Proplatycoma tsukubae, Shimada & Kakui, 2020

Shimada, Daisuke & Kakui, Keiichi, 2020, A New Species of Free-living Marine Nematode, Proplatycoma tsukubae sp. nov. (Enoplida: Leptosomatidae), from Japan, Species Diversity 25, pp. 205-211 : 206-209

Subfamily Platycominae Platonova, 1976 Genus Proplatycoma Platonova, 1976 Proplatycoma tsukubae sp. nov. ( Figs 1–4 )

Material examined. Holotype, adult male ( ICHUM 5965 ), whole mount slide, off Shimoda , Shizuoka Prefecture, Japan (34°42.153′N 139°02.378′E – 34°42.216′N 139°02.621′E), 117–202 m deep, 12 October 2017. Two paratypes, adult male ( ICHUM 5966 ) and immature female ( ICHUM 5967 ), same collection data as holotype. Two nontype specimens, immature females, used for molecular studies, same collection data as holotype.

Etymology. The specific name tsukubae is a noun in the genitive case derived from R/V Tsukuba II.

Diagnosis. Proplatycoma tsukubae sp. nov. is characterized by the largest body size (longer than 20 mm) in the genus, short, unilobed amphidial flaps just reaching the anterior edge of the amphidial apertures, long (1.5 cloacal body diameter) spicules, uniquely shaped accessory structures of spicules constricted at the middle and tapering toward both ends, a gubernaculum with long dorso-caudal apophyses equal to the accessory structure length, two precloacal ventromedian supplements: anterior one with a papilliform sensillum, posterior one with a pair of ventrolateral setae, the presence of postcloacal ventrolateral supplements, and a blunt (not acute) tail tip.

Measurements. See Table 1.

Description of males. Body ( Fig. 1A, B ) longer than 20 mm, almost uniform in diameter except in cervical and caudal regions. Cuticle finely striated only anterior to nerve ring; each striation 0.8–1.2 µm wide. Papilliform somatic sensilla sparse. Numerous gland cells (largest 60×25 µm) surrounding intestine. Ocellus-like pigment spots absent. Head ( Figs 1C, D , 2A ) truncated at anterior end, 40–45% of maximum body diameter at level of cephalic setae, and constricted at level of amphids. Six inner labial sensilla papilliform. Six outer labial setae (0.5 cephalic diameter) and four cephalic setae (0.6 cephalic diameter) arranged in single circle. Subcephalic setae just behind amphids absent. Cervical setae ( Fig. 1E ) slightly shorter than outer labial setae, arranged in eight longitudinal rows, and present anterior to nerve ring. Cephalic capsule ( Figs 1C, D , 2A ) weakly developed, 12 µm long and 1.5 µm thick, located at level of cephalic setae. Amphids ( Figs 1C, D , 2B, C ) pocket-like, located at 0.5 cephalic diameter from anterior body end: aperture transversely elliptical, 0.4–0.5 cephalic diameter wide, with a short flap expanded from middle of posterior edge; flap unilobed, anterior tip reaching at anterior edge of aperture in holotype but shorter in paratype male; fovea cupshaped, 10–12 µm deep. Buccal cavity minute and tooth-like structure absent. Buccal aperture surrounded by low three microlabia; each microlabium equal in shape and length. Excretory pore and ventral gland not observed. Nerve ring ( Fig. 1E ) located at 16–19% of pharyngeal length from anterior body end. Reproductive system ( Fig. 3A, B ) diorchic. Testes opposed and outstretched, each on left side of intestine: anterior testis beginning at 62–64% of body length; posterior testis ending at 68–69% of body length. Seminal vesicle situated between both testes, connecting with anterior testis through short duct and with posterior testis directly. Vas deferens muscular, more conspicuous posteriorly. Spicules ( Figs 2D , 3C ) paired, 1.5–1.6 cloacal body diameter or 56–67% of tail length, bent ventrally, tapering toward both ends, slightly constricted and bluntly rounded at proximal end, not acute at distal end. Pair of large accessory structures ( Fig. 3C, D ) present on ventrolateral side of each spicule, constricted at middle and tapering toward both ends, 0.4–0.5 spicule length, finely striated at distal end. Gubernaculum ( Figs 2D , 3C ) with paired apophyses; each apophysis almost straight, directed dorso-caudally, gradually tapering toward caudal end, 0.75 cloacal body diameter or 45% of spicule length. Two precloacal ventromedian supplements ( Figs 2E , 3C ) present as papilliform elevation of body cuticle: anterior one larger, located at 2.7–3.0 cloacal body diameter from cloaca, with papilliform sensillum at top; posterior one at 1.2–1.6 cloacal body diameter from cloaca, with a pair of posterior ventrolateral setae. A postcloacal ventromedian supplement ( Figs 2D , 3C ) also present, similar to precloacal ventromedian supplements but smaller in size, located at 0.6–0.7 cloacal body diameter from cloaca, with a setiform sensillum at top. A row of smaller supplements ( Figs 2D–F , 3C ) present at each ventrolateral field: 31–39 supplements anterior to precloacal ventromedian supplements; 4–6 supplements situated between precloacal ventromedian supplements; 8–9 supplements posterior to postcloacal ventromedian supplement. Tail ( Fig. 3C ) coni- co-cylindrical, anterior part 80% tapering and posterior part slightly expanded with blunt tip. Caudal setae consisting of several short (ca. 10 µm) setae at distal end of conical part. At tail tip ( Fig. 2G ), three short setae present. Spinneret and caudal glands not observed.

