Introduction
Echinaster is one of the speciose genera of the family Echinasteridae. This genus mainly occurs in the shallow and tropical waters in the Pacific, Indian, and Atlantic Oceans (Mah and Blake Reference Mah and Blake2012) and is characterized by slender and subcylindrical arms, a reticulated skeleton covered by a thick integument, and long spines (usually 0.5–1.5 mm in length) widely spaced out from each other (Clark and Downey Reference Clark and Downey1992; Liao and Clark Reference Liao and Clark1995). Since the establishment of the family Echinasteridae by Verrill (Reference Verrill1867), the classification of Echinaster has been debated for a long time because it was difficult to separate this genus from the confamilial genera, Henricia and Othilia, due to the presence of some intermediate-form species that have blurred the border between them (Clark and Downey Reference Clark and Downey1992; Fisher Reference Fisher1940; Mortensen Reference Mortensen1933). Indeed, the taxonomic status of the genus Othilia has been argued; whether the genus should be ranked as a subgenus of the genus Echinaster (e.g. Clark Reference Clark1987; Clark and Downey Reference Clark and Downey1992; Verrill Reference Verrill1914) or as a separate genus (e.g. Fisher Reference Fisher1911, Reference Fisher1919; Tortonese and Downey Reference Tortonese and Downey1977). A recent molecular phylogenetic study indicated the divergence of these genera (Lopes et al. Reference Lopes, Pérez-Portela, Paiva and Ventura2016), and Echinaster was separated from Othilia as a different genus (Cunha et al. Reference Cunha, Martins, Menegola and Souto2021). The Echinaster fauna is most diverse in the Indo-West Pacific Region, containing 10 endemic species of a total of 21 species in this genus (Clark Reference Clark1996). However, only two species, Echinaster callosus (Marenzeller Reference Marenzeller1895) and Echinaster luzonicus (Gray Reference Gray1840), have been known in Japanese waters (Saba et al. Reference Saba, Irimura and Soyama2002).
During the faunal surveys in Japanese subtropical waters, we collected 10 Echinaster and two Metrodira specimens from southern Japan, resulting in the discovery of two undescribed species and two species newly recorded from Japanese waters. In addition, we inferred the phylogenetic position of these species among the family Echinasteridae. Based on the result of the phylogenetic analysis, we also found that the genus Metrodira was recognized as a junior synonym of the genus Echinaster.
Materials and methods
Sample collection and morphological observations
The 12 examined specimens in this study were collected by hand during snorkelling and scuba diving, or with dredges by the research vessels R/V Toyoshio-maru and R/V Koyo-maru, and the diving boat Dainihayabusa-maru in southern Japan (Figure 1). Some specimens were photographed using a digital camera on-board or in laboratory, and all specimens were preserved with 99.5% ethanol. All specimens are deposited in the National Museum of Nature and Science, Tsukuba, Japan (NSMT).
Locality of sampling sites of Echinaster species. (a) Echinaster callosus, (b) Echinaster crystallus n. sp., (c) Echinaster luzonicus, (d) Echinaster stereosomus, (e) Echinaster subulatus n. comb., (f) Echinaster toyoshiomaruae n. sp.
The ethanol-preserved specimens were observed using the binocular microscopes MZ 8 (Leica Microsystems, Germany) and Swift S7 (Swift Optical Instruments, USA). Lengths of major radius (R) and minor radius (r) were measured from the centre of the mouth opening to unregenerated and unbroken arm tips and the connection of each proximal part of two arms, respectively. Removing the epidermis by applying commercial bleach (about 5% sodium hypochlorite), the shape and the arrangement of the underlying plates were observed. The proximal superomarginal plates of NSMT E-12494, 14935, and 14937 were extracted for the observation of crystal bodies under a scanning electron microscope (SEM). These plates were immersed in a drop of commercial bleach for a few minutes, followed by washing with deionized water to clean up their surface. The samples were then mounted on brass or aluminium SEM stubs and coated with gold–palladium. Finally, these plates were observed under a JSM-6380LV SEM (JEOL, Japan). Eight specimens (NSMT E-9360, 12494, and 14935–14941) were scanned using an inspeXio SMX-225CT FPD HR micro-computed tomography (micro-CT) scanner (Shimadzu, Japan), at a tube voltage of 115 kV and a tube current of 70 μA for 30 min, to count the number of spinose supero- and inferomarginal plates obscured by a thick dermal layer. Two specimens of the type species of the genus Echinaster, Echinaster sepositus (NSMT E-14942) were also scanned similarly for morphological comparison. Three-dimensional images were reconstructed using VGSTUDIO Max 3.2 (Volume Graphics, Germany).
DNA extraction, gene amplification, and molecular analysis
The tube feet of six preserved specimens (Table 1) were excised for DNA extraction. Total DNA was extracted by boiling the tissues with Chelex-100 resin (Bio-Rad, USA) in ultrapure water for 30 min, followed by rapid cool down on the ice for 10 min. Polymerase chain reaction (PCR) was performed by using 5.0 μl of 2 × PCR Buffer (Takara Bio, Japan), 0.2 μl TKs Gflex Polymerase (Takara Bio, Japan), 0.2 μl of each primer pair (10 μM), 1.0 μl of extracted DNA, and 3.4 μl of ultrapure water (3.4 μl). Following the prior phylogenetic study for Echinasteridae (Lopes et al. Reference Lopes, Pérez-Portela, Paiva and Ventura2016), three primer pairs, COIceF (5′-ACT GCC CAC GCC CTA GTA ATG ATA TTT TTT ATG GTN ATG CC-3′) and COIceR (5′-TCG TGT GTC TAC GTC CAT TCC TAC TGT RAA CAT RTG-3′) (Hoareau and Boissin Reference Hoareau and Boissin2010), 16SaL (5′-CGC CTG TTT ATC AAA AAC AT-3′) and 16SAN-R (5′-GCT TAC GCC GGT CTG AAC TCA G-3′) (Palumbi Reference Palumbi1994; Zanol et al. Reference Zanol, Halanych, Struck and Fauchald2010), and 28SF (5′-AGA AAC TAA CMA GGA TTC CYY TAG TA-3′) and 28SR (5′-ACT TTC CCT CAY GGT ACT TGT-3′) (Foltz et al. Reference Foltz, Bolton, Kelley and Nguyen2007), were used to amplify the partial sequences of the mitochondrial cytochrome c oxidase subunit I (COI) and 16S rRNA (16S) genes and the nuclear 28S rRNA (28S) gene, respectively. The protocol used for PCR amplification was as follows: preheating at 94 °C for 1 min; 30 cycles of 98 °C for 10 s, 55 °C for 15 s, and 68 °C for 30 s. Following purification using ExoSAP-IT Cleanup Regent (Thermo Fisher Scientific, Inc., USA), Big Dye Terminator v3.1 Cycle Sequencing Kit (Thermo Fisher Scientific, Inc., USA) was used for sequencing the PCR products using the Applied Biosystems 3500xL Genetic Analyzer (Life Technologies, Inc., USA). Consequently, a partial sequence of COI (454–626 bps), 16S (422–618 bp), and 12S (231–314 bps) genes were obtained. Fifty sequences of 19 Echinasteridae species (Table 1) were obtained from GenBank, and four asteroids, Luidia foliolata, Odontaster validus, Patiria miniata, and Tosia australis, were selected as out-groups following the molecular phylogenetic analyses in Lopes et al. (Reference Lopes, Pérez-Portela, Paiva and Ventura2016). A total of 27, 28, and 23 sequences were aligned by MAFFT ver. 7.222 (Katoh and Standley Reference Katoh and Standley2013) for COI, 16S, and 28S, respectively. For the COI dataset, the stop codons were removed by manually editing the sequence errors using MEGA ver. 7.0.26 (Kumar et al. Reference Kumar, Stecher and Tamura2016). The 16S and 28S datasets were trimmed with the gappyout option by using trimAL 1.2rev59 (Capella-Gutiérrez et al. Reference Capella-Gutiérrez, Silla-Martínez and Gabaldón2009). The suitable models for the substitution ratio for concatenated sequences were determined using Kakusan4 (Tanabe Reference Tanabe2011), resulting in a separate model for the Maximum likelihood (ML) analysis and a proportional model for the Bayesian inference (BI) analysis. ML tree was inferred under the GTR + G model for each gene using RAxML ver. 8.2.9 (Stamatakis Reference Stamatakis2014). Bootstrap values (BS) were calculated from 1000 replicates. BI analysis was conducted using MrBayes 3.2.6 (Ronquist et al. Reference Ronquist, Teslenko, van der Mark, Ayres, Darling, H€ohna, Larget, Liu, Suchard and Huelsenbeck2012). Based on the Bayesian information criteria calculated by Kakusan4, the best-fit substitution models for this analysis were determined as the GTR + G, HKY85 + G, K80 + G, F81 + Homogeneous, and GTR + G models for the 16S, 28S, and the first, second, and third codon positions of COI, respectively. Markov-chain Monte-Carlo analysis (MCMC) was simulated for 10 million generations and sampled every 100 generations. The first 10% of trees were discarded as burn-in. MEGA ver. 7.0.26 (Kumar et al. Reference Kumar, Stecher and Tamura2016) was used to calculate the interspecific Kimura 2-parameter (K2P) distance.
