five species of sea stars of the families Ophidiasteridae and Goniasteridae including two new species, Bathyferdina caelator sp. nov. and Fromia labeosa sp. nov., were collected by dredging from the mesophotic zone of the Ogasawara Islands, Japan. Bathyferdina caelator is distinguished from B. aireyae Mah, 2017 by the presence of glassy bosses (crystal bodies) on actinal and adambulacral plates and the absence of them on marginal plates. Fromia labeosa has large, elliptical pedicellariae on the actinal plate and is further distinguished from its congeners based on characters of the abactinal and superomarginal plates, granules, actinal papulae, furrow spines, and the subambulacral spines. We also provide descriptions for three mesophotic species, Fromia eusticha Fisher, 1913, Ogmaster capella (Müller and Troschel, 1842), and Tamaria tenella (Fisher, 1906), which are poorly studied in Japanese waters.


Introduction
The Ogasawara Islands are oceanic islands located approximately 1000 km south of Japan's mainland. The islands are in the subtropics and inhabited by tropical/subtropical marine animals (Imajima 1970). The sea star fauna of the Ogasawara Islands was first reported by Hayashi (1938), who described six shallow-water species. Subsequent reports on the marine diversity further recorded shallow-water sea stars from this region (Imajima 1969;Shigei 1970;Amemiya and Yanagi sawa 1991). However, these reports provide no morphological description, pictures, or information on preserved specimens of each species. Pope and Rowe (1977) described a single individual of Thromidia catalai Pope and Rowe, 1977 from Chichi-jima Island of the Ogasawara Islands. Saba (2011) listed ten species occurring in the islands with photographs and short notes on their morphological characters. Fujita et al. (2015) recorded fourteen species from the islands with images of live individuals and vouchered specimens. All studies of asteroids from this region were conducted primarily in shallow water (<45 m), with only a single deep sea record (550 m) in this region by Kogure and Tachikawa (2009), who described Astroceramus boninensis Kogure and Tachikawa, 2009 from east of Haha-jima Island.
In this study, we sampled sea stars from a previously unstudied depth ranging 50-160 m with biological dredges. This depth range corresponds to the mesophotic zone, which is often recognized as depth range from 30 to 150 m (Baldwin et al. 2018). The mesophotic zone is known to harbor characteristic fauna differentiated from neighboring communities in shallower reef or deep water for fish, scleractinian corals, and other invertebrate taxa including echinoderms (Laverick et al. 2017;Semmler et al. 2017;Baldwin et al. 2018). Although there has been no study explicitly mentioning the significance of sea stars in the mesophotic zone, Mah (2003) suggested high possibility of finding new sea star species from the depth range of 60-200 m, which is difficult to explore because of the depth limit of conventional SCUBA diving and steep, rocky substrate to prevent dredging. Several new or rarely found taxa of sea stars have been reported thereafter from the subtropical/tropical mesophotic zone with various sampling methods such as mixed-gas diving, manned and unmanned submersibles, or dredging (Mah 2003;Pawson 2007;Kogure and Fujita 2012). Here we describe five species of sea stars of the families Ophidiasteridae and Goniasteridae, expanding knowledge of the sea stars occurring in the mesophotic zone of the Ogasawara Islands.

Materials and Methods
The examined specimens were collected from the Ogasawara Islands, Japan, using biological dredges lined with 5 mm mesh operated by three vessels: R/V Koyo of Tokyo Metropolitan Ogasawara Fisheries Center, TR/V Shin'yo-maru of the Tokyo University of Marine Science and Technology, and R/V Published online 1 January 2021 DOI: 10.12782/specdiv.26.7 Tansei-maru of the Japan Agency for Marine-Earth Science and Technology. Locality of the sampling stations is shown in Table 1. The collected specimens were on most occasions anesthetized by immersion in MgCl 2 solution (34.2 g/L), photographed, and then preserved in 99% ethanol. Otherwise they were just preserved in 99% ethanol.
Specimens were photographed using a digital camera with a macro lens or mounted on a stereoscopic microscope. Some of the images were generated by focus stacking performed by StackReg plug-in and Extended Depth of Field plug-in (http://bigwww.epfl.ch/) on ImageJ 1.48v (https:// imagej.nih.gov/ij/). Observation by scanning electron microscope (SEM) was also conducted on some specimens.
