Sphaerolaimus, Bastian, 1865

Zograf, Julia K., Pavlyuk, Olga N., Trebukhova, Yulia A. & Tu, Nguyen Dinh, 2020, Description of three new Sphaerolaimus species (Nematoda: Sphaerolaimidae) form the South China Sea, Zootaxa 4755 (3), pp. 491-514 : 502-506

Sphaerolaimus io sp. nov.

( Figs. 7–9 ; Table 1 )

Diagnosis. Sphaerolaimus io sp. nov. is characterized by body length 1481–1768 µm, six outer labial papillae and four 2 µm long cephalic setae in one circle, eight groups of 4 subcephalic setae 5–19 µm long, aphideal fovea 0.26 corresponding body diameter in males and 0.21 corresponding body diameter in females and located at level of posterior part of buccal cavity, arcuate spicules 1.7 anal body diameter long, gubernaculum with apophyses, vulva located at 70% of body length from anterior, and conico-cylindrical tail 3.5–4 anal body diameter long with three terminal setae 10 µm long.

Etymology. Species named after the satellite of Jupiter, Io.

Holotype. VIETNAM: ♂, formalin–fixed, mounted on slide ( MIMB 39994 ), in glycerin; deposited in the Zoo- logical Museum NSCMB FEB RAS.

Paratypes. VIETNAM: 2♂♂, 3♀♀ formalin–fixed, mounted on slide ( MIMB 39995 –39999), in glycerin; de- posited in the Zoological Museum NSCMB FEB RAS.

Type locality. Northern Vietnam, Tien Yen Estuary, 21°18,997’ N, 107°36,075’ E; fine silty sands, water depth 0.5–0.8 m.

Description. Males. Body moderately long. Cuticle finely striated with lateral alae 12–14 µm wide, which begins in the cervical region and extends to the base of tail. Anterior end blunt. Inner labial sensilla papilliform, six in number. Outer labial and cephalic setae in one circle. Outer labial setae 1 µm long, cephalic setae 2 µm long. Below, the circle of eight bunches of subcephalic setae are situated, 4 setae (5–7 µm long) in each bunch. Rows of cervical setae 8–10 µm long are situated behind the subcephalic setae. Amphideal fovea circular, with cuticularized outline, 9 µm in diameter or 27% of corresponding body diameter. Distance from the anterior end to amphid 30–35 µm. Numerous longitudinal cheilostomal ridges immediately anterior to gymnostom. Gymnostom barrel–shaped, 29–31 µm long and 23–25 µm wide. Stegostom funnel shaped and surrounded by pharyngeal tissue. Pharynx cylindrical, with heavily cuticularized lumen along entire length. Cardia small, conoid, surrounded by intestine. Nerve ring at

approximately 10% of total body length from anterior end. Excretory pore behind nerve ring, 11.5% of total body length from anterior end.

Testes paired. Spicules curved, equal to 1.7 anal body diameters in length. Gubernaculum with huge apophyses, lengths of which equals to 20% of total spicule length. Tail consists of anterior conical and posterior cylindrical parts, with sparse setae 7–8 µm long. Tail equal to 12.4% of total body in length or 3.5 anal body diameters with three terminal setae 10 µm long.

Females. Similar to males. Body slightly longer than that of males. Amphideal fovea slightly smaller than that in males, 8 µm in diameter or 21% of corresponding body diameter. Reproductive system monodelphic, prodelphic, with single anterior ovary located to the right of intestine extending to base of pharynx; no spermatheca was found. Vulva situated far posterior to mid - body (V=70%); vaginal glands present on either side of proximal region of va- gina. Tail equal to 12% of total body length or 4 anal body diameters, with few setae 7–8 µm long and three terminal setae 10 µm long.

Relationships. Sphaerolaimus io sp. nov. is close to S. pacificus described by Wieser (1965) and Timm (1963) in body size, de Man indexes, amphid size and length of spicules. However, S. pacificus sensu Wieser (1965) and Timm (1963) differs dramatically from S. pacificus sensu Allgén, 1947 . Population described by Allgén differs from population described by Wieser (1956) by smaller body (1144 µm vs 1550–1880 µm), index a (14.3 vs 20.4–23.5), index c (14.3 vs 8.3–9.4), shorter cephalic setae, vulva situated far anterior vs vulva shifted to the posterior half of the body. Sphaerolaimus pacificus sensu Allgén, 1947 differs from S. pacificus sensu Timm, 1961 by the smaller body (1140 µm vs 1230–1810 µm), index c (14.3 vs 7–9.4), shorter cephalic setae, vulva situated far anterior vs vulva shifted to the posterior half of the body. Based on these features we propose that S. pacificus sensu Wieser (1965) and Timm (1963) belongs to Sphaerolaimus io sp. nov.

Sphaerolaimus pacificus sensu Pastor de Ward (1984) is also noticeably different from S. pacificus sensu Allgen, 1947 . It is almost twice bigger (2130–2520 µm vs 1144 µm), indexes a and b are higher (18.2–22.9 vs 14.3 and 4.5–6.3 vs 2.98 correspondingly), index c is lower (9.7–10.1 vs 14.3), and the depth of buccal cavity is much smaller than in S. pacifics sensu Allgen, 1947 . Probably, S. picificus sensu Pator de Ward (1984) is a new species of Sphaerolaimus .

Figures

FIGURE 7. Sphaerolaimus io sp. nov. A. General view of male. B. General view of female. C.Anterior end of male. D.Anterior end of female. E. Posterior end of male. F. Copulatory apparatus.Abbreviation: vs—ventral side. Scale bars: A, B—100 µm; C, D, F—20 µm; E—50 µm.

FIGURE 8. Sphaerolaimus io sp. nov. Light microscopy (DIC). A. General view of male. B. Anterior end of male. C. Anterior end of male. D. Anterior end of female. E. Anterior end of female. F. Copulative apparatus of male. G. Posterior end of female. Abbreviation: vs—ventral side. Scale bars: A, G—100 µm; B–F—20 µm.

FIGURE 9. Sphaerolaimus io sp. nov. SEM. A. anterior end of female. B. Anterior end of male, ventro–lateral view. C. Head of male, apical view. D. Lateral alae. E. Posterior end of male. Abbreviations: a—amphid; cs—cephalic setae; ils—inner labial sensillae; ols—outer labial sensillae; scs—subcephalic setae; vs—ventral side. Scale bars: A, E—20 µm; B, C—10 µm; D—2 µm.