List of the species used for the molecular phylogenetic analysis, with the GenBank accession numbers of COI, 16S, and 28S sequences
ND indicates data not available. GenBank accession numbers of sequences obtained in this study are shown as bold.
Results
Phylogenetic analysis
Both ML and BI analyses produced almost identical topologies with only minor differences in terminal nodes (Figure 2). The resulting phylogenetic trees resolved the three distinct clades with strong supporting values (98–100% BS; 1.00 PP [posterior probability]), corresponding to the three genera Echinaster, Henricia, and Othilia. Among these, the Echinaster clade included the majority of Echinaster species, including five Echinaster and one Metrodira species newly sequenced in this study. One Echinaster species, E. farquhari, was placed within the Henricia clade.
Maximum likelihood tree of 22 Echinasteridae species based on concatenated sequences of COI, 16S, and 28S genes. The values on branches indicate bootstrap support (left, >70%) and the Bayesian posterior probabilities (right, >0.95). The scale bar indicates substitutions per site in branch length. Operational taxonomic units (OTUs) newly sequenced in this study are shown in bold with catalogue numbers. See Table 1 for accession numbers of each OTUs.
Systematics
Family Echinasteridae Verrill, Reference Verrill1867
Genus Echinaster Müller & Troschel, Reference Müller and Troschel1840
(Japanese name: Ruson-hitode-zoku)
Echinaster Müller & Troschel, Reference Müller and Troschel1840: 321 (in part); Müller & Troschel, Reference Müller and Troschel1842: 22 (in part); Dujardin & Hupe, Reference Dujardin and Hupe1862: 344, 345 (in part); Perrier, Reference Perrier1869: 57–58 (in part); Lütken, Reference Lütken1871: 281 (in part); Perrier, Reference Perrier1875: 100 (in part); Viguier, Reference Viguier1879: 62–64 (in part); Sladen. Reference Sladen1889: 553, 554 (in part); Perrier, Reference Perrier and Perrier1893: 849; 1894: 146, 147; Fisher, Reference Fisher1913: 194; Verrill, Reference Verrill1915: 35 (in part); Fisher, Reference Fisher1919: 426–428; Hayashi, Reference Hayashi1940: 123; H.L. Clark, Reference Clark1946: 145; Hayashi, Reference Hayashi1973: 75; Walenkamp, Reference Walenkamp1976: 17 (in part); Tortonese & Downey, Reference Tortonese and Downey1977: 829, 830 (in part); Clark & Downey, Reference Clark and Downey1992: 350–357 (in part); Liao & Clark, Reference Liao and Clark1995: 135; McKnight Reference McKnight2006: 27.
Echinaster (Echinaster): Clark, Reference Clark1987: 66; Clark & Downey, Reference Clark and Downey1992: 357.
Metrodira Gray, Reference Gray1840: 282; Gray, Reference Gray1866: 12; Perrier, Reference Perrier1875: 180; Fisher, Reference Fisher1919: 405, 406; Blake, Reference Blake1980: 181; Liao & Clark, Reference Liao and Clark1995: 134; Purwati & Lane, Reference Purwati and Lane2004: 100. New synonymy.
Rhopia Gray, Reference Gray1840: 282; Gray, Reference Gray1866: 12.
Scaphaster Loriol, Reference Loriol1899: 27, 28.
Verrillaster Downey, Reference Downey1973: 89.
Type species
Asterias seposita Retzius, 1783.
Other species included
Echinaster arcystatus HL Clark, Reference Clark1914, Echinaster callosus Marenzeller, Reference Marenzeller1895, Echinaster colemani Rowe & Albertson, Reference Rowe and Albertson1987, Echinaster cribella Lütken, Reference Lütken1871, Echinaster cylindricus Meissner, Reference Meissner1892, Echinaster deplanatus Grube, Reference Grube1857, Echinaster glomeratus Clark, Reference Clark1916, Echinaster lacunosus Grube, Reference Grube1857, Echinaster luzonicus (Gray Reference Gray1840), Echinaster modestus Perrier, Reference Perrier1881, Echinaster panamensis Leipoldt (Reference Leipoldt1895), Echinaster parvispinus AH Clark (Reference Clark1916), Echinaster purpureus (Gray Reference Gray1840), Echinaster reticulatus Clark, Reference Clark1923; Echinaster rigidus Grube, Reference Grube1857, Echinaster smithi Lüdwig, Reference Lüdwig1903, Echinaster stereosomus Fisher, Reference Fisher1913, Echinaster superbus Clark, Reference Clark1916, Echinaster varicolor Clark, Reference Clark1938; Echinaster subulatus (Gray Reference Gray1840) n. comb.; Echinaster crystallus n. sp., Echinaster toyoshiomaruae n. sp.
Excluded species
Henricia farquhari (Benham Reference Benham1909) n. comb.
Diagnosis
Genus of Echinasteridae with small disc and tapering arms, usually subcylindrical but sometimes slightly flattened; abactinal skeleton reticular, ranging from open to close, with/without regular longitudinal arrangement, obscured by thick skin; pentagon of linked primary radial plates and interradial plates on disc around anus absent; abactinal spines bullet-shape, rarely exceeding 1.5 mm long, usually 0.5–1.0 mm long, always relatively sparse and usually single on each plate; crystal bodies either present or absent; abactinal and sometimes intermarginal meshes occupied by papulae; inferomarginal plates showing proximal imbrication; intermarginal plates present between supero- and inferomarginal series proximally; actinal plates either present or absent; adambulacral plates with single transverse series of subambulacral spines across plate with the one or more small modified inset furrow spines (modified from Clark and Downey Reference Clark and Downey1992).
Echinaster callosus Marenzeller, Reference Marenzeller1895
(Japanese name: Ōtoge-ruson-hitode)
Figures 3a, 4a, 5a
Denuded abactinal surface of the proximal portion of arm. (a) Echinaster callosus, NSMT E-6709; (b) Echinaster crystallus n. sp., holotype, NSMT E-12494; (c) Echinaster luzonicus, NSMT E-14941;, (d) Echinaster stereosomus, NSMT E-9360; (e) Echinaster subulatus n. comb., NSMT E-14937; (f) Echinaster toyoshiomaruae n. sp., holotype, NSMT E-14935. Abbreviations: sa, straight abactinal series; za, zigzag abactinal series. Scale bars indicate 1 mm.