Total DNA was extracted from tubefeet using DNeasy Blood & Tissue Kit (QIAGEN). We amplified partial sequences (655 bp) of mitochondrial cytochrome c oxidase subunit I (COI) by polymerase chain reaction (PCR) using Ex Taq (Takara Bio). PCR was carried out with the primers COIceF and COIceR (Hoareau and Boissin 2010) following the cycle condition they provide. Amplified DNA was purified by ExoSAP-IT (Affymetrix) and sequenced using BigDye Terminator v3.1 Cycle Sequencing Kit and 3500xL genetic analyzer (Applied Biosystems). We obtained partial COI sequences from each species to facilitate future identification. The sequence data were deposited in DNA Data Bank of Japan (DDBJ).
The specimens examined in this study are deposited in the Department of Zoology, National Museum of Nature and Science, Tsukuba, Japan (NSMT).
We generally followed the morphological terminology in A. M. Clark and Downey (1992). R and r represent the length from the center of disc to the arm tip and the interradial edge, respectively.

Taxonomic Accounts
Family Goniasteridae Forbes, 1841 Subfamily Ferdininae  Diagnosis. A species of Bathyferdina with glassy bosses on abactinal, actinal and adambulacral plates. Disc broad with R/r 2.0-2.3. Abactinal plates are homogeneous in size and shape. Superomarginal plates rectangular to barrel-like in outline, and regularly decreasing in size from the inter-radius to the tip of arms. Glassy boss/ridge present on abactinal and actinal plates. One or two glassy bosses on the center of each adambulacral plate. Three furrow spines on each adambulacral plate.
Description of holotype. R=23.2 mm, r=9.9 mm, R/r= 2.3, width of arm is 11.1 mm at base, 6.6 mm at half of R, and 3.1 mm at 1/10 R from the tip. Body is flat on both abactinal and actinal sides. Arms are five and tapering more greatly near the disc than near the arm tip (Fig. 3A).
Abactinal plates are polygonal to elliptical in shape, not lobed, variable in size, and tessellated so that only small spaces are left for papulae around the corners of these plates (Fig. 3B). Those on the disc are about 1.5-2.0 times larger than those on the arms. The arrangement of abactinal plates is regular, and median rows of plates are conspicuous. Between two upper interradial corners of first superomarginal plates on a ray, there are seven to nine abactinal plates. The entire abactinal surface of abactinal plates is rough with glassy bosses which are circular around the center of the plate and elongated around the periphery (Fig. 3D). Madreporite is single, 1 mm in diameter, flat and pentagonal with rounded corners; gyri extend radially from the central area where nine discontinuous, short gyri lie in parallel to one another. Terminal plates are conical with a broad base and a narrow rounded apex, smaller than the distalmost marginal plates, smooth and bare without skin or granules (Fig. 3C).
Superomarginal and inferomarginal plates correspond in number and size. There are six (seven on one arm) plates to each side of an arm. Superomarginal plates are rectangular in outline, longer than width, rounded at corners, and conspicuous on the abactinal side; first superomarginal plates are about 3 mm in length and 2 mm in width on the abactinal surface. They gradually decrease in size toward the arm tip where the distalmost superomarginal plates are more squarish, measuring about 1-1.5 mm in both length and width. No glassy boss was observed on any marginal plates.
Actinal plates are quadrangular, regularly arranged, and normally flat except several small plates slightly convex between larger actinal plates (Fig. 3F) and inferomarginal plates are slightly convex. The surface of actinal plates is rough with glassy bosses when denuded of granular skin.
Adambulacral armature is composed of three (distally two) furrow spines on each adambulacral plate (Fig. 3F). Furrow spines are truncated at the tip, prismatic, and quadrangular or triangular in a cross section. The adambulacral plates are rectangular with slightly rounded corners. On the proximal first to tenth adambulacral plates, there are one or two glassy bosses at the center of the plates.
