1. INTRODUCTION

The mesophotic zone is typically defined as being from 30 m to 150 m depth, intermediate in illumination between its shallower and deeper counterparts (Turak and DeVantier 2019). It is characterized by light-dependent communities surviving in light-limited environments and harbors unique biota that include both eurybathic organisms and those exclusively adapted to deep-water habitats (Schneider et al. 2019). The challenges and economic constraints associated with conducting in situ surveys and underwater sampling have historically hindered scientific research at these depths. In the previous centuries, marine biodiversity surveys predominantly focused on infralittoral and shallow depths. However, advancements in submersible technology during the 1970s enabled exploration of the deep sea (Grassle et al. 1975;Heirtzler and Grassle 1976;Barnes et al. 1977;Corliss et al. 1979;Tunnicliffe et al. 1986), resulting in remarkable discoveries of marine life and a significant re-evaluation of marine biodiversity patterns (e.g., Rex (1981) and Grassle (1985)). Nevertheless, the mesophotic zone has received limited attention due to its depth range, being too shallow for safe submersible operation yet too deep for conventional SCUBA diving (Pyle 1996;Hinderstein et al. 2010;Kahng et al. 2010).

Genus AmphiophiuraMatsumoto, 1915 comprises 57 extant species (Stöhr et al. 2023). Among them, only one species, A. sculpta (Duncan, 1879), had been recorded in the Korean marine fauna prior to this study (Yi and Irimura 1988;MABIK 2023). Amphiophiura exhibits distinct morphological characteristics compared to other genera within the same family, Ophiopyrgidae Perrier, 1893 (Matsumoto 1915, 1917;Fell 1960): 1) disk high, and arms not very high (higher than broad), 2) disk and ventral interbrachal covered with prominent scales, and 3) arm spines arranged in a single row. Numerous species of Amphiophiura inhabit the lower sublittoral and upper abyssal benthic zone (ranging from 100-3,000 m), and sometimes the lower abyssal benthic zone (below 4,000 m) (Lyman 1878;Matsumoto 1917;McKnight 2003;Liu 2008). In September 2022, a trimix diving team for scientific research conducted a survey in the upper mesophotic zone (below 70 m, upper sublittoral benthic zone) of the East Sea, Korea. During this survey, an Amphiophiura specimen was discovered, marking an extraordinary finding in the upper mesophotic zone.

In this study, a newly recorded brittle star found during a mesophotic zone survey is presented, accompanied by a comprehensive taxonomic description and high-resolution images. Furthermore, a taxonomic key to species within the genus Amphiophiura in Korea is provided.

2. MATERIALS AND METHODS

A sample was collected from the water around Namae- ri in Gangwon-do, in the East Sea, Korea (Fig. 1). The sample was collected by trimix SCUBA diving (Fig. 1), and preserved in ethyl alcohol solution (>95%) after sorting in land. The collected sample was deposited in the National Marine Biodiversity Institute of Korea (MABIK; Seocheon, Korea). The sample for observation was lightly bleached following the procedure by Stöhr et al. (2012) and bleached specimens were dried in an oven for six hours at 55°C. Specimen observation and photography for figures were performed using a Nikon SMZ1000 stereomicroscope (Nikon, Tokyo, Japan), a DP22 digital camera (Olympus, Tokyo, Japan), and a Helicon Focus 7.7.5 (Helicon Soft Ltd., Oakland, CA, USA) for combinations of images with varying focus. Observation of major morphological characteristics and measurement of body part for ophiuroids such as length of an arm and a disk were performed following the method of Shin (2012). The sample was identified using traditional taxonomic characters described by Clark (1911). All the abbreviations in this study are referred from Hendler (2018) and Goharimanesh et al. (2021).

