Oxystomina hakureiae Shimada, 2020

Shimada, Daisuke, Takeda, Naoya, Tsune, Akira & Murakami, Chisato, 2020, Three new species of free-living marine nematodes (Nematoda: Enoplida) from the Clarion-Clipperton Fracture Zone (CCFZ), North Pacific, Zootaxa 4859 (4), pp. 507-526 : 516-523

Oxystomina hakureiae Shimada , sp. nov.

Figures 5–7 , Table 4 .

Type material. Holotype. Adult male (NSMT-As 4614a), formalin-fixed, permanent whole mounts. Paratype. An adult female (NSMT-As 4614b), formalin-fixed, permanent whole mounts, same collection data as for holotype.

Type locality. CCFZ (9° 14.9264′ N, 146° 15.5178′ W, 5342 m depth) with a spade corer by Yasushi Miyazaki and Yoshiro Sekido ( MBRIJ) on 16 Nov. 2011 .

Diagnosis. Oxystomina hakureiae sp. nov. is characterized by the short (less than 0.3 corresponding body diameters), setiform outer labial and cephalic sensilla; the amphids located 2.2–2.3 cephalic diameters posterior to level of the cephalic sensilla; the conico-cylindrical tail with the posterior portion almost the same length as conical portion in both sexes; the arcuate spicules with a velum and a capitulum; the presence of the paired gubernacula; the presence of two precloacal setae in male; the diorchic reproductive system in male; and the monodelphic reproductive system with the posterior ovary in female.

Etymology. The specific name hakureiae is a noun in the genitive case after the research vessel, Hakurei-maru No. 2.

Description. Male. Body tapering toward both ends. Cuticle smooth, 2–4 μm thick. Somatic sensilla papil- liform or very short setiform, arranged in eight longitudinal rows, with large (10–30 μm in diameter) unicellular epidermal glands (cf. Decraemer et al. 2014) present from posterior to amphids to caudal region. Head rounded, diameter at cephalic sensilla bases equal to 0.16 maximum body diameters, slightly constricted between outer labial and cephalic sensilla. Inner labial sensilla indistinct. Six outer labial and four cephalic sensilla setiform, arranged in two well-separated circles: outer labial sensilla ca. 2.5 μm long, 0.17–0.18 labial diameters or 0.14–0.15 cephalic di- ameters, located at 0.35 cephalic diameters from anterior body end; cephalic sensilla shorter, ca. 2.0 μm, 0.11–0.13 labial diameters or 0.09–0.11 cephalic diameters, located at 1.2 cephalic diameters from anterior end. Amphids pocket-like: fovea double-contoured, longitudinally elongated; aperture circular or oval, 0.4 corresponding body diameters wide, located at 3.5 cephalic diameters from anterior body end or 2.3 cephalic diameters posterior to level of cephalic sensilla. Buccal cavity minute. Pharynx thin (0.2–0.3 corresponding body diameters wide), conspicuously enlarged at posterior end. Pore of secretory-excretory system distinct and sclerotized, situated at 13.2 cephalic diameters or 0.3 pharyngeal lengths from anterior body end. Renette cell large, maximally 0.4 corresponding body diameters wide, posterior end at 0.93 pharyngeal lengths from anterior end. Nerve ring situated at 0.45 pharyngeal lengths from anterior end. Cardia inconspicuous. Tail conico-cylindrical, 3.7 cloacal body diameters long: anterior conical portion 40% of tail length; posterior cylindrical portion straight, ca. 0.1 cloacal body diameters wide; tip very slightly expanded, with spinneret and no terminal setae. Three caudal glands located in conical portion of tail. Caudal sensilla papilliform, same as somatic sensilla. Reproductive system diorchic, present on right side of intestine: anterior testis outstretched, situated from 23% of body length; posterior testis reflexed at 44% of body length, ending at 38% of body length. Sperms elongated in shape (cf. Jensen 1979). Vas deferens conspicuous, filled with sperms. Spicules equal, arcuate with ventral velum and proximal capitulum, 1.4 cloacal body diameters or 0.4 tail lengths. Gubernacula paired, rounded proximally and acute distally, 0.33 cloacal body diameters or 0.25 spicules lengths. Two pairs of precloacal ventromedian setae (15–18 μm long) present at 0.8 cloacal body diameters anterior to cloaca.