Abactinal, actinal and adambulacral plates are covered with a thin skin concealed by fine granules. On the marginal plates, the skin and granules are limited at the periphery, leaving the rest of the plate surface exposed (Fig. 3B). Papulae are isolated, confined around the abactinal plates except in the abactinal interradial areas where they are absent. There are no papulae on the actinal surface. Most papulae occur in spaces where corners of three abactinal plates meet. There are pedicellariae on some of the first or second superomarginal plate and actinal interradial plates (Fig. 3B). Each pedicellaria with a single fan-shaped valve rather than two or more valves normally observed in Ferdininae. Some alveoli lacking a fan-shaped valve are present on some marginal plates.
Color in life is brownish on the abactinal surface and white on the actinal. Bare areas on marginal plates are darker brown ( Fig. 2A).
Notes on paratypes. The body dimensions in the paratypes are: R=10.0 mm, r=5.0 mm, and R/r=2.0 (NSMT E-8266); R=20.4 mm, r=9.7 mm, and R/r=2.1 (NSMT E-8267). The other characters are consistent with the description of the holotype.
DNA sequence. A partial sequence of COI (655 bp) was obtained from NSMT E-8265 and deposited in DDBJ (Acc. No. LC427072).
Remarks. Mah (2017) established Bathyferdina with a de-scription of B. aireyae. Bathyferdina caelator sp. nov. is devoid of any subambulacral spines or enlarged granules while it shows small R/r, a uniform granular skin covering abactinal and actinal surface, marginal plates continuous in size, large quadrate bare area on marginal plates, homogenous abactinal plates completely covered with skin, and glassy bosses on abactinal plates. These characters support placement of the present species in Bathyferdina. Mah (2017) mentioned that Bathyferdina has no glassy bosses on marginal plates in his key of Ferdininae but he also stated that it has glassy bosses on both abactinal and marginal plates. According to Mah (personal communication), B. aireyae in fact has glassy bosses on the abactinal and marginal surfaces but not on the actinal surface. We have emended the diagnosis of Bathyferdina accordingly. Bathyferdina caelator is distinguished from the single congeneric species, B. aireyae, in having glassy bosses on not only abactinal but actinal and adambulacral plates while lacking them on marginal plates. Bathyferdina caelator lacks granules or tubercles on the inferomarginal plates, while they are occasionally present in B. aireyae. Mah (2017) described B. aireyae as lacking any pedicellariae but one of the paratypes reexamined in this study (CASIZ 219696) possesses a large number of pedicellariae on marginal and actinal plates. Although these pedicellariae varied in number of valves from one to five, the shape of the valves closely resembled those in B. caelator.
Etymology. The specific epithet, caelator, is a Latin masculine noun in apposition meaning sculptor, and alluding to the complex patterns of glassy bosses on abactinal, actinal and adambulacral plates.
Japanese name. Koyo is taken from R/V Koyo, the vessel which collected the holotype, and akamon-hitode comes from the Japanese name of Neoferdina cumingi (Gray, 1840), another species of Ferdininae which occurs commonly in Japanese waters. Fisher, 1913 [Japanese name: Nameraka-juzuberi-hitode] ( 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).

Fromia eusticha
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, lack- ing 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.
Japanese name. Nameraka means smooth, referring to its fine granules and graduating superomarginal plates. Juzuberi-hitode is a Japanese name for the genus Fromia. Diagnosis. A species of the genus Fromia with a small disc and slender, slightly arched arms. At R=31.3 mm, abactinal plates arranged in five longitudinal rows at base of arms. Superomarginal plates only slightly convex, not alternating but regularly decreasing in size toward the tip of arms. Coarse granules enlarged at center of plates. Papulae single and confined on the abactinal surface. Adambulacral plates bear three (rarely two or four) furrow spines and five to nine thicker subambulacral granules graduating in size toward those on neighboring actinal/marginal plates. Oral plates bear five oral spines on the margin and four to six suboral spines on the rest of the plate. Large, elliptical pedicellariae on many actinal plates with their major axis oblique to the ambulacra.
Description of holotype. R=31.3 mm, r=8.6 mm, R/r= 3.6, width of arm is 9.0 mm at base, 5.4 mm at half of R, and 2.9 mm at 1/10 R from the tip. Body is slightly arched at both sides, and the abactinal interradial areas are sunken. Arms taper to blunt tips.