Denuded lateral to actinal surface of the proximal portion of the arm. (a) Echinaster callosus, NSMT E-6709; (b) Echinaster crystallus n. sp., holotype, NSMT E-12494; (c) Echinaster luzonicus, NSMT E-14941; (d) Echinaster stereosomus, NSMT E-9360; (e) Echinaster subulatus n. comb., NSMT E-14937; (f), Echinaster toyoshiomaruae n. sp., holotype, NSMT E-14935. Abbreviations: ac, actinal series; ad, adambulacral series; i, inferomarginal series; s, superomarginal series. Scale bars indicate 1 mm.
Denuded actinal surface of the proximal portion of the arm. (a) Echinaster callosus, NSMT E-6709, showing actinal spine on unpaired actinal plate (arrowhead); (b) Echinaster crystallus n. sp., holotype, NSMT E-12494; (c) Echinaster luzonicus, NSMT E-14941; (d) Echinaster stereosomus, NSMT E-9360, showing actinal spine on unpaired actinal plate (arrowhead); (e) Echinaster subulatus n. comb., NSMT E-14937; (f), Echinaster toyoshiomaruae n. sp., holotype, NSMT E-14935. Abbreviations: ac, actinal series; ad, adambulacral series; i, inferomarginal series. Scale bars indicate 1 mm.
Echinaster callosus Marenzeller, Reference Marenzeller1895: 531; Fisher, Reference Fisher1919: 428, pl. 112 fig. 2, pl. 122 figs 4, 5, pl. 132 fig. 5; Domantay & Roxas, Reference Domantay and Roxas1938: 229–230; Clark & Rowe, Reference Clark and Rowe1971: 40, 72; Marsh, Reference Marsh1977: 277–278, fig. 11; Jangoux & de Ridder, Reference Jangoux and de Ridder1990: 210; Liao & Clark, Reference Liao and Clark1995: 66, 136; VandenSpiegel et al., Reference VandenSpiegel, Lane, Stampanato and Jangoux1998: 435, 448, 462, fig. 30, pl. 2 fig. 4.
Material examined
NSMT E-6709, 25 June 2008, Majanohama beach, Akajima Island, Kerama Islands, Okinawa, Japan, 4 m in depths, fixed in 99.5% ethanol: R = 111 mm, r = 18 mm.
Diagnosis
Arms five with single madreporite; abactinal skeleton irregular open meshwork; each papular area contains over 10 papulae; primary abactinal plates bear one spaced spines; supero- and inferomarginal plates arranged irregularly or in longitudinal series, bearing zero or one spine; intermarginal plates bear spines; papulae absent between supero-, inter-, and inferomarginal plates; actinal plates arranged in longitudinal series with single spine only on proximal-most unpaired plate; adambulacral plates bear one furrow spine deep in furrow and one subambulacral spine; crystal bodies absent.
Description
Arms are five, subcylindrical in shape, slender and gradually tapering to the arm tip. R/r ratio is 6.2r. The abactinal skeleton is an irregular open meshwork, constituted by trilobate or quadrilobate primary abactinal plates and oval to rod-shaped secondary abactinal plates (Figure 3a). Abactinal plates are partially imbricated with each other. Each abactinal papular area is larger than the surrounding plates, containing 1–50 papulae. Each primary abactinal plate bears one spaced abactinal spines, but secondary abactinal plates lack spines. A madreporite is located at the margin of the disc without spines.
Abactinal spines are 1.3–2.3 mm in length. The lateral surface of the spines is uniformly smooth and is gradually tapered to the bluntly pointed tip, providing bullet-shaped appearance (Figure 3a).
Supero- and inferomarginal plates are irregular in shape, bearing zero or one spine and arranged in intermittent longitudinal series only at the proximal portion of the arms (Figure 4a). Supero- and inferomarginal series are gradually irregular towards the arm tips and almost indiscernible from middle to distal portion of the arms. Supero- and inferomarginal series are separated by each other at proximal portion of the arms. The superomarginal series gradually come closer to the inferomarginal series towards distally, but the meeting point of these two series is unrecognizable. The interspace formed between these two series is filled with ovate, triangular, or quadrilobate intermarginal plates, which irregularly arranged and bear zero or one spine. Papulae are absent between the supero-, inter-, and inferomarginal plates.
Actinal plates are round to ovate and irregularly filled the space between inferomarginal and adambulacral series (Figures 4a & 5a). A few actinal plates adjacent to the adambulacral plates arranged in longitudinal series. Actinal plates lack spines, but each first unpaired plate bears one spine. Papulae are absent between the inferomarginal, actinal, and adambulacral plates.
Adambulacral plates are arranged in longitudinal series along ambulacral furrows (Figures 4a & 5b). All adambulacral plates bear one furrow spine deep in the furrow and one subambulacral spine on the adradial corner.
Crystal bodies are absent (Figures 3a, 4a, & 5a).
The colour in life is unknown in observed specimen.
Distribution
Indian and West Pacific Ocean; Faransan Bank, Sri Lanka, Andaman Island, Philippines, Indonesia, Solomon Islands, Caroline Island, Amami Islands, Okinawa Islands, and Kerama Islands, bathymetrical range is 4–62 m in depths (Clark Reference Clark1996; Saba et al. Reference Saba, Irimura and Soyama2002; Lopes et al. Reference Lopes, Pérez-Portela, Paiva and Ventura2016; present study).
Remarks
The observed specimen aligns with the original description (Marenzeller Reference Marenzeller1895) of E. callosus in having an open-meshed abactinal skeleton composed of irregularly arranged abactinal plates (Figure 3a), zero or single abactinal spine on each abactinal plate, up to 50 papulae in each papular area, first unpaired actinal plates bearing spine (Figure 5a) and one subambulacral spine on each adambulacral plate (Figure 4a). Notably, the observed specimen lacks regular supero- and inferomarginal series at the middle to the distal portion of the arms. However, Fisher (Reference Fisher1919) reported that the young E. callosus (unknown body size) had regularly arranged supero- and inferomarginal series. The regularity of the arrangement of supero- and inferomarginal plates of E. callosus appears to be associated with its growth stage.
Our phylogenetic analysis (Figure 2) supported the close relationship of E. callosus and E. stereosomus with high support values (100% BS and 1.00 PP). This is also supported by the following shared morphologies: the presence of the dermal callosity investing abactinal spines (Figure 6b; VandenSpiegel et al. Reference VandenSpiegel, Lane, Stampanato and Jangoux1998), the first unpaired actinal plates bearing spine (Figure 5a, d) and the absence of crystal bodies (Figures 3a, d, 4a, d, & 5a, d). However, E. stereosomus differs from E. callosus in possessing the longitudinally arranged abactinal plates (Figure 3d) and one or two subambulacral spines on each adambulacral plate (Figures 4d, 5d, & 7d).
Live specimens of Echinaster species. Abactinal and actinal views. (a) Echinaster crystallus n. sp., holotype, NSMT E-12494; (b) Echinaster stereosomus, NSMT E-9360; (c) Echinaster subulatus n. comb., NSMT E-14937; (d) Echinaster toyoshiomaruae n. sp., holotype, NSMT E-14935. Scale bars indicate 10 mm.
Volume rendering images by micro-CT scanner of lateral to actinal surface of arms. (a) Echinaster crystallus n. sp., holotype, NSMT E-12494; (b) Echinaster sepositus, NSMT E-14942; (c) Echinaster luzonicus, NSMT E-14940; (d) Echinaster stereosomus, NSMT E-14936; (e) Echinaster toyoshiomaruae n. sp., holotype, NSMT E-14935. Abbreviations: ac, actinal series; ad, adambulacral series; i, inferomarginal series; s, superomarginal series.
