1. INTRODUCTION

Inglis (1969) established two genera, Austranema and Parapinnanema, within the subfamily Euchromadorinae. Warwick and Coles (1975) concluded that AustranemaInglis, 1969 should be considered synonymous with ParapinnanemaInglis, 1969, recognizing them as equivalent. This decision was based on the sole distinction between them being the arrangement of the cephalic setae. Austranema was identified by having these cephalic setae forming a single circle (10 setae), while Parapinnanema had them organized in two circles (6+4 setae) near the external labial setae. However, this characteristic proved inconsistent. For instance, both A. pectinatum (Wieser and Hopper 1967) and A. colesi (Inglis 1968) displayed two circles of cephalic setae, blurring the differentiation.

The genus Parapinnanema is identified by a thick cuticle in the pharyngeal region and amphids encircled by a fringed layer of cuticle. The buccal cavity takes on a conical form and is equipped with one dorsal plate and two subventral plates. The tail exhibits a conical- cylindrical shape. In males, there are elevated precloacal and caudal modifications to the cuticle. In females, the vulva appears elongated, and the vagina opens widely, featuring an internal double sphincter around the uterine chamber.

At present, the genus Parapinnanema encompasses twelve species, distributed as follows: P. alii (Murphy 1965) was identified in the Maldives, P. babletiGourbault and Vincx, 1994 in the Tuamotu Archipelago, P. colesi (Inglis 1968) in New Caledonia, P. harveyiWarwick and Coles, 1975 in the Isles of Scilly, P. hawaiiensisSemprucci and Sorensen, 2014 in Hawaii, P. imbricatum Belogurov, Belogurova, and Smolyanko, 1985 in the Sea of Japan, P. mexicanum (Jensen 1985) in the Gulf of Mexico, P. pectinatum (Wieser and Hopper 1967) in Florida, and both P. rhipsoidesGourbault and Vincx, 1994 and P. ritaeGourbault and Vincx, 1994 in Guadeloupe. Parapinnanema shirleyae (Coles 1965) was discovered in South Africa, and P. wilsoniInglis, 1969 was recorded in Western Australia.

As part of an ongoing investigation into the biodiversity of free-living marine nematodes in Korea, P. imbricatum was identified in the subtidal sediment of the Ulleungdo Island in the East Sea, Korea. This paper presents a detailed redescription of the newly discovered specimens collected from the East Sea, Korea, accompanied by illustrations and differential interference contrast (DIC) photomicrographs.

2. MATERIALS AND METHODS

Samples of marine nematodes were acquired from the subtidal sediment of the Ulleungdo Island in the East Sea, Korea, using the Smith-McIntyre Grab. In the initial stage, meiobenthos were roughly separated from the sediment through decantation techniques, employing a 67 μm mesh sieve in the field. This process occurred after a brief freshwater rinse to minimize sediment attachment (Kristensen and Higgins 1989). The collected samples were subsequently preserved in a 5% neutralized formalin solution in seawater. In the laboratory, the meiobenthos were further isolated from these initial samples using the Ludox flotation method in Ludox^® (DuPont) HS 40 (Burgess 2001). The concentrated samples obtained through this procedure were once again preserved in a 5% neutralized formalin solution.

Marine nematode specimens were carefully chosen under high magnification using a dissecting microscope (LEICA M205 C). For morphological examination under a light microscope, the specimens underwent dehydration through a gradual series of glycerin (Seinhorst 1959). These specimens were then mounted on an HS slide (Shirayama et al. 1993). The nematodes were observed and photographed using a LEICA DM2500 LED microscope equipped with a LEICA K5C color CMOS camera from LEICA, Wetzlar, Germany. The camera was employed for all sketches and measurements.

All measurements are recorded in micrometers along the arc. Ratios a, b, c are determined following de Man’s method (de Man 1880). The abbreviations utilized are as follows: cbd=corresponding body diameter; mbd= maximum body diameter; a=body length divided by maximum body diameter; b=body length divided by esophagus length; c=body length divided by tail length; c′=tail length divided by anal body diameter.