Female. Similar to male except for the following characteristics. Outer labial sensilla longer (ca. 4 μm, 0.27 labial diameters or 0.2 cephalic diameters) than in male; cephalic sensilla same as in male. Amphidial aperture oval (length: width = 3:2), smaller (0.25 corresponding body diameters wide) than in male, located at 3.4 cephalic diameters from anterior body end or 2.2 cephalic diameters from level of cephalic sensilla. Pore of secretory-excretory system situated at 10.9 cephalic diameters or 0.25 pharyngeal lengths from anterior body end. Renette cell larger than in male, maximally 0.5 corresponding body diameters wide; posterior end at 0.97 pharyngeal lengths from anterior end. Nerve ring situated at 0.42 pharyngeal lengths from anterior end. Tail shorter than in male, 2.7 anal body diameters long; anterior conical and posterior cylindrical portions almost equal in length. Reproductive system monodelphic. Only posterior ovary present at left side of intestine, reflexed at 43% of body length and ending at 31% of body length from anterior body end. Uterus situated from 23% to 30% of body length, with a short branch anterior to vulva. An egg present in uterus, ca. 265 μm long and 90 μm wide. Distal part of oviduct to proximal part of uterus filled with elongated sperms, possibly functioning as spermatheca. Vulva pore-like, surrounded by strongly developed cuticular wall, located at 25% of body length. Vagina sclerotized.

Remarks. A male and a female were identified in the genus Oxystomina based on the indistinct inner labial sensilla, the separate circles of the outer labial and cephalic sensilla, the minute buccal cavity, the circular or oval amphidial apertures, and non-rounded tail end. Oxystomina contains 38 valid species ( Schuurmans Stekhoven 1935; Gerlach & Riemann 1974; Lorenzen 1981; Platonova 1984; Quang et al. 2008; Gagarin & Nguyen 2014; Nguyen et al. 2016). Oxystomina filicauda ( Kreis, 1929) Schuurmans Stekhoven, 1935 and O. mirabilis Allgén, 1959 should be treated as species inquirendae, because they were established only based on a juvenile ( O. filicauda ) or broken male ( O. mirabilis ) without descriptions of reproductive characteristics. Nemanema arenaria ( Bussau, 1990) comb. nov. was established as Oxystomina arenaria Bussau, 1990 , however it should belong to the genus Nemanema Cobb, 1920 based on the conical tail with rounded tip (vs. conical tail with acute tip or conico-cylindrical tail in Oxystomina ).

Oxystomina hakureiae sp. nov. differs from 28 congeners described based on male specimens by the following characteristics: from O. antarctica Mawson, 1956 , O. caspica Tchesunov, 1978 , O. clavicauda ( Filipjev, 1918) Filipjev, 1921 , O. cobbi ( Filipjev, 1927) Wieser, 1953 , O. elongata ( Bütschli, 1874) Filipjev, 1921 , O. minor Nguyen, Gagarin., Phan, Nguyen & Nguyen, 2016 , O. paraclavicaudata Quang, Nguyen, Chau, Smol & Vanreusel, 2008 , and O. vipriensis Gagarin & Nguyen, 2014 by having two testes; from O. nuda ( Filipjev, 1927) Schuurmans Stekhoven, 1935 by the curving spicules (vs. straight in O. nuda ); from O. alpatovi ( Filipjev, 1927) Wieser, 1953 , O. alpha Chitwood, 1937 , O. islandica ( De Coninck, 1943) Filipjev & Kreis, 1926 , O. miranda Wieser, 1953 , O. nidrosiensis Allgén, 1933 , O. oxycaudata ( Ditlevsen, 1926) Allgén, 1929 , O. tenuicollis Allgén, 1959 , and O. unguiculata Schuurmans Stekhoven, 1935 by the presence of velum; from O. acuta Gerlach, 1957 and O. elegans Platonova, 1971 by the presence of gubernacula; from O. asetosa ( Southern, 1914) Filipjev, 1921 , O. chitwoodi Timm, 1967 , O. exilis ( Cobb, 1920) Filipjev & Kreis, 1926 , and O. tenuis (Cobb in Thorne, 1939) Lorenzen, 1981 by the setiform anterior sensilla (vs. absent or papilliform in the latter); from O. astridae ( Jensen, 1979) Lorenzen, 1981 by the posterior position of amphids (2.3 cephalic diameters from level of cephalic sensilla in O. hakureiae sp. nov. vs. just posterior to cephalic sensilla in O. astridae ); from O. affinis Gerlach, 1956 , O. demani Platonova, 1984 , O. insulaealbae Filipjev, 1927 , and O. pulchella Vitiello, 1970 by the much shorter outer labial sensilla (shorter than 0.2 labial diameters in our specimen vs. equal to or longer than 0.4 labial diameters in the latter).

