INTRODUCTION

The genus Admirandus was created by Belogurov and Belogurova (1979) based on the type species, A. multicavus. So far, four species within the genus Admirandus have been described previously from various localities and habitats of the estuary of the river, intertidal and shallow subtidal sediments in oceans as follows: A. beloguroviTchesunov, Mokievsky and Thanh, 2010 from the estuary of Be River, intertidal flat with Avicennia mangrove trees of the Nha Trang City, Central Vietnam; A. multicavusBelogurov and Belogurova, 1979 from the intertidal muddy-sand sediment of Huangdao, Qingdao and Rizhao of Shandong province near the Yellow Sea, China, and from the muddy sediment of Vladivostok, Peter the Great Bay, the Sea of Japan, Russia (Russkiy Island); A. papillatus (Kreis, 1932) from the intertidal muddy sediments of Sundays River Estuary, South Africa; and A. sanjuliensis Lo Russo, Villares and Pastor de Ward, 2016 from the littoral coasts of the San Antonio Bay, Río Negro Province and Santa Cruz Province, Argentina (Kreis 1932;Furstenberg and Vincx 1989;Huang and Zhang 2009;Tchesunov et al. 2010;Mordukhovich et al. 2015;Lo Russo et al. 2016) (Table 2). However, no biodiversity studies around the Korean seawaters with respect to the genus Admirandus have been carried out so far.

During a continuous investigation on the species composition and the distribution of the free-living marine nematofauna around the Korea, A. multicavus was discovered from the intertidal sediments habitat. This report provides illustrations and differential interference contrast (DIC) photomicrographs of A. multicavus with a comparative table on the biogeographical and ecological characteristics of the members of the genus Admirandus. This is the first taxonomic report of the genus Admirandus from Korea.

MATERIALS AND METHODS

Field collection. We gathered the meiobenthos from the sieving of the bulk intertidal sediments, which was collected around the western and southern coasts of Korea (Fig. 1).

Sample processing. Initially, meiobenthos were roughly separated from the sediment by decantation methods using a 67 μm mesh sieve in the survey field after freshwater rinsing for less than a few minutes to reduce the attachment from the sediment (Kristensen 1989), and then fixed in 4% neutralized formalin in sea water. After the first step procedure, meiobenthos were separated once again from the rough samples still including a little detritus using the Ludox flotation method (Burgess 2001) in the laboratory, and the concentrated samples were subsequently fixed again in 4% neutralized formalin solution.

Permanent slide preparation. The marine nematode specimens were sorted out under a high magnification of dissecting microscope (LEICA M205 C; Wetzlar, Germany). Specimens for making permanent slides were dehydrated through a graded series of glycerin according to Seinhorst (1959) and mounted on a HS slide with two coverslips (Shirayama et al. 1993).

Microscopic observation. The permanently mounted nematodes specimens were observed and photographed using an Olympus BX53 microscope equipped with an Olympus DP26 digital camera (Olympus, Tokyo, Japan). All drawings and measurements were made with the aid of a camera lucida.

Terminology and abbreviations. The terminology used for the description and measurements was followed by Mordukhovich et al. (2015). Abbreviations used in the text are as follows: a=body length divided by maximum body diameter; a.b.d.=anal body diameter; b=body length divided by pharynx length; c=body length divided by tail length; cʹ=tail length divided by anal body diameter; c.b.d.=corresponding body diameter; L=total body length; Spic=spicule or length of spicule measured along the arc; T=tail length; V=distance of vulva from anterior end as percentage of body length, in %.