3. SYSTEMATIC ACCOUNTS

Phylum Nematoda Potts, 1932

Class Chromadorea Inglis, 1983

Order Chromadorida Chitwood, 1933

Superfamily Chromadoroidea Filipjev, 1917

Family Chromadoridae Filipjev, 1917

Subfamily Euchromadorinae Gerlach and Riemann, 1973

Genus ParapinnanemaInglis, 1969

Type species. Parapinnanema wilsoniInglis, 1969.

Catalog of the twelve valid species within the genus ParapinnanemaInglis, 1969

Parapinnanema alii (Murphy, 1965)

Nygmatonchus aliiMurphy, 1965, p. 204, figs. 1-3, three males and four females, Maldives, Ari Atoll, subtidal at 0.7 m depth.

Austranema alii: Inglis, 1969, p. 195, figs. 38-40, 100, 101, three males and two females, Western Australia, Shark Bay, weed and sand from trawl at 12 m depth.

Parapinnanema alii: Warwick and Coles, 1975, p. 405; Gourbault and Vincx, 1994, p. 153, figs. 8, 9, thirtyfive males, thirty-five females, and twenty-eight juveniles, New Caledonia, Îlot Goéland, Récif Mbere, Rocher à la Voile, Sèche Croissant, baie Maa, fine grey sand at 9-13 m depth, silt or muddy bottoms at 12 m depth.

Parapinnanema babletiGourbault and Vincx, 1994

Parapinnanema babletiGourbault and Vincx, 1994, p. 142, figs. 1-4, sixteen males and six females, Polynesia, Fangataufa Atoll, fine to medium sand in lagoonal samples at 6-38 m depth.

Parapinnanema colesi (Inglis, 1968)

Euchromadora colesiInglis, 1968, p. 36, figs. 6-13, seven males and eleven females, New Caledonia, St Vincent’s Bay, fine to coarse sand in sublittoral at 4-14 m depth.

Austranema colesi: Inglis, 1969, p. 187.

Parapinnanema colesi: Warwick and Coles, 1975, p. 405.

Parapinnanema harveyiWarwick and Coles, 1975

Parapinnanema harveyiWarwick and Coles, 1975, p. 409, fig. 3A-F, three males and one female, England, St. Martin’s, Isles of Scilly, coarse gravelly sand.

Parapinnanema hawaiiensisSemprucci and Sorensen, 2014

Parapinnanema hawaiiensisSemprucci and Sorensen, 2014, p. 501, figs. 1-3, three males and one female, Hawaii, Honolulu, coastal at 10 m depth.

Parapinnanema imbricatumBelogurov, Belogurova and Smolyanko, 1985

Parapinnanema imbricata Belogurov, Belogurova and Smolyanko, 1985, p. 50, figs. 1, 2, seven males and six females, Sea of Japan, Moneron Island, coarse sand in the sublittoral at 40-80 m depth.

Parapinnanema imbricatum: Gourbault and Vincx, 1994, p. 155.

Parapinnanema mexicanum (Jensen, 1985)

Austranema mexicanumJensen, 1985, p. 250, fig. 4AK, five males and three females, Gulf of Mexico, East Flower Garden Bank, medium to coarse sand in sublittoral.

Parapinnanema mexicanum: Gourbault and Vincx, 1994, p. 155.

Parapinnanema pectinatum (Wieser and Hopper, 1967)

Euchromadora pectinataWieser and Hopper, 1967, p. 285, fig. 53A-D, eight specimens, Florida, Biscayne Bay, dredgings in Thalassia beds at 4 m depth.

Austranema pectinata: Inglis, 1969, p. 187.

Parapinnanema pectinata: Warwick and Coles, 1975, p. 405.

Parapinnanema pectinatum: Gourbault and Vincx, 1994, p. 157.