Oxystomina hakureiae sp. nov. also differs from other 10 congeners known only by females: from O. borealis ( Steiner, 1916) Schuurmans Stekhoven, 1935 and O. filicaudata Allgén, 1959 by the single ovary (vs. two ovaries in the latter); from O. brevicaudata ( Kreis, 1929) Gerlach & Riemann, 1974 and O. propria Sergeeva, 1973 by the conico-cylindrical tail (vs. conical in the latter); from O. breviceps Sergeeva, 1973 , O. greenpatchi Allgén, 1959 , O. tenuicaudata Filipjev, 1946 , and O. vespertilio Wieser, 1953 by the setiform outer labial and cephalic sensilla (vs. absent or papilliform in the latter); from O. orientalis Platonova, 1971 by the much shorter outer labial sensilla (ca. 1/4 of labial diameter in O. hakureiae sp. nov. vs. as long as labial diameter in O. orientalis ); from O. novozemelica ( Filipjev, 1927) Wieser, 1953 by the cylindrical posterior portion of tail (vs. clavate in O. novozemelica ).

The taxonomic key for males of the species of Oxystomina is as follows. We did not use the redescriptions of O. elongata by Lorenzen (1969), Platt & Warwick (1983), Maqbool et al. (1999), Huang & Zhang (2006), and Riera et al. (2010) to make the key, because they do not agree well with the original description by Bütschli (1874) in the length of anterior sensilla and the shape of spicules (cf. Platonova 1984):

1 Spicules almost straight........................................................................... O. nuda

- Spicules arcuate...................................................................................... 2

2 Cephalic sensilla inconspicuous or absent.................................................................. 3

- Cephalic sensilla conspicuous.......................................................................... 11

3 Outer labial sensilla setiform................................................................. O. nidrosiensis

- Outer labial sensilla papilliform or absent.................................................................. 4

4 Spicules without velum................................................................................ 5

- Spicules with velum................................................................................... 7

5 Spicules with capitulum..................................................................... O. unguiculata - Spicules without capitulum............................................................................. 6

6 Gubernacula present......................................................................... O. tenuicollis

- Gubernacula absent........................................................................ O. oxycaudata

7 Amphid width almost equal to corresponding body diameter............................................. O. exilis

- Amphid width equal to or less than 0.5 corresponding body diameters........................................... 8

8 Posterior portion of tail filiform, much longer than anterior conical portion............................... O. chitwoodi

- Posterior portion of tail clavate, equal to or shorter than anterior conical portion.................................... 9

9 Gubernacula absent............................................................................ O. elegans

- Gubernacula present.................................................................................. 10

10 Precloacal papilla present........................................................................ O. asetosa

- Precloacal papilla or seta absent.................................................................. O. elongata

11 Amphids located just posterior to cephalic sensilla.......................................................... 12

- Amphids located much (more than cephalic diameter) posterior to cephalic sensilla................................ 15

12 Outer labial and cephalic sensilla papilliform......................................................... O. tenuis

- Outer labial and cephalic sensilla setiform................................................................. 13

13 Spicules without velum....................................................................... O. islandica

- Spicules with velum.................................................................................. 14

14 Reproductive system monorchic................................................................... O. cobbi

- Reproductive system diorchic................................................................... O. astridae

15 Spicules without velum............................................................................... 16

- Spicules with velum.................................................................................. 22

16 Spicules without capitulum....................................................................... O. minor

- Spicules with capitulum............................................................................... 17

17 Spicules with ventral apophysis......................................................................... 18

- Spicules without ventral apophysis...................................................................... 19

18 Reproductive system monorchic................................................................ O. vipriensis

- Reproductive system diorchic................................................................... O. miranda