3. SYSTEMATIC ACCOUNTS

Phylum Echinodermata Klein, 1778

Class Ophiuroidea Gray, 1840

Superorder Euryophiurida O’Hara, Hugall, Thuy, Stöhr and Martynov, 2017

Order Ophiuroida Müller and Troschel, 1840 sensuO’Hara et al., 2017

Suborder Ophiurina Müller and Troschel, 1840 sensuO’Hara et al., 2017

Family Ophiopyrgidae Perrier, 1893

Genus AmphiophiuraMatsumoto, 1915

Key to the species of genus Amphiophiura in Korea

1. Arm slightly long (R=over 2.9r), radial shield almost separated from each other·····························A. megapoma

- Arm short (R=below 2.0r), radial shield adjoined each other two-thirds of the length·······························A. sculpta

Amphiophiura megapoma (H.L. Clark, 1911)

큰뚜껑양편빗살거미불가사리 (신칭) (Fig. 2)

Ophiura megapoma H.L. Clark, 1911: 79, 80, fig. 23.

Amphiophiura megapoma: Matsumoto, 1917: 264; Fujita et al., 2014: 210; Stöhr et al., 2023: 242734.

Material examined. One specimen, off the water of Namae- ri, Hyeunnam-myeon, Yangyang-gun, Gangwondo, Korea (37°58ʹ42.7ʺN 128°48ʹ42.9ʺE) (Fig. 1), 1 September 2022, depth of 53.6 m, water temperature of 2.7°C, inhabits muddy to fine sand sediments, deposited in MABIK (MABIKIV00172890).

Description. Disk rather thick, flat, rounded pentagonal outline, and covered with numerous conspicuous scales (Fig. 2A). Arms five, rather high, and slightly long. Radial shield moderate sized, bared, rather longer than broad, separated each other with small scales, but slightly connected at middle part (Fig. 2B). Genital slits long and conspicuous (Fig. 2E). Genital scales short, wider than long, rounded square or oval shape, and gradually larger toward aboral side (Fig. 2B, C). Tentacle pore covered with four to six large scales at outer side of pore (Fig. 2D, E), and number of scales gradually decrease toward tip. Oral shields longer than broad, rounded distally but angular proximally (Fig. 2D). Adoral shields rather longer than broad (Fig. 2D). Oral plate smaller then adoral shield, longer than broad, much wider distal part than proximal part, and one to two rounded granules on inner side of plate or not (Fig. 2D). Dorsal arm plate hexagonal, wider than long at basal, and passing basal part rapidly longer than broad (Fig. 2B). Lateral arm plate moderate size with six arm spines at basal, and number of arm spine gradually decrease toward tip. Tooth papilla one, lateral oral papillae five to seven, and adoral shield spines one to two (Fig. 2D).

Size. Diameter of disk (r)=16.4 mm, length from the middle of disk to the tip of arm (R)=50.1 mm, width of arm at proximal=3.3 mm, and R=3.1r.

Color. The specimen color in life is dark olive on the aboral side and pale gray on the oral part and ventral side of arm.

Distribution. Korea (East Sea), Japan (Toyama Bay, Hokkaido), Kuril Islands, Sea of Okhotsk.

Remarks.Amphiophiura megapoma was initially reported as Ophiura megapoma by Hebert L. Clark (1911). Matsumoto (1915) subsequently established a new genus, Amphiophiura, which included O. megapoma and 29 other species. The specimen in this study exhibits the same morphological characteristics, although there are a few minor differences from the original description by Clark (1911): 1) the number of proximal arm spines (holotype: four or five; this study: five), and 2) the number of proximal tentacle scales (holotype: four to seven; this study: five to seven). Around the waters of Korea, five Amphiophiura species were recorded (Table 1): A. megapoma, A. oediplax (Clark, 1911), A. penichra (Clark, 1911), A. pomphophora (Clark, 1911), and A. sculpta (Duncan, 1879). Among them, A. sculpta has been previously recorded from a depth of 50 m in the Yellow Sea, Korea (Yi and Irimura 1988). However, this species distinctly differs from A. megapoma in two major morphological characteristics: 1) the ratio of R/r (A. megapoma=2.9-3.1; A. sculpta=1.5), and 2) the radial shields are joined or separated (A. megapoma=separated; A. sculpta=joined) (Table 1). Furthermore, A. megapoma has more arm spines compared to A. oediplax, A. penichra, and A. pomphophora, and the radial shields of these three species are joined (Table 1). Consequently, the collected Amphiura specimen was identified as A. megapoma, representing the first record in the Korean marine fauna, and the first collection rec-ord at depths below 100 m.