Parapinnanema rhipsoidesGourbault and Vincx, 1994

Parapinnanema rhipsoidesGourbault and Vincx, 1994, p. 151, fig. 7A-G, one male and two females, Guadeloupe, Riviere Salee, coarse sediments, Le Moule, coarse coral sand, south-western Ilet a Fajou, lagoonal coarse coral sand at 3 m depth.

Parapinnanema ritaeGourbault and Vincx, 1994

Parapinnanema ritaeGourbault and Vincx, 1994, p. 147, figs. 5, 6, eight males, seven females, and one juvenile, Guadeloupe, Pointe Lambis, mangrove sediments.

Parapinnanema shirleyae (Coles, 1965)

Euchromadora shirleyaeColes, 1965, p. 163, figs. 2, 8, 19-21, 34, twelve males, twelve females, and two juveniles, South Africa, coarse white sand in sublittoral at 27 m depth.

Austranema shirleyae: Inglis, 1969, p. 172, figs. 44-46, 85-90.

Parapinnanema shirleyae: Warwick and Coles, 1975, p. 405.

Parapinnanema wilsoniInglis, 1969

Parapinnanema wilsoniInglis, 1969, p. 171, figs. 51-55, 66-75, 97, 98, four males and two females, Western Australia, Cape Leeuwin, Sarge Bay, weed and associated sand without silt at 0.6 m depth, Cape Naturaliste, Bunker Bay, green algae in rock-pools.

Parapinnanema imbricatumBelogurov, Belogurova and Smolyanko, 1985 (Figs. 1-4; Table 1)

Parapinnanema imbricatumBelogurov, Belogurova and Smolyanko, 1985, p. 50, figs. 1, 2.

Locality. The nematodes were collected from the subtidal benthic sediments of the East Sea (37°27ʹ24.08ʺN, 130°51ʹ47.08ʺE) at Ulleungdo Island, Ulleung-gun, Gyeongsangbuk-do, Korea, on May 23, 2023.

Material examined. One male (MABIK NA00158134) and one female (MABIK NA00158135), mounted in glycerin on a HS slide, are archived in the nematode collection of the Marine Biodiversity Institute of Korea (MABIK), Seochun, Korea. Additionally, nine male specimens (KIOST NEM-1-2700 to KIOST NEM-1- 2708) and nine female specimens (KIOST NEM-1-2710 to KIOST NEM-1-2718), also mounted on an HS slide in glycerin, have been placed 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.

Measurements. Refer to Table 1 for measurements and morphometric data.

Description. Adults. The body gradually tapers towards both ends, exhibiting a relatively lengthy size (3,317-4,338 μm long) (Figs. 1A, 2A, 3A, 4A). The oral opening is broad, leading to the vestibulum, which features twelve buccal rugae in its walls. Six internal labial sensillae are present as papillae. Additionally, there are six outer labial and four cephalic sensillae arranged in a single circle (Figs. 1B, 2B, 3B, 4B). The dorsal tooth is relatively large, accompanied by two subventral comblike plates, each bearing three teeth. All armament, including the comb-like teeth, is situated in the anterior part of the stoma (Figs. 1C, 2C, 3C, 4C). The esophagus lacks bulbs and concludes with a smooth expansion. The amphideal transverse fovea, resembling a slit, is positioned posterior to the cephalic setae. At the head end, the cuticle is divided into two layers, with the inner layer forming an endocuticle, which splits into anterior and posterior rings. These rings cover the middle and end of the vestibulum, overlapping each other like tiles on the surface (Fig. 2D). Overlapping occurs both inside and forward in the front half of the body and inside and at the back in the back half. In the first case, each subsequent ring covers a part of the previous one, while in the second case, each front ring overlaps the next one. The internal contours of the ring are visible in the middle, between the external contours of this ring and the next or previous one. Consequently, it seems that the cuticle comprises two layers of rings shifted relative to each other by half the width of the external part of the ring (Figs. 1G, 4E). Along the body, four narrow stripes run, which widen later and are referred to as ‘the non-ornamented zones’ (Figs. 2E, 4D). The tail exhibits a conical shape with dense cuticles displaying ornamentation. Towards the tail end, there are no rings, and the cuticle appears smooth (Figs. 1F, 4G). The evacuative spinneret is composed of a tube with a distinctive cone and two pores. A smaller pore opens at the tail’s apex, penetrating the cuticle and forming a channel approximately half the length of the tube. The second pore, significantly larger than the first, is positioned at the tube’s apex, often extending beyond the tail by almost half its length. The apex of the tube is slightly truncated, and the inner part of the tube has expanded to form a cone. The boundary between the cone and the tube is not clearly defined, and the wall of the cone smoothly transitions into the wall of the tube (Fig. 4G).

