Sphaerolaimus ganymede, Zograf & Pavlyuk & Trebukhova & Tu, 2020

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 : 498-502

Sphaerolaimus ganymede sp. nov.

( Figs. 4–6 ; Table 1 )

Diagnosis. Sphaerolaimus ganymede sp. nov. is characterized by body length 1329–1456 µm, finely striated cuticle with lateral alae, six outer labial setae and four 5 µm long cephalic setae in one circle, amphideal fovea 0.44 corresponding body diameter in males and 0.2 corresponding body diameters in females and located at level of posterior part of buccal cavity, short spicules 1.1 anal body diameters in length, gubernaculum with dorsal apophyses, vulva located at 70% of body length from anterior, conico-cylindrical tail 3.2–3.9 anal body diameters in length with tree terminal setae.

Etymology. The species named after the satellite of Jupiter, Ganymede.

Holotype. VIETNAM: ♂, formalin–fixed, mounted on slide ( MIMB 39990 ), in glycerin; deposited in the Zoo- logical Museum NSCMB FEB RAS.

Paratypes. VIETNAM: 1♂, 2♀♀ formalin–fixed, mounted on slide ( MIMB 39991 –39993), in glycerin; depos- ited 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, which begins in the cervical region and extends to the base of the tail. Inner labial sensilla papilliform, six in number. Outer labial (1 µm long) and cephalic setae in one circle. Cephalic setae 5 µm long. Below, the circle of eight bunches of subcephalic setae are situated, 2–3 setae (14 µm long) in each bunch. Rows of cervical setae 11–14 µm long are situated behind the subcephalic setae. Amphideal fovea circular, with cuticularized outline, 15 µm in diameter or 44% of correspond- ing body diameter. Distance from the anterior end to amphid 23 µm. Numerous longitudinal cheilostomal ridges immediately anterior to gymnostom. Gymnostom barrel–shaped, 15–17 µm long and 20–22 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 9.2% of total body length from anterior end. Excretory pore behind nerve ring, 12% of total body length from anterior end.

Testes paired. Spicules small, weakly curved, 1.1 anal body diameters in length. Gubernaculum short, with curved apophyses. Tail consists of anterior conical and posterior cylindrical parts, with sparse setae 8–10 µm long. Tail equal to 10.3% of total body in length or 3.2–3.3 anal body diameter, with three terminal setae 12 µm long.

Females. Similar to males. Amphids smaller than in males, 7 µm in diameter or 20% of corresponding body diameter. Reproductive system monodelphic, prodelphic, with single anterior ovary located to the right of intestine extending to the renette; no spermatheca was found. Vulva situated far posterior to mid–body (V=70.5%). Tail equal to 11.6% of total body in length or 3.9 anal body diameters with sparse setae 8–10 µm long and three terminal setae 12 µm long.

Relationships. Sphaerolaimus ganymede sp. nov. is closest to S. maeoticus sensu Gerlach (1957) and Timm (1963) in body length, indexes a, b, c, size and position of amphideal fovea, vulva position, length of spicules and shape of gubernaculum. On the other hand, newly described species differs from S. maeoticus sensu Filipjev, 1922 by smaller index a (12.3–17.5 vs 20–27), index c in male (7.9 vs 10), bigger amphid in male (15–16 µm vs 11 µm), and longer spicules (58–61.5 µm vs 50 µm) and presence of gubernaculum.

In our opinion, descriptions by both, Gerlach (1957) and Timm (1963) do not quite fit the description by Filipjev (1922). Gerlach expressed some doubts about similarity of his species with S. maeoticus sensu Filipjev, 1922 , and assumed that his species might be separate species. Population of S. maeoticus described by Gerlach (1967) differs from population described by Filipjev (1922) by the indexes a (16–18 vs 20–27) and c (7.2–7.6 vs 9–10), amphid diameter in male (14 vs 11 or 40% of corresponding body diameter vs 33% of corresponding body diameter), spicule length (58 µm vs 50 µm) and presence of gubernaculum. Timm (1963) based his description on Gerlach (1967) work and his species also differed from S. maeoticus sensu Filipjev, 1922 by smaller body (1200 µm vs 1300–1600 µm), index a (15.6–17 vs 20–27) and the presence of gubernaculum. So, S. maeoticus sensu Gerlach (1957) and Timm (1963) refers to S. ganymede sp. nov.

Figures

FIGURE 4. Sphaerolaimus ganymede 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. Copulative apparatus of male.Abbreviation: vs—ventral side. Scale bars: A, B—200 µm; C, D, F—20 µm; E—50 µm.

FIGURE 5. Sphaerolaimus ganymede sp. nov. Light microscopy (DIC). A. General view of male. B. General view of female. C. Anterior end of male. D. Anterior end of male. E. Posterior end of male. F. Anterior end of female. G. Anterior end of female. Abbreviation: vs—ventral side. Scale bars: A, B—100 µm; C–G—20 µm.

FIGURE 6. Sphaerolaimus ganymede sp. nov. SEM. A. General view of male. B. Anterior end of male, lateral view. C. Posterior end of male. D. Anterior end of female, lateral view. Abbreviations: a—amphid; cs—cephalic setae; ils—inner labial sensilla; l—lateral alae; ols—outer labial sensilla; scs—subcephalic setae; vs—ventral side. Scale bars: A—100 µm; B, D—10 µm; C—20 µm.