SYSTEMATIC ACCOUNTS

Phylum Nematoda Potts, 1932

Class Enoplea Inglis, 1983

Order Enoplida Filipjev, 1929

Family Oncholaimidae Filipjev, 1916

Subfamily Adoncholaiminae Gerlach and Riemann, 1974

Genus AdmirandusBelogurov and Belogurova, 1979

Type species of the genus Admirandus

Admirandus multicavusBelogurov and Belogurova, 1979

Other species of the genus Admirandus

Admirandus beloguroviTchesunov, Mokievsky and Thanh, 2010

Admirandus papillatus (Kreis, 1932) Shimada and Kijihara, 2014

Admirandus sanjuliensis Lo Russo, Villares and Pastor de Ward, 2016

Admirandus multicavusBelogurov and Belogurova, 1979 (Figs. 2-4; Table 1)

Admirandus multicavus: Tchesunov et al., 2010, p. 157, Fig. 1;Mordukhovich et al., 2015, p. 1231, Figs. 1, 2.

Adoncholaimus chinensisHuang and Zhang, 2009, p. 1084, Figs. 1, 2.

Material. Two males (MABIK NA00156664, MABIK NA00156665), in glycerin on HS slide, from Jimak-ri, Gogun- myeon, Jindo-gun, Jeollanam-do, 20 June 2018 (leg. H. G. Kim and H. J. Lee) was deposited in the nematode collection at the specimen conservation room of the Marine Biodiversity Institute of Korea (MABIK), Seochun, Korea. One male (KIOST NEM-1-2549) and one female (KIOST NEM-1-2609), in glycerin on HS slide, from Jangseon Beach, Daeseo-myeon, Goheung-gun, Jeollanam-do, 28 May 2020 (leg. H. S. Rho. and H. Lee) was deposited in the nematode collection at the specimen conservation room of the Bio-Resources Bank of Marine Nematodes (BRBNM), East Sea Research Institute, Korea Institute of Ocean Science & Technology (KIOST), Korea.

Locality and Habitat. Intertidal mud flat habitat of Jangseon Beach (34°44ʹ59.00ʺN, 127°15ʹ02.00ʺE), Daeseo-myeon, Goheung-gun, Jeollanam-do, Korea. Intertidal muddy sand habitat of Jimak-ri (34°28ʹ17.02ʺN, 126°22ʹ05.64ʺE), Gogun-myeon, Jindo-gun, Jeollanam-do, Korea.

Measurements. See Table 1 for measurements and morphometrics.

Description. Males. Body 3,380-3,976 μm long, elongate, nearly cylindrical except head and tail region, tapering very slightly towards both end; 64-89 μm wide at level of pharynx. Head 26-28 μm wide at level of cephalic setae. Maximum body width 70-99 μm wide. Cuticle transparent, smooth. Cephalic end not off-set from remaining body (Figs. 2A, 3A). Six lips each with a minute inner conical labial papilla at their base; six outer labial papillae arranged in one circle. Four conical cephalic setae 2 μm long. In labial region, marginal lamellae horseshoe shaped features around inner labial papilla (Fig. 2A). Very short (1-2 μm) conical somatic setae extended posteriorly from anterior sensilla; posterior to nerve ring somatic setae becoming scanty and decrease in size.

Amphidial fovea transparent, semicircular, pocket-like with transverse slit-like aperture, 25-33% of c.b.d.; distance from anterior body end to aperture 15-18 μm long (Figs. 2A, 3C). Buccal cavity cylindroid, very large with thick wall, length to width ratio 2.5-2.9 times as long as wide. Buccal cavity with the largest right ventrosublateral onchium (34-38 μm long); left ventrosublateral onchium and dorsal onchium smaller and equal in size (27-29 μm long) (Figs. 2A, 3B). Excretory pore opened at 74-86 μm from anterior body end. Pharynx 504-583 μm long, evenly muscular, gradually widening to posterior end. Cardia large, triangular shaped. Nerve ring situated at 241-266 μm long from anterior body end. Males diorchic with opposed outstretched testes, located right side of intestine. Spicules paired, 103-121 μm long, 2.0-2.3 a.b.d. long or 1.1-1.3 times tail length. Gubernaculum slender, curved parallel to spicules, 27-28 μm long (Figs. 2C, 3E). Two rows of 7-8 circumcloacal setae (4-6 μm long) on each subventral side of body (Figs. 2C, 3D). Several short setae on tail. Tail composed of distinct proximal conical portion and distal slender cylindrical one (Fig. 3E).