Echinaster crystallus n. sp.
(New Japanese name: Shiratsuyu-ruson-hitode)
Figures 3b, 4b, 5b, 6a, 7a, 8a
SEM images of crystal bodies (arrowheads) on the superomarginal plates. (a) Echinaster crystallus n. sp., holotype, NSMT E-12494; (b) Echinaster subulatus n. comb., NSMT E-14937; (c) Echinaster toyoshiomaruae n. sp., holotype, NSMT E-14935. Scale bars indicate 0.1 mm.
https://zoobank.org/48277DBC-CCB2-43DE-906E-534C28DD15ED
Echinaster sepositus: Wakita et al. (Reference Wakita, Fujita and Kajihara2019: 120).
Material examined
Holotype. NSMT E-12494, 24 May 2007, west of Iheyajima Island, Okinawa, Japan, 27°01.440′ N, 127°51.540′ E, 81–82 m in depth, collected with dredge by the training vessel Toyoshio-maru, fixed in 99.5% ethanol: R = 33 mm, r = 6 mm.
Etymology
The specific epithet, crystallus, derives from the Latin word crystallus meaning crystal. The species has well-marked crystal bodies on abactinal, superomarginal, and inferomarginal plates. The former Japanese name ‘shiratsuyu’ means water droplets on grass alluding to the nature of the beautiful crystal bodies, and the latter name ‘ruson-hitode’ is commonly used for the species name of the genus.
Diagnosis
Arms five with single madreporite; abactinal skeleton irregular open meshwork; each papular area contains less than 10 papulae; abactinal plates bear one or rarely two spaced spines less than 1 mm long; supero- and inferomarginal plates arranged in longitudinal series, bearing spines on half of them; intermarginal plates lack spines; papulae absent between supero-, inter-, and inferomarginal plates; actinal plates arranged in longitudinal series without spines; adambulacral plates bear one (sometimes two) furrow spines deep in furrow and one or two subambulacral spines; crystal bodies protuberated from plate surface, present on primary abactinal and supero- and inferomarginal plates.
Description
Arms are five, subcylindrical in shape, slender and gradually tapering to the arm tip (Figure 6a). R/r ratio is 5.8r. The abactinal skeleton is an irregular open meshwork, constituted by ovate to rod-shaped, trilobate, quadrilobate, or stellate abactinal plates (Figure 3b). Abactinal plates are partially imbricated with each other, and zero or one ovate to rod-shaped abactinal plates linked two trilobate, quadrilobate, or stellate abactinal plates. Each abactinal papular area is larger than the surrounding plates, containing one to four papulae. Each trilobate, quadrilobate, or stellate abactinal plate bears one or rarely two spaced abactinal spines, but the ovate to rod-shaped abactinal plates lack spines. A madreporite is located at the margin of the disc with two spines on its periphery.
Abactinal spines are 0.7–0.8 mm in length. The lateral surface of the spines is uniformly smooth and is gradually tapered to the bluntly pointed tip, providing bullet-shaped appearance (Figure 3b).
Supero- and inferomarginal plates are round, ovate, trilobate, quadrilobate, or stellate, proximally imbricated and arranged in regular longitudinal series at the actinal to the lateral side of the arms (Figures 4b, 5b, & 7a). First and second supero- and inferomarginal plates consistently lack spines in all series, and the remaining supero- and inferomarginal plates alternately bear one (sometimes two on inferomarginal plates) spine. The proportions of supero- and inferomarginal plates with spines in each series are 45–53% and 45–58%, respectively. Supero- and inferomarginal series are separated by each other from proximal-most to one-third of the arm’s length. The superomarginal series gradually come closer to the inferomarginal series towards the arm tips. Supero- and inferomarginal series meet at one-third of the arm’s length and adjoin completely to the terminal plate. The interspace formed between these two series is filled with round intermarginal plates, which irregularly arranged and lack spines. Papulae are absent between the supero-, inter-, and inferomarginal plates.
Actinal plates are arranged in longitudinal series, forming one series on both sides of the arms, respectively (Figures 4b, 5b, & 7a). The series are confined within one-third of the arm’s length. All actinal plates lack spines. Papulae are absent between the inferomarginal, actinal, and adambulacral plates.
Adambulacral plates are arranged in longitudinal series along ambulacral furrows (Figures 4b, 5b, & 7a). All adambulacral plates bear one (sometimes two) furrow spines deep in the furrow and one subambulacral spine on the adradial corner, and 47–53% adambulacral plates bear one more subambulacral spine on the abradial surface. First to fourth adambulacral plates consistently lack abradial subambulacral spines in all adambulacral series, and the remaining plates alternately bear them.
Crystal bodies are present only on the trilobate, quadrilobate, or stellate abactinal and supero- and inferomarginal plates (Figures 3b, 4b, 5b, & 8a). They are well-marked and protuberated from the plate surface (Figure 8a).
The colour in life is white to pale orange with red-brown bands and orange papular areas on the abactinal side (Figure 6a). The actinal side also shows white to pale orange with oval red-brown areas on each interradius and many orange transverse stripes.
Distribution
Echinaster crystallus n. sp. is only known from the type locality, west of Iheyajima Island, Japan, at a depth of 81–82 m.
Remarks
Echinaster crystallus n. sp. has an open-meshed abactinal skeleton constituted by the irregularly arranged abactinal plates (Figure 3b). This feature is different from the following seven congeners having a longitudinally arranged or close-meshed abactinal skeleton: E. deplanatus, E. modestus, E. panamensis, E. parvispinus, E. rigidus, E. stereosomus and E. subulatus n. comb. (Grube Reference Grube1857; Leipoldt Reference Leipoldt1895; Koehler Reference Koehler1910a, Reference Koehler1910b; Fisher Reference Fisher1913, Reference Fisher1919; AH HL Clark Reference Clark1916; Turner and Campbell Reference Turner and Campbell1981). Out of the remaining 14 known species, 6 species, E. glomeratus, E. luzonicus, E. purpureus, E. superbus, E. reticulatus, and E. varicolor have papulae between supero-, inter-, and inferomarginal plates (HL Clark Reference Clark1914, Reference Clark1923, Reference Clark1938; Fisher Reference Fisher1919; HL Clark Reference Clark1916; Rowe and Albertson Reference Rowe and Albertson1987), and 3 species, E. arcystatus, E. colemani, and E. cribella, have many abactinal spines on each abactinal plate (Lütken Reference Lütken1871; HL; Clark Reference Clark1914; Rowe and Albertson Reference Rowe and Albertson1987), while the new species has no papulae between supero-, inter-, and inferomarginal plates (Figures 4b & 5b) and a single abactinal spine on each abactinal plate (Figure 3b). Moreover, three species, E. callosus, E. cylindricus and E. lacunosus, have only one subambulacral spine on each adambulacral plate (Grube Reference Grube1857; Marenzeller Reference Marenzeller1895; Meissner Reference Meissner1892) unlike the new species have more (Figure 4b). The poorly described species, E. smithi, is readily distinguished from the new species in having no regular superomarginal series (Lüdwig Reference Lüdwig1903). Consequently, E. crystallus n. sp. most resembled the Mediterranean to Atlantic type species, E. sepositus, in agreement with the previous identification of the specimen examined (holotype, NSMT E-12494) by Wakita et al. (Reference Wakita, Fujita and Kajihara2019). However, the new species can be distinguished from E. sepositus by characteristics mentioned below.