Description of immature female. Body ( Fig. 4A ) similar to males except for following characters. Cervical setae in eight longitudinal rows slightly shorter than those in males. Cephalic capsule inconspicuous. Amphids ( Figs 2H , 4B ) smaller than those in males: aperture elliptical, 1/8 of cephalic diameter wide, without flap; fovea cup-shaped, 8 µm deep. Reproductive system ( Fig. 4C ) not developed. Anteri- or ovary present, beginning at 61% of body length; however posterior end inconspicuous. Posterior ovary and uteri not observed. Vulva pore-like, situated at 68% of body length, without gland cells. Posterior region of intestine twisted as illustrated in Fig. 4D , probably due to damage during sampling. Tail ( Fig. 4D ) conical in anterior part 70% and cylindrical in posterior part, with sparse caudal setae.

Sequence data. The partial COI and 18S sequences were determined from two immature females. Two COI sequences (342 bp: accession numbers LC495491 and LC495492 ) differed by 20 nucleotide substitutions, which represented the first COI sequences from any Proplatycoma species . The 18S sequences from two specimens (1706 bp: accession numbers LC495489 and LC495490 ) were identical and the most similar to the partial 18S sequence (1625 bp: accession number KR265047 ) of P . fleurdelis among the sequences in the DDBJ/EMBL/GenBank database.

Remarks. Our specimens are identified as a member of the genus Proplatycoma based on the presence of the amphidial flaps, the accessory structures of the spicules, and the dorso-caudal apophyses of the gubernaculum [amended from Smol et al. (2014)]. Proplatycoma tsukubae sp. nov. is clearly distinguishable from three congeners, P. curiosa , and longer apophyses of the gubernaculum (3/4 of cloacal body diameter or 1/2 of spicule length in P. tsukubae sp. nov. vs. 1/4 of cloacal body diameter and spicule length in P. africana ). Furthermore, P. tsukubae sp. nov. differs from all four congeners based on the presence of the anterior precloacal ventromedian supplement [absent in P. africana and P. curiosa ( Gerlach 1955, 1959)] without subventral setae [present in P. fleurdelis and P. sudafricana ( Inglis 1966; Hope 1988a)], the accessory structure of the spicules constricted at the middle and tapering toward both ends [conical tapering toward proximal end in P. curiosa and P. fleurdelis , almost cylindrical in P. africana , and with a proximal head in P. sudafricana ( Gerlach 1955, 1959; Inglis 1966; Hope 1988a)], and blunt tail tip [acute in all of the congeners ( Gerlach 1955, 1959; Inglis 1966; Hope 1988a)].

P. fleurdelis , and P. sudafricana , based on its unilobed amphidial flaps [trilobed in the three congeners ( Gerlach 1955; Inglis 1966; Hope 1988a)]. In addition, P. tsukubae sp. nov. has a simple, not convoluted seminal vesicle ( Fig. 3A, B ), although Hope (1988a) reported the highly convoluted seminal vesicle in P. fleurdelis . Proplatycoma tsukubae sp. nov. is similar to P. africana in the unilobed amphidial flaps, however it differs based on the much shorter amphidial flaps just reaching the anterior edge of the amphidial aperture

Figures

Fig. 1. Proplatycoma tsukubae sp. nov. A, male body (holotype); B, male body (paratype); C, male head (holotype); D, male head (paratype); E, male anterior region (holotype). Abbreviations: a.t., anterior testis; c.c., cephalic capsule; m.l., microlabium; p.t., posterior testis.

Fig. 2. Differential interference contrast micrographs of Proplatycoma tsukubae sp. nov. A, male head (holotype); B, male amphidial flap (holotype), C, male amphidial flap (paratype); D, male cloacal region (holotype); E, precloacal ventromedian supplements (holotype); F, precloacal ventrolateral supplements (holotype); G, tip of male tail with terminal setae (holotype); H, immature female amphidial aperture. Abbreviations: a.p.s., anterior precloacal ventromedian supplement; c., cloaca; m.l., microlabium; c.c., cephalic capsule; p.p.s., posterior precloacal ventromedian supplement; p.s., postcloacal ventromedian supplement.

Fig. 3. Proplatycoma tsukubae sp. nov. A, male reproductive system (holotype); B, male reproductive system (paratype); C, male posterior region (holotype); D, accessory structure of spicule (holotype). Abbreviations: a.p.s., anterior precloacal ventromedian supplement; p.p.s., posterior precloacal ventromedian supplement; p.s., postcloacal ventromedian supplement.

Fig. 4. Proplatycoma tsukubae sp. nov. A, immature female body; B, immature female head; C, immature female reproductive system; D, immature female posterior region. Abbreviations: a.o., anterior ovary; m.l., microlabium; v., vulva.