Males. The spicules exhibit a curved form (81-95 μm long along the arc) (Figs. 1D, 2G). The lateral pieces of the gubernaculum are configured in an L-shape, featuring a sharp bend that creates an almost perfect right angle (Fig. 2H, 2J). The median segment of the gubernaculum takes the form of a laterally expanded plate, reaching the L-shaped pieces (Figs. 1E, 2I). In the precloacal region, an elevated cuticular structure is noticeable along a ventral line (Figs. 1H, 2F).

Females. Resembling male characteristics in most aspects (Figs. 3A-F, 4A-H). The female genital system is didelphic and amphidelphic, with both ovaries reflexed and developed as a sphincter around the uterine chamber (Figs. 3F, 4D). The vulva is elongated, and the vagina is open. A shift in the orientation of the epicuticle sculpture is noticeable in the vulvar region of the female (Fig. 4E). No cuticular elevation was observed in the female (Figs. 3E, 4F).

4. DISCUSSION

As indicated by Gourbault and Vincx (1994), the genus Parapinnanema can be categorized into two species groups based on overall body length. The first species group, which includes the recently reported species P. hawaiiensis, exhibits shorter body lengths ranging from 1,300-2,250 μm. In contrast, the second species group features longer body lengths, with species like P. colesi, P. harveyi, P. imbricatum, and P. shirleyae reaching up to 3,000 μm. Nevertheless, certain members of P. alii, classified within the first species group characterized by shorter body lengths, have shown a significant variation in body length. This variation almost doubled the species’ population average (3,325 μm compared to the typical 1,900 μm). Considering the variation in body length within the P. alii population discussed earlier, Gourbault and Vincx (1994) suggested that categorizing the genus Parapinnanema into two species groups based solely on body length may not be appropriate.

Species of Parapinnanema are distinguished by subtle characteristics, including features such as the shape of the buccal cavity and the structure of the male’s copulatory apparatus (Wieser and Hopper 1967;Warwick and Coles 1975; Gourbault and Vincx 1994; Semprucci and Sorensen 2014). In particular, Parapinnanema imbricatum can be differentiated from other species within the genus by the observable teeth in the buccal cavity arranged in a series of comb-like structures. However, in some individuals, the structure of the buccal cavity may not be clearly observable, and Murphy (1965) mentioned the following regarding this trait: ‘when the stoma is widely opened, with the lips turned outward, the dorsal tooth projects beyond the contours of the head.’

Parapinnanema imbricatum, originally documented on Moneron Island in the Sea of Japan, closely resembles the Korean specimens examined in this research in terms of general characteristics. These include the buccal cavity with a series of comb-like teeth, layers of the cuticle that overlap, sharply bent lateral pieces of the gubernaculum with a laterally expanded median plate reaching the L-shaped pieces, a spinneret comprising a tube with a distinct cone and two pores, and the female amphidelphic genital system with both ovaries reflexed and forming a sphincter around the uterine chamber (Belogurov et al. 1985). Nevertheless, Korean specimens of P. imbricatum exhibit a relatively elongated body (3,317-4,339 μm compared to the original range of 3,100-4,200 μm) and a slender body width (66- 77 μm as opposed to the initial range of 71-85 μm). Additionally, the head diameter is relatively shorter (24-29 μm compared to the original 28-36 μm). No notable variations were detected in the remaining specimens we examined.