Echinaster sepositus has two subspecies, E. s. sepositus and E. s. madseni. Of these two subspecies, only E. s. madseni shares the crystal bodies with E. crystallus n. sp. (Figures 4b & 8a); however, E. s. madseni has crystal bodies only on the proximal abactinal, marginal, and actinal plates (Clark and Downey Reference Clark and Downey1992; Clark and Tortonese Reference Clark and Tortonese1986), unlike E. crystallus n. sp. has crystal bodies on almost all abactinal and marginal plates. In addition, E. s. madseni has spines on most inferomarginal plates (Clark and Tortonese Reference Clark and Tortonese1986), whereas the new species has spines only on half (45–58%) of the inferomarginal plates (Figure 7a). From our observation of E. s. sepositus (NSMT E-14942), the specimen has no crystal bodies on any plates, supero- and inferomarginal plates mostly (90–100%) bear spines (Figure 7b), and some intermarginal and actinal plates have one spine. These features are distinct from those of the new species.
Another new species, E. toyoshiomaruae n. sp., is different from E. crystallus n. sp. in having flat crystal bodies only on the proximal superomarginal plates (Figures 4f & 8c), spines on most (71–89%) supero- and inferomarginal plates (Figure 7e) and one spine on each proximal-most intermarginal plate (Figure 5f). Table 2 summarizes some diagnostic characteristics to distinguish between the six Japanese Echinaster species.
Tabular key to the Japanese species of Echinaster examined in this study
Our molecular analysis (Figure 2) demonstrated that E. crystallus n. sp. was most closely related to a specimen of E. stereosomus from Western Australia (Ward et al. Reference Ward, Holmes and O’Hara2008) with a small K2P distance (0.3%). The Ward et al.’s (Reference Ward, Holmes and O’Hara2008) specimen may possibly belong to this new species (see also Remarks of Echinaster stereosomus).
Echinaster luzonicus (Gray Reference Gray1840)
(Japanese name: Ruson-hitode)
Figures 3c, 4c, 5c, 7c
Echinaster affinis Perrier, Reference Perrier1869: 58
Echinaster eridanella Müller & Troschel, Reference Müller and Troschel1842: 24.
Echinaster luzonicus: Clark, Reference Clark1921: 98–100; Domantay & Roxas, Reference Domantay and Roxas1938: 230; Hayashi, Reference Hayashi1940: 123–125; Clark & Rowe, Reference Clark and Rowe1971: 40, 42, 72, pl. 10 Figures 1, 2; Hayashi, Reference Hayashi1973: 75–76; Marsh, Reference Marsh1974: 93; Marsh Reference Marsh1977,: 278, 279; Soota & Sastry, Reference Soota and Sastry1979: 345–348, pl. 8 figs 4–9; Liao & Clark, Reference Liao and Clark1995: 136–137, fig. 56; Purwati & Lane, Reference Purwati and Lane2004: 100.
Echinaster multipapillatus Hoffman in Rowe, Reference Rowe1974: 189.
Echinaster purpureus Domantay & Roxas, Reference Domantay and Roxas1938: 230.
Othilia luzonica Gray, Reference Gray1840: 282; Clark, Reference Clark1952: 290.
Othilia purpurea: Fisher, Reference Fisher1919: 432–435, pl. 12 fig. 1, pl. 132 fig. 7; Djakonov, Reference Djakonov1930: 251.
Materials examined
NSMT E-14943, 25 July 2017, Maesato Beach, Ishigaki Island, Japan, 24°19.625′ N, 124°11.126′ E, collected by hands during snorkeling, fixed in 99.5% ethanol, 1 specimen: R = 34 mm, r = 5 mm. NSMT E-14941, 3 August 2015, Yadori Beach, Amami Islands, Japan, 3 m in depth, collected by hands during snorkeling, fixed in 99.5% ethanol, 1 specimen: R = 49 mm, r = 9 mm. NSMT E-14940, 24 January 2014, Sedake, Okinawa Islands, Japan, 26°32.651′ N, 128°02.880′ E, 7–13 m in depth, collected by hands during scuba diving, fixed in 99.5% ethanol, 1 specimen: R = 40 mm, r = 8 mm. NSMT E-14939, 21 August 2015, Nishidomari, Ootsuki, Kochi, Japan, collected by hands during scuba diving, fixed in 99.5% ethanol, 1 specimen: R = 27 mm, r = 7 mm. NSMT E-14938, 3 August 2015, Yadori Beach, Amami Islands, Japan, 1 m in depth, collected by hands during scuba diving, fixed in 99.5% ethanol, 1 specimen: R = 54 mm, r = 8 mm.
Diagnosis
Arms five or more with one or more madreporites; abactinal skeleton is irregular open meshwork; each papular area contains less than 10 papulae; most abactinal plates bear one or rarely two spaced spines less than 1 mm long; supero- and inferomarginal plates arranged in longitudinal series, bearing spines on most of them; intermarginal plates bear spines; papulae present between supero-, inter-, and inferomarginal plates; actinal plates absent; adambulacral plates bear one furrow spines deep in furrow and 1–3 subambulacral spines; crystal bodies absent.
Description
Arms are five to seven, subcylindrical in shape, slender and gradually tapering to the arm tip. R/r ratio is 3.9–6.8r. The abactinal skeleton is an irregular open meshwork, constituted by round to ovate, trilobate, or quadrilobate abactinal plates (Figure 3c). Abactinal plates are partially imbricated with each other, and zero or one smaller rod-shaped to ovate abactinal plates linked larger two ovate, trilobate, or quadrilobate abactinal plates. Each abactinal papular area is larger than the surrounding plates, containing 1–6 papulae. Most abactinal plates bear one or rarely two spaced abactinal spines, but some round to ovate abactinal plates bear no spines. Madreporites are one or two, located at the margin of the disc with one to six spines on its periphery.
Abactinal spines are 0.5 mm in length. The lateral surface of the spines is uniformly smooth and is gradually tapered to the bluntly pointed tip, providing bullet-shaped appearance (Figure 3c).
Supero- and inferomarginal plates are round, trilobate, quadrilobate, or stellate, bearing zero to two spines (rarely three on inferomarginal plates), proximally imbricated and arranged in regular longitudinal series at the actinal to the lateral side of the arms (Figures 4c, 5c, & 7c). The proportions of supero- and inferomarginal plates with spines in each series are 77–100% and 82–100%, respectively. These marginal series are separated by each other from proximal-most to one-third of the arm’s length. The superomarginal series gradually come closer to the inferomarginal series towards the arm tips. Supero- and inferomarginal series meet at one-third of the arm’s length and adjoin completely to the terminal plate. The interspace formed between these two series is filled with round to ovate or rod-shaped intermarginal plates, which irregularly arranged and bear zero or one spine. Papulae are present between the supero-, inter-, and inferomarginal plates, but absent between inferomarginal and adambulacral plates.
Actinal plates are absent (Figures 4c, 5c, & 7c).
Adambulacral plates are rectangular, arranged in longitudinal series along ambulacral furrows (Figures 4c, 5c, & 7c). All adambulacral plates bear one furrow spines deep in the furrow and one subambulacral spine on the adradial corner, and 75–94% adambulacral plates bear one and 0–12% adambulacral plates bear two subambulacral spines on the abradial surface in each series.
Crystal bodies are absent on all plates (Figures 3c, 4c, & 5c).
The colour in life is unknown in all observed specimens.
Distribution
East Indian Ocean and West Pacific Ocean: north of Australia, Caroline Islands, East Indies, Bay of Bengal, Andaman Sea, China Sea, and Japan, bathymetrical range is 0–729 m (Clark Reference Clark1996; Soota and Sastry Reference Soota and Sastry1979).