Abactinal plates are polygonal, weakly lobate, arranged in staggered longitudinal series where there are five rows at the base of arms (Fig. 5C). Hemispherical glassy bosses (crystal bodies) occur on the abactinal plates. Madreporite is single, circular, and about 0.9 mm in diameter, located at about one half r from the anal aperture. Gyri do not regularly radiate from the center of the madreporite. The anal aperture is single and surrounded by six slightly larger ossicles. Terminal plates are conical, truncated at the tip. Several tubercles encircle the top of terminal plates.
Superomarginal and inferomarginal plates are block-like, rectangular, only slightly convex and not alternating but regularly decreasing in size toward the tip of arms. There are sixteen superomarginal and seventeen inferomarginal plates on one side of the ray when skin and granules are removed. The second to fourth superomarginal plates average 1.7 mm in length.
Actinal plates are polygonal, without lobes, and leave no space in between. No glassy bosses occur on actinal plates. These are arranged in three longitudinal series at the base of a denuded arm with a few odd plates on the interradial area.
Adambulacral plates bear three (exceptionally two or four), slender, flattened, cylindrical and bluntly pointed furrow spines and five to nine thicker, more sharply pointed subambulacral granules (Fig. 5F). The latter are not in regular rows parallel to the ambulacrum, and are grading in size to the granules on neighboring actinal or marginal plates. Tubefeet are biserial with a stout terminal disc.
Each oral plate bears five oral spines on the adoral margin and four to six suboral spines on the rest of the plate. Oral spines are conical or pyramidal, larger than furrow spines. Suboral spines are similar to oral spines in the adradial portion but abradially grading into actinal granules.
The abactinal, marginal, and actinal plates are concealed with a skin which is covered coarsely with granules (Fig.  5B). These granules are domed at the top, polygonal in shape, and slightly enlarged at the center of plates, but never spinous. The diameter is ca. 260-340 µm at center and ca. 140-200 µm on periphery. There are 20 granules per square millimeter on abactinal plates at the base of arms and actinal interradial plates. The papular pores are isolated and confined around abactinal plates except on the interradial areas and tips of arms, where the pores are absent.
Many actinal plates possess a large elliptical pedicellaria (Fig. 5D). It is typically 1 mm in length and 400 µm in width. The pedicellariae are loosely arranged in a parallel series to an ambulacrum on arms, and their major axis is often oblique to the ambulacrum (Fig. 5A, E).
Color in life is vermilion with white granules and terminal plates (Fig. 2C).
Notes on paratypes. The difference in the numeric characters among type specimens is shown in Table 2. R/r generally increases and the number of granules per square millimeter decreases as R increases. Other characters in the paratypes are consistent with the description of holotype.
DNA sequence. A partial sequence of COI (655 bp) was obtained from NSMT E-9295 and deposited in DDBJ (Acc. No. LC427074).
Remarks. The present species is distinguished from its congeners by the following characters: superomarginal plates regularly decreasing in size toward the arm tip in F. labeosa sp. nov. while they are alternating large and small, or small superomarginal plates are occasionally intercalated between larger ones, in F. pacifica H. L. Clark, 1921, F. heffernani (Livingstone, 1931, F. monilis (Perrier, 1869), and F. nodosa A. M. Clark, 1967; there are five abactinal plates between first superomarginal plates on each side of arms in F. labeosa while there are more than seven such plates in F. hemiopla Fisher, 1913, F. milleporella (Lamarck, 1816, F. polypora H. L. Clark, 1916, and F. schultzei (Döderlein, 1910); abactinal plates are uniform in size in F. labeosa while several elliptical plates are larger than others in F. elegans H. L. Clark, 1921 andF. indica (Perrier, 1869); abactinal granules are granular in F. labeosa while they are spinuous in F. armata Koehler, 1910; they are also coarse (20/mm 2 ) in F. labeosa while they are fine (80-90/mm 2 ) in F. hadracantha H. L. Clark, 1921; papulae are lacking on the actinal surface in F. labeosa while there are two rows of actinal papulae in F. balansae Perrier, 1875 andF. ghardaqana Mortensen, 1938. Five to nine subambulacral tubercles continuous in size and not arranged in definite rows in F. labeosa while subambulacral spines are conspicuously larger than outer granules on the adambulacral plate, and arranged in a straight row in F. eusticha. In addition to these differences, large elliptical pedicellariae on actinal plates are specific to F. labeosa.