Female. Similar to male in most respects (Figs. 2B, D, 4AF). Body 3,443 μm long, elongate, tapering very slightly towards both end; 68 μm wide at level of pharynx. Head 25 μm wide at level of cephalic setae. Maximum body width 75 μm wide. Four conical cephalic setae 2 μm long. Amphidial fovea 30% of c.b.d.; distance from anterior body end to aperture 16 μm long (Figs. 2B, 4C). Buccal cavity cylindroid, length to width ratio 3.2 times as long as wide. Buccal cavity with the largest right ventrosublateral onchium (40 μm long); left ventrosublateral onchium and dorsal onchium smaller and equal in size (31 μm long) (Figs. 2B, 4B). Excretory pore opened at 74 μm from anterior body end. Pharynx 510 μm long. Nerve ring situated at 245 μm long from anterior body end. Ovaries paired, antidromously reflexed, both situated right side of intestine. Three fertilized eggs with coarse granulated cytoplasm in uteri (Fig. 4D). Demanian system well developed, tubular, situated dorsally; duct extends to anal region terminating with slitlike external pores on tail (Fig. 2D).

Remarks. The genus Admirandus was established by Belogurov and Belogurova (1979) and classified within the subfamily Adoncholaiminae Gerlach and Riemann, 1974 of the family Oncholaimidae Filipjev, 1916 belongs to the order Enoplida Filipjev, 1929 based on the following combination of the generic characteristics: outer labial and cephalic sensilla papilliform; cephalic sensilla surrounded by marginal lamellae in form of hook-like plates; buccal capsule elongate, length to width ratio c. 1.5:1 to 3:1; three onchia in buccal cavity, the largest being located right ventrosublaterally and other two lower ones equal in size; ovaries paired; demanian tube system generally of the Adoncholaimus type, but with precloacal or postanal terminal canals and pores situated at one preclocal and two on the tail; presence of terminal canals as long ducts; the spicules curved, relatively long, slender, proximally cephalated, distally pointed, slightly broadening only in front of the distal end; gubernaculum present; midventral preanal supplementary organ present or not; tail anterior conical and posterior cylindrical portions; and precloacal and postcloacal sensilla long, setiform (Mordukhovich et al. 2015;Lo Russo et al. 2016).

Furthermore, Admirandus multicavus Belogurov and Bel- ogurova, 1979, the type species of the genus, is characterized by the following combination of species identification characteristics, which are summarized on the basis of the original description of Belogurov and Belogurova (1979), the redescription of Mordukhovich et al. (2015), and a key for the species of genus Admirandus of Lo Russo et al. (2016): (1) the relative length of cephalic setae, spicules and gubernaculum; (2) the presence/absence of midven- tral preanal supplementary organ; (3) the number of gland opening of the demanian system; and (4) the relative ratio of the stoma length to width.

Recently, A. multicavus was redescribed from the intertidal muddy sediment samples of the Russkiy Island, Peter the Great Bay, Russia (Mordukhovich et al. 2015). Mordukhovich et al. (2015) demonstrated the individuals of A. multicavus of the Russkiy Island are similar to the measurements of the original description in general, but there are some discrepancies only in the length of gubernaculum, in the absence of midventral preanal supplementary organ and in the length of cephalic setae.

The Korean A. multicavus specimens discovered from the intertidal mud flat habitat around the western and southern coasts of Korea are also shares several similar features in general body shape with the original description and the Russkiy Island specimens of A. multicavus, however, we found some differences among them such as the combination of the following characteristics in the male specimens: the body length (3,380-3,976 μm vs. 2,060- 2,280 μm and 2,451-2,781 μm, respectively); the length of cephalic setae (2 μm vs. 1 μm and 1.5 μm, respectively); the spicules length (103-121 μm vs. 71-110 μm and 105-126 μm, respectively); and the gubernaculum length (27-28 μm vs. 15 μm and 19-31 μm, respectively). This is the first taxonomic report of the genus Admirandus from the Korean seawaters.