Remarks
The specimens we observed are referable to E. luzonicus in possessing unique morphological characteristics: six or seven arms (in NSMT E-14943, and 14939–14941) and/or two madreporites (in NSMT E-14943, 14938, and 14941) (Liao and Clark Reference Liao and Clark1995). According to Soota and Sastry (Reference Soota and Sastry1979) and Liao and Clark (Reference Liao and Clark1995), E. luzonicus is a morphologically variable species that is possibly conspecific with E. varicolor and E. purpureus; however, our specimens more resemble E. luzonicus in having most adambulacral plates with subambulacral spines on their abradial surface and in lacking the series of actinal plates (Figures 4c, 5c, & 7c).
Our phylogenetic analysis (Figure 2) showed the close relationship of E. luzonicus and E. purpureus with K2P distance of 6.3%. Further molecular and morphological approaches are required to investigate the conspecificity of both species.
Echinaster stereosomus Fisher, Reference Fisher1913
(New Japanese name: Utakata-ruson-hitode)
Figures 3d, 4d, 5d, 6b, 7d
Echinaster stereosomus Fisher, Reference Fisher1913: 195, 196; Fisher, Reference Fisher1919: 430–432, pl. 122 figs 2, 3, pl. 132 fig 6; Marsh, Reference Marsh1976: 220; Jangoux, Reference Jangoux1978: 298, 299, pl. 3; Jangoux, Reference Jangoux1981: 475; Liao & Clark, Reference Liao and Clark1995: 137, pl. 18 figs 1, 2; VandenSpiegel et al., Reference VandenSpiegel, Lane, Stampanato and Jangoux1998: 448, 462, pl. 31, pl. 2 fig. 5.
Echinaster acanthodes Clark, Reference Clark1916: 61, 62, fig. 7, pl. 19 figs 1, 2; Clark, Reference Clark1946: 146.
Material examined
NSMT E-9360, 25 June 2014, east of Ototojima Island, Ogasawara Islands, Japan, 27°9.247′ N, 142°11.980′ E, 59 m in depth, collected with dredge by the R/V Koyo-maru, fixed in 99.5% ethanol, 1 specimen: R = 60 mm, r = 13 mm. NSMT E-14936, 12 July 2016, East of Ototojima Island, Ogasawara Islands, Japan, 27°9.407′ N, 142°12.163′ E, 57–63 m depth, collected with dredge by the R/V Koyo-maru, fixed in 99.5% ethanol, 1 specimen: R = 37 mm, r = 8 mm.
Etymology for Japanese name
The Japanese name ‘utakata’ means bubbles in Japanese, implying the dermal callosity investing the abactinal spines.
Diagnosis
Arms five with single madreporite; abactinal skeleton open meshwork with two straight and three zigzag longitudinal series; each papular area contains less than 10 papulae; abactinal plates bear one spaced spines; supero- and inferomarginal plates arranged in longitudinal series, bearing spines on half of them; half of supero- and inferomarginal plates bear spines; most intermarginal plates lack spines, but proximal-most plate bears single spine; papulae absent between supero-, inter-, and inferomarginal plates; actinal plates arranged in longitudinal series with single spine only on proximal-most unpaired plate; adambulacral plates bear one furrow spines deep in furrow and one or two subambulacral spines; crystal bodies absent.
Description
Arms are five, subcylindrical in shape, slender and gradually tapering to the arm tip (Figure 6b). R/r ratio is 4.6r. The abactinal skeleton is a regular open meshwork, constituted by rod-shaped to oval, trilobate, or quadrilobate abactinal plates (Figure 3d). Abactinal plates are partially imbricated with each other and arranged in two straight and three zigzag longitudinal series. Zigzag abactinal series intervened between two straight abactinal series and straight abactinal and superomarginal series. Each abactinal papular area is larger than the surrounding plates, containing 1–5 papulae. Each trilobate or quadrilobate abactinal plate bears one spaced abactinal spine, but the other abactinal plates lack spines. A madreporite is located at the margin of the disc without spines.
Abactinal spines are 1.4–1.6 mm in length, invested within the translucent dermal callosity in the living specimen (Figure 6b). The lateral surface of the spines is uniformly smooth and is gradually tapered to the bluntly pointed tip, providing bullet-shaped appearance (Figure 3d).
Supero- and inferomarginal plates are trilobate or quadrilobate, proximally imbricated and arranged in regular longitudinal series at the lateral side of the arms (Figures 4d, 5d, & 7d). Every second or third marginal plates bear one (sometimes two on inferomarginal plates) spines. The proportions of supero- and inferomarginal plates with spines in each series are 31–51% and 41–59%, respectively. These marginal series are separated by each other from proximal-most to one-third of the arm’s length. The superomarginal series gradually come closer to the inferomarginal series towards the arm tips. Supero- and inferomarginal series meet within one-third of the arm’s length and adjoin completely to the terminal plate. The interspace formed between these two series is filled with oval and irregularly arranged intermarginal plates. Most of the intermarginal plates lack spines, however, one proximal-most intermarginal plates on each interbrachial area bears one spine. Papulae are absent between the supero-, inter-, and inferomarginal plates.
Actinal plates are arranged in longitudinal series, forming one series on both sides of the arms, respectively. The series are confined within one-third of the arm’s length (Figures 4d, 5d, & 7d). Actinal plates lack spines, but each first unpaired plate bears one spine. Papulae are absent between the inferomarginal, actinal, and adambulacral plates.
Adambulacral plates are rectangular and arranged in longitudinal series along ambulacral furrows (Figures 4d, 5d, & 7d). All adambulacral plates bear one furrow spines deep in the furrow and one subambulacral spine on the adradial corner, and 52–70% adambulacral plates bear one subambulacral spine on the abradial surface.
Crystal bodies are absent on all plates (Figures 3d, 4d, & 5d).
The colour in life is reddish brown (NSMT E-9360) or orange (NSMT E-14936) with darker and lighter bands on the abactinal side (Figure 6b). The actinal side shows uniform white to pale orange.
Distribution
West Pacific Ocean: Queensland, Australia; Surigao Strait, Philippines; Gulf of Thailand, Thailand; Hinan Island, China; Sultan Shoal, Singapore; Ogasawara Islands, Japan; bathymetrical range is 57–685 m (Liao and Clark Reference Liao and Clark1995; Clark Reference Clark1996; VandenSpiegel et al. Reference VandenSpiegel, Lane, Stampanato and Jangoux1998; present study).
Remarks
This is the first record of E. stereosomus from Japanese waters, extending the northern-most distributional range. The morphology of the specimens examined corresponds well to the descriptions of the holotype of E. stereosomus (Fisher Reference Fisher1913, Reference Fisher1919, plate 122 Figure 2) in having longitudinally arranged abactinal plates (Figure 3d), abactinal plates bearing single spine, the first unpaired actinal plates bearing spine (Figure 5d), and adambulacral plates bearing one or two subambulacral spines (Figures 4d, 5d, & 7d). Of 25 Echinaster species including the new species described in this paper, only six species, E. deplanatus, E. modestus, E. parvispinus, E. panamensis, E. rigidus and E. stereosomus, have longitudinally arranged abactinal plates (Grube Reference Grube1857; Leipoldt Reference Leipoldt1895; Fisher Reference Fisher1913; AH HL Clark Reference Clark1916; Turner and Campbell Reference Turner and Campbell1981). Of these species, E. modestus, E. parvispinus and E. rigidus differ from E. stereosomus in lacking the first unpaired actinal plates bearing spine (Grube Reference Grube1857; AH HL Clark Reference Clark1916; Turner and Campbell Reference Turner and Campbell1981). In addition, E. deplanatus, E. modestus, and E. panamensis have crystal bodies on abactinal plates (Grube Reference Grube1857; Leipoldt Reference Leipoldt1895; Turner and Campbell Reference Turner and Campbell1981), while our specimens lack crystal bodies on any plates (Figure 4d & 5d). Echinaster cribella, which is unknown in the arrangement of abactinal plates, differs from our specimen in having a linear group of abactinal spines on each abactinal plate (Lütken Reference Lütken1871).