Etymology. The species epithet, labeosa, is a Latin feminine adjective meaning having large lips and referring to the large pedicellariae on the actinal surface.
Japanese name. Ogasawara is taken from the type locality and juzuberi-hitode is a Japanese name for the genus Fromia.
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 pen-tagonal, 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.
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.
Description. R=30.0 mm, r=6.2 mm, R/r=4.8, width of arm is 7.3 mm at base, 4.6 mm at half of R, and 3.0 mm at 1/10 R from the tip in NSMT E-8285. R=26.0 mm, r=5.4 mm, R/r=4.8, width of arm is 6.7 mm at base, 3.6 mm at half of R, and 2.1 mm at 1/10 R from the tip in NSMT E-9277. Body is arched on the abactinal side and flat on the actinal side. Arms are five and tapering toward the tip evenly from the base (Fig. 7A).
Abactinal plates are four-lobed and arranged in three regular longitudinal rows which reach the tip of arms. The rows are partly disrupted at base of an arm of NSMT E-8285, and the shortest arm of NSMT E-9277. Five to six carinal plates in the middle of arms and a few distal actinolateral plates are devoid of skin and granules in an elevated circular area at the center (Fig. 7F). Madreporite is circular, not elevated from the abactinal surface of disc; gyri radiate from the center in a complex pattern. Tubefeet are biserial with a terminal disc. Terminal plates are spherical, devoid of granules, and bearing about ten tubercles at the apex in NSMT E-9277 (Fig. 7F). In NSMT E-8285, the tubercles are very low and faint, and the number is about three.
Superomarginal and inferomarginal plates are similar in shape and size to abactinal plates, and arranged in two regular longitudinal series. Distal superomarginal plates (twelve to thirteen plates from the arm tip in NSMT E-9277 and seven to nine in NSMT E-8285) are devoid of a skin and granules like some of the abactinal plates (Fig. 7F). More distal plates have a larger, more swollen and more elliptical naked area. In NSMT E-8285, three to four distalmost inferomarginal plates are denuded and swollen like distalmost superomarginal plates.
Actinal plates are rectangular and arranged in three longitudinal series at the base of arms: the innermost series reach about 3/4R, the outer 1/2R, and the outermost 1/6R in NSMT E-8285 (in NSMT E-9277, there are two series reaching 2/3R and 1/4R respectively, and a few interradial plates).
Each adambulacral plate bears two furrow spines and a subambulacral spine (Fig. 7D). Furrow spines are truncated, flattened, and polygonal but rounded at corners and slightly concave at the furrow surface. Each plate carries a pair of subequal spines. Subambulacral spines are obovate but more circular in the distal part of arms. Between two furrow spines and a subambulacral spine, there are granules one of which is occasionally larger than the surroundings to appear like a tubercle.
Oral plates are covered with granules like actinal plates. A plate bears five to six oral spines and a suboral spine (Fig.  7B). They are identical in shape to furrow spines and subambulacral spines respectively.
The entire body is overlaid with a skin except some abactinal plates and distal superomarginal plates as mentioned above. The skin is covered with fine granules homogenous in size (Fig. 7B, C). The number of granules per square millimeter is 379 on proximal carinal plates and 167 on proximal actinal plates in NSMT E-9277; 281 and 295 respectively in NSMT E-8285.
A papular area bears one to three pores on the disc and one to five on the arms. The areas occur between carinal, abactinolateral, superomarginal and inferomarginal plates but never below the inferomarginal plates. They are arranged in six longitudinal series although the numbers of pores change irregularly.
Pedicellariae presents on superomarginal plates (from the base to the middle of arms), abactinal interradial plates, and exceptionally on an abactinolateral plate in NSMT E-9277, on actinolateral, superomarginal, and inferomarginal plates in NSMT E-8285. Each pedicellaria with two valves, which fit in an entrenched piece of ossicle with an alveolus at the center (Fig. 7E). The ossicle is merged with the subjacent plate. The valve is gradually broadening from the base to form a smooth, unserrated, and slightly curved contour.