19 Gubernacula absent.......................................................................... O. antarctica

- Gubernacula present.................................................................................. 20

20 Outer labial and cephalic sensilla longer than corresponding body diameter................................. O. alpha

- Outer labial and cephalic sensilla shorter than corresponding body diameter...................................... 21

21 Posterior portion of tail as long as anterior conical portion............................................. O. alpatovi

- Posterior portion of tail much longer than anterior conical portion................................ O. paraclavicaudata

22 Tail conical.................................................................................... O. acuta

- Tail conico-cylindrical or clavate........................................................................ 23

23 Gubernacula absent............................................................................ O. caspica

- Gubernacula present.................................................................................. 24

24 Spicules without capitulum............................................................................ 25

- Spicules with capitulum............................................................................... 26

25 Gubernacula large, as long as 0.4 spicule lengths.................................................. O. clavicauda

- Gubernacula minute............................................................................ O. demani

26 Outer labial and cephalic sensilla much shorter than 0.5 corresponding body diameters.............. O. hakureiae sp. nov.

- Outer labial and cephalic sensilla approximately 0.5 corresponding body diameters................................ 27

27 Posterior portion of tail filiform................................................................. O. pulchella

- Posterior portion of tail clavate......................................................................... 28

28 Precloacal setae present.......................................................................... O. affinis

- Precloacal setae absent..................................................................... O. insulaealbae

Another taxonomic key for female of the species of Oxystomina is as follows:

1 Tail conical.......................................................................................... 2

- Tail conico-cylindrical or clavate......................................................................... 4

2 Outer labial and cephalic sensilla absent.......................................................... O. breviceps

- Outer labial sensilla present............................................................................. 3

3 Cephalic sensilla absent.................................................................... O. brevicaudata

- Cephalic sensilla present........................................................................ O. propria

4 Cephalic sensilla setiform............................................................................... 5

- Cephalic sensilla papilliform, inconspicuous, or absent....................................................... 6

5 Outer labial sensilla as long as cephalic diameter................................................... O. orientalis

- Outer labial sensilla much shorter than cephalic diameter.......................................... O. novozemelica

6 Outer labial and cephalic sensilla absent or papilliform....................................................... 7

- Outer labial sensilla setiform............................................................................ 8

7 Cephalic sensilla absent..................................................................... O. greenpatchi

- Cephalic sensilla present..................................................................... O. vespertilio 8 Reproductive system monodelphic............................................................ O. tenuicaudata

- Reproductive system didelphic........................................................................... 9

9 Outer labial sensilla 1.0 cephalic diameter long...................................................... O. borealis

- Outer labial sensilla 0.5 cephalic diameters long................................................... O. filicaudata

Figures

FIGURE 5. Oxystomina hakureiae sp. nov. Holotype male: A, body; B, cervical region; C, head; D, tail; E, left spicule and gubernaculum. Scale bars:A, 1 mm; B, D, 100 μm; C, E, 10 μm.Abbreviations: a.t., anterior testis; c.s., cephalic sensillum; gub., gubernaculum; o.l.s., outer labial sensillum; p.s., precloacal setae; p.t., posterior testis; v.d., vas deferens; vel., velum.

FIGURE 6. Oxystomina hakureiae sp. nov. Paratype female: A, body; B, head; C, tail; D, reproductive system. Scale bars: A, 1 mm; B, 10 μm; C, D, 100 μm. Abbreviations: c.s., cephalic sensillum; o.d., oviduct; o.l.s., outer labial sensillum; ova., ovary; spr., spermatheca; vul., vulva.

FIGURE 7. Oxystomina hakureiae sp. nov. Holotype male: A, head, inner view; B, somatic sensilla; C, gland cells of somatic sensilla; D, amphidial aperture; E; amphidial fovea; F, sclerotized pore of secretory-excretory system; G, left spicule; H, left gubernaculum; I, precloacal ventromedian setae; J, sperms in testis. Paratype female: K, sclerotized vagina with cuticular wall surrounding vulva; L, sperms in spermatheca. Scale bars:A–F, J, K, 10 μm; G, I, 20 μm; H, L, 5 μm.Abbreviations: c.s., cephalic sensillum; o.l.s., outer labial sensillum; p.s., precloacal ventromedian setae; s.s., somatic sensilla.