The phylogenetic tree (Figure 2) showed that the specimen examined by Ward et al. (Reference Ward, Holmes and O’Hara2008) from Australia was separated from our specimen of E. stereosomus. It is very closely related to E. crystallus n. sp. (see Remarks of E. crystallus n. sp.). Morphological observation of Ward et al.’s specimen is required to reveal the specific status.
Echinaster subulatus (Gray Reference Gray1840) n. comb.
(New Japanese name: Yoroi-ruson-hitode)
Figures 3e, 4e, 5e, 6c
Metrodira subulata Gray, Reference Gray1840: 282; Gray, Reference Gray1866: 12; Perrier, Reference Perrier1875: 180–183; Koehler, Reference Koehler1910a: 172–174, pl. 4 figs 1, 2, pl. 18 fig. 9; Koehler, Reference Koehler1910b: 284–288, pl. 15 fig. 3, pl. 17 fig 3–5; Fisher, Reference Fisher1919: 406; Clark, Reference Clark1921: 100–101; Clark, Reference Clark1938: 187, pl. 11 fig. 3; Clark & Rowe, Reference Clark and Rowe1971: 39, 57; Blake, Reference Blake1980: 181; Liao & Clark, Reference Liao and Clark1995: 134, 135, pl. 17 figs 5, 6; VandenSpiegel et al., Reference VandenSpiegel, Lane, Stampanato and Jangoux1998: 443, pl. 2, fig. 17; Purwati & Lane, Reference Purwati and Lane2004: 100; Sastry, Reference Sastry2005: 46; Nguyen & Dao, Reference Nguyen and Dao2013: 178, fig. 4.
Fromia (Metrodira) subulata: Perrier, Reference Perrier1878: 78.
Linckia (Metrodira) subulata: Martens, Reference Martens1867: 109.
Scaphaster humberti Loriol, Reference Loriol1899: 28, pl. 3 fig. 1.
Scytaster subulatus: Müller & Troschel, Reference Müller and Troschel1842: 36–37; Dujardin & Hupe, Reference Dujardin and Hupe1862: 368; Perrier, Reference Perrier1869: 62.
Material examined
NSMT E-14937, 29 May 2019, Ikomo Bay, Kakeromajima Island, Amami Islands, Japan, 28°05.435′ N, 129°14.940′ E, 74–75 m in depth, collected with dredge by the diving boat Dainihayabusa-maru, fixed in 99.5% ethanol, 2 specimens: R = 11 mm, r = 3 mm; R = 7 mm, r = 2 mm.
Etymology
The Japanese name ‘yoroi’ means the armour derived from the tessellated skeleton of this species.
Diagnosis
Arms five with single madreporite; abactinal skeleton irregular close meshwork almost tessellated; each papular area contains single papulae; abactinal plates bear one to six spaced spines; supero- and inferomarginal plates arranged in longitudinal series, bearing spines on most of them; intermarginal plates lack spines; papulae absent between supero-, inter-, and inferomarginal plates; actinal plate single in each interradius without spines; adambulacral plates bear two furrow spines directed proximally/distally on adradial apophysis and three to five subambulacral spines; crystal bodies protuberated from plate surface, present on abactinal, supero-, and inferomarginal and actinal plates.
Description
Arms are five, elliptical cylindrical in shape, slender and gradually tapering to the arm tip (Figure 6c). R/r ratio is 3.5–3.7r. The abactinal skeleton is a close meshwork almost tessellated, constituted by oval, rectangular, or rhomboid abactinal plates (Figure 3e). Abactinal plates are partially imbricated with each other. Each abactinal papular area is smaller than the surrounding plates, containing one papula. Each abactinal plate bears one or sometimes two to six abactinal spines. A madreporite is located at the centre of one of the interradial rhomboid plates without spines.
Abactinal spines are 0.1 mm in length. The lateral surface of the spines is uniformly smooth and is gradually tapered to the bluntly pointed tip, providing bullet-shaped appearance (Figure 3e).
Supero- and inferomarginal plates are oval to rectangular, proximally imbricated and arranged in regular longitudinal series at the lateral side of the arms (Figures 4e & 5e). First inferomarginal plates consistently lack spines in all series, and the remaining supero- and inferomarginal plates bear zero to four spines. The proportions of supero- and inferomarginal plates with spines in each series are 100% and 86–90%, respectively. Supero- and inferomarginal series are separated by each other from proximal-most to one-third of the arm’s length. The superomarginal series gradually come closer to the inferomarginal series towards the arm tips. Supero- and inferomarginal series meet within one-third of the arm’s length and adjoin completely to the terminal plate. The interspace formed between these two series is filled with oval intermarginal plates arranged in a longitudinal series. Intermarginal plates lack spines. Papulae are absent between the supero-, inter-, and inferomarginal plates.
Actinal plates are single in each interradius, lacking spines and located between oral, adambulacral and first inferomarginal plates. Papulae are absent between the inferomarginal, actinal, and adambulacral plates (Figures 4e & 5e).
Adambulacral plates are oval to rectangular, possessing adradial apophyses and arranged in longitudinal series along ambulacral furrows (Figures 4e & 5e). Each adambulacral plate bears two furrow spines on the apex of the apophysis and three to five subambulacral spines arranged in an oblique transverse series (Figure 5e). Two furrow spines on each adambulacral plate are directed distally and proximally, respectively, to provide the C-shaped frames for the tube feet together with the stem of adambulacral apophyses.
Crystal bodies are present on abactinal, marginal, and actinal plates (Figures 3e, 4e, 5e, & 8b). They are well-marked and protuberated from the plate surface (Figure 8b).
The colour in life is light brown with darker spots on the abactinal side (Figure 6c). The actinal side shows uniform white.
Distribution
East Indian and West Pacific Oceans: north of Australia, Sri Lanka and Bay of Bengal to East Indies, South China Sea, Taiwan Strait, and Japan, bathymetrical range is 0–100 m (Liao and Clark Reference Liao and Clark1995; Clark Reference Clark1996; VandenSpiegel et al. Reference VandenSpiegel, Lane, Stampanato and Jangoux1998; Purwati and Lane Reference Purwati and Lane2004; present study).
Remarks
This is the first record of E. subulatus n. comb. from Japanese waters, extending its northernmost distributional range. Although the two observed specimens were small (R = 7, 11), their morphological characteristics corresponded well to the descriptions of Metrodira subulata (Perrier Reference Perrier1875; Koehler, Reference Koehler1910a; Blake Reference Blake1980; VandenSpiegel et al. Reference VandenSpiegel, Lane, Stampanato and Jangoux1998) in possessing the following characteristics: the crystal bodies on abactinal plates (Figure 3e), single actinal plate in each interradius (Figure 5e), almost tessellated abactinal skeleton (Figure 3e), and the adambulacral apophysis with distally/proximally directed furrow spines (Figure 5e), and the latter two morphological features are unique in this species.
Based on its phylogenetic position within the Echinaster clade (Figure 1) and sharing the diagnostic characteristics with the genus Echinaster (see Phylogenetic analysis and taxonomic remarks), we transferred this species to the genus Echinaster.
Echinaster toyoshiomaruae n. sp.
(New Japanese name: Toyoshio-ruson-hitode)
Material examined
Holotype. NSMT E-14935, 21 May 2017, Oshima-Shin-Sone Bank, north of Amami Islands, Japan, 28°52.978′ N, 129°32.833′ E, 159–182 m in depth, collected with dredge by the T/V Toyoshio-maru, fixed in 99.5% ethanol: R = 47 mm, r = 8 mm.