Color in life is unknown. The ex-ethanol specimens show brown transverse bands on arms with darker papular areas and transverse lines between plates on the abactinal surface.
DNA sequence. A partial sequence of COI (655 bp) was sequenced from NSMT E-9277 and deposited in DDBJ (Acc. No. LC427076).
Remarks. Fisher (1906) described two closely similar species, Ophidiaster sclerodermus Fisher, 1906 and O. tenellus, distinguishing them by the length of arms, granulation, thickness of skin, number of papular pores in each area, and number of pedicellariae. Both species were subsequently transferred to the genus Tamaria by H. L. Clark (1921). The present Ogasawara specimens resemble these two species, but show discordance in some characters from each species. Tamaria scleroderma has a larger number of papu- lar pores (9-14) and shorter arms (R/r=4.8) than Tamaria tenella. The Ogasawara specimens are consistent with T. tenella in the number of papular pores (1-3) while it has longer arms (R/r=6.9). The New Zealand specimens of T. tenella described by McKnight (1975McKnight ( , 2001 differ from the Ogasawara specimens in the proportion of arms, number of papulae, distribution of pedicellariae, and spines on actinal plates. The rays are longer in proportion to the disc: R/r=5.5 at R=22 mm and 7.5 at R=48 mm. All the papulae are isolated, and the pedicellariae occur on actinal plates. In the larger specimen with R=48 mm, spines similar to the subambulacral ones occur on some actinal plates. In addition to these differences, the Ogasawara specimens also have a central naked area on the distal carinal and superomarginal plates which is not present in the Hawaiian or New Zealand specimens. The present specimens are temporarily considered to be T. tenella, based on the small number of papular pores per area. However, we believe that these species require a detailed revision in regard to morphological variation with a greater number of specimens. Distribution. Hawaii Islands, 238-276 m (Fisher 1906). Three Kings Rise andNorfolk Ridge, New Zealand, 403-503 m (McKnight 1993, 2001). Ogasawara Islands, Japan, 159-161 m (Arai et al. 2018;this study).
Japanese name. Birodo means velvet, referring to the smooth skin with minute granules. Hime-hokiboshi means a small comet.

Discussion
Five species of sea stars from the Ogasawara Islands are described in this study; two are new to science. Summarizing the published records of sea stars in the Ogasawara Islands and this study, we recognize 31 species of 23 genera in 10 families (Table 3). It should be noted that most of the historical records were not verified by specimens, images, or other data. This number of asteroid species is lower than those in the neighboring subtropical/tropical islands such as the Nansei Islands and the Mariana Islands, which are Table 3. Sea stars reported from the coasts of the Ogasawara Islands and adjacent waters. References as follows: a Hayashi (1938); b Imajima (1969); c Shigei (1970); d Pope and Rowe (1977); e Amemiya and Yanagisawa (1991); f Kogure and Tachikawa (2009) Fisher, 1913 i, j Fromia indica (Perrier, 1869) a, e, h, i Fromia labeosa sp. nov.
In the Ogasawara Islands, the five species described in this study occurred only in the mesophotic zone between 56-161 m. None of them have ever been reported from the shallower zone (0-30 m) of the Ogasawara Islands (Arai et al. 2018). This suggests that the mesophotic zone has a unique sea star fauna in the Ogasawara Islands. Moreover, the five species have not been recorded from shallower water in any localities save for Fromia eusticha, which is known from 0-55 m of Philippines, Indonesia and Marshall Islands (Fisher 1913;Domantay and Roxas 1938;A. H. Clark 1952;Jangoux 1978). Tamaria tenella is reported from deeper waters below the mesophotic zone in the Hawaii Islands and New Zealand (Fisher 1906;McKnight 1993McKnight , 2001.
The two new species, Bathyferdina caelator sp. nov. and Fromia labeosa sp. nov., were collected only in the Ogasawara Islands, and it is possible that they are endemic to the islands. There is only a single species of sea star currently known to be endemic in the Ogasawara Islands: Astroceramus boninensis. This species has never been reported from other localities since the original description. Extensive surveys in the neighboring islands such as the Izu Islands are required to clarify the geographic distribution of these species and to understand diversity and endemism of the sea stars of the Ogasawara Islands.