Etymology
The specific and Japanese name comes from the name of the T/V Toyoshio-maru which collected the specimen.
Diagnosis
Arms five with single madreporite; abactinal skeleton irregular open meshwork; each papular area contains less than 10 papulae; abactinal plates bear one or two spaced spines; supero- and inferomarginal plates arranged in longitudinal series, bearing spines on most of them; most intermarginal plates lack spines, but proximal-most plate bears single spine; papulae absent between supero-, inter-, and inferomarginal plates; actinal plates arranged in longitudinal series without spines; adambulacral plates bear one furrow spines deep in furrow and one to three subambulacral spines; crystal bodies not protuberated from plate surface, present on the proximal superomarginal plates.
Description
Arms are five, subcylindrical in shape, slender and gradually tapering to the arm tip (Figure 6d). R/r ratio is 5.8r. The abactinal skeleton is an irregular open meshwork, constituted by round to ovate or triangular abactinal plates (Figure 3f). Abactinal plates are partially imbricated with each other, and zero to three smaller round to ovate abactinal plates linked two larger ovate or triangular abactinal plates. Each abactinal papular area is larger than the surrounding plates, containing one to five papulae. Each large ovate or triangular abactinal plate bears one or rarely two or three abactinal spines, but the other plates lack spines. A madreporite, which is partially damaged, is located at the margin of the disc with one spine on its periphery.
Abactinal spines are 0.5–0.6 mm in length. The lateral surface of the spines is uniformly smooth and is gradually tapered to the bluntly pointed tip, providing bullet-shaped appearance (Figure 3f).
Supero- and inferomarginal plates are round to ovate, bearing one (sometimes two) spine, proximally imbricated and arranged in regular longitudinal series at the lateral side of the arms (Figures 4f, 5f, & 7e). The proportions of supero- and inferomarginal plates with spines in each series are 71–89% and 75–89%, respectively. Supero- and inferomarginal series are separated by each other from proximal-most to one-third of the arm’s length. The superomarginal series gradually come closer to the inferomarginal series towards the arm tips. Supero- and inferomarginal series meet within one-third of the arm’s length and adjoin completely to the terminal plate. The interspace formed between these two series is filled with round and irregularly arranged intermarginal plates. Most of the intermarginal plates lack spines, however, one proximal-most intermarginal plates in each interradius bears one spine. Papulae are absent between the supero-, inter-, and inferomarginal plates.
Actinal plates are arranged in longitudinal series, forming two series on both sides of the arms, respectively. The series are confined within one-third of the arm’s length (Figures 4f, 5f, & 7e). All actinal plates lack spines. Papulae are absent between the inferomarginal, actinal, and adambulacral plates.
Adambulacral plates are arranged in longitudinal series along ambulacral furrows (Figures 4f, 5f, & 7e). All adambulacral plates bear one furrow spine deep in the furrow and one subambulacral spine on the adradial corner, and 70–71% adambulacral plates bear one or two subambulacral spine on the abradial surface.
Crystal bodies are present only on the proximal superomarginal plates (Figures 3f, 4f, 5f, & 8c). They are inconspicuous and flat, not protuberated from the plate surface (Figure 8c).
The colour in life is uniform reddish yellow (Figure 6d).
Distribution
Echinaster toyoshiomaruae n. sp. is only known from the type locality. Oshima-Shin-Sone Bank, north of Amami Island, Japan, at a depth of 159–182 m.
Remarks
Echinaster toyoshiomaruae n. sp. has irregularly arranged abactinal plates (Figure 3f), one abactinal spine on most abactinal plates, no papulae between supero-, inter-, and inferomarginal plates (Figure 4f) and two or three subambulacral spines on most (70–71%) adambulacral plate. These features are only shared by E. crystallus n. sp. and E. sepositus (see Remarks of E. crystallus n. sp.). Echinaster toyoshiomaruae n. sp. is distinguished from them in having flat crystal bodies only on the proximal superomarginal plates (Figure 8c), one to two spines on most (71–89%) supero- and inferomarginal plates (Figure 7e) and one spine on each proximal-most intermarginal plate (Figure 5f).
Discussion
In our phylogenetic tree (Figure 2), five Echinaster and one Metrodira species found in Japanese waters formed a clade with six Echinaster species including its type species, E. sepositus. Given that seven Othilia species were isolated from this Echinaster clade, we supported the generic separation of Othilia from Echinaster suggested by Lopes et al. (Reference Lopes, Pérez-Portela, Paiva and Ventura2016). Clark and Downey (Reference Clark and Downey1992) indicated four distinct diagnostic characteristics that separate Othilia from Echinaster: the presence of a pentagon of linked primary radial and interradial plates on the disc, longitudinal series of abactinal plates, crystal bodies, and the absence of actinal plates. In addition, Othilia has distal imbrication pattern of the inferomarginal plates (Campbell and Turner Reference Campbell and Turner1984; Clark and Downey Reference Clark and Downey1992; Downey Reference Downey1973; Hopkins et al. Reference Hopkins, Fontanella, Ventura, Féral and David2003; Turner and Campbell Reference Turner and Campbell1981), while Echinaster sensu stricto is diagnosed by the inferomarginal plates showing proximal imbrication (Figure 4b–f). Echinaster farquhari forms a clade with Henricia species (Figure 2) and has actinal papulae like Henricia (McKnight Reference McKnight2006), so we transferred E. farquhari to the genus Henricia. The monotypic genus Metrodira is within the Echinaster clade, and we newly propose that Metrodira is a junior synonym of Echinaster. Metrodira subulata shares no actinal papulae, the absence of the pentagon of linked primary radial and interradial plates, and the inferomarginal plates showing proximal imbrication with Echinaster. Meanwhile, the tessellated abactinal skeleton of Metrodira contradicts the reticulated skeleton of Echinaster. However, Metrodira’s abactinal skeleton is composed of partially imbricated abactinal plates with close gaps (Figure 3e), rather should be considered a tight reticulation as Blake (Reference Blake1980) noted.
Through revisiting the classification of the genus Echinaster based on the molecular and morphological data, we finally recognized six Echinaster species in Japan. Besides two new species with scarce occurrence records, the remaining four species are known to be widespread from the north of Australia to the East Indies, which means that the Japanese Echinaster fauna is continuous with those of the Indian and West Pacific Oceans. Further faunal surveys may reveal the wide distribution of two new species described in this study among the Indian and West Pacific regions.
Acknowledgements
We would like to express our sincere gratitude to Taiju Kitayama (National Museum of Nature and Science), Hayate Tanaka (ex. University of Tokyo), Kenji Iwao (Akajima Marine Science Laboratory), Mikihito Arai (ex. University of Tokyo), Sadao Yokoyama (Setouchi Town, Amami-oshima Island), Susumu Ohtsuka (Hiroshima University), Takato Izumi (Fukuyama University), Kuroshio Biological Research Foundation, and the captains and the crew of R/V Koyo-maru and T/V Toyoshio-maru for their help in collecting specimens. We are grateful to Gento Shinohara, Shūhei Nomura, Takahiko Kutsuna, and Yasunari Shigeta (National Museum of Nature and Science) for their help in manipulating micro-CT. We also thank anonymous reviewers for their constructive comments. We thank anonymous reviewers for the constructive and valuable comments.
Author contributions
IK and TF performed morphological observations, molecular analyses, drafted this manuscript, and commented on and approved the final manuscript.
Funding
This study was partly supported by the National Museum of Nature and Science research projects ‘Geological, biological, and anthropological histories in relation to the Kuroshio Current,’ ‘Biological Properties of Biodiversity Hotspots in Japan,’ and ‘Collection of mitochondrial genome data of invertebrates and its application to taxonomy and phylogeny,’ and by JSPS KAKENHI (21K06327).
Competing interests
None.
