Sphaerolaimus callisto, 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 : 494-496

Sphaerolaimus callisto sp. nov.

( Figs. 1–3 ; Table 1 )

Diagnosis. Sphaerolaimus callisto sp. nov. is characterized by body length 2349–2692 µm, finely striated cuticle, six outer labial sensilla and four 1.3–1.6 µm long cephalic setae in one circle, amphideal fovea 0.12–0.13 of corresponding body diameter located at level of posterior part of buccal cavity, and spicules 4.5 anal body diameters long, vulva located at 78–84% of body length from anterior, and conico-cylindrical tail 3.5–3.7 anal body diameter long with tree terminal setae.

Etymology. The species named for the satellite of Jupiter, Callisto.

Holotype. VIETNAM: ♂, formalin–fixed, mounted on slide ( MIMB 39985 ), in glycerin; deposited in the Zoologi- cal Museum NSCMB FEB RAS.

Paratypes. VIETNAM 2♂♂, 3♀♀, formalin–fixed, mounted on slide ( MIMB 39986 –39989), 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 stout, tapering to both extremities. Cuticle finely striated with lateral alae 24–26 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 0.6 µm long, cephalic setae 1.6 µm long. Below, the circle of eight bunches of subcephalic setae are situated, 2–3 setae (9–11 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, 11 µm in diameter or 12.9% of corresponding body diameter. Distance from the anterior end to amphid 34.5 µm. Numerous longitudinal cheilostomatal ridges immediately anterior to gymnostom. Gymnostom barrel - shaped, 30–33 µm long and 25–27 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 6.7% of total body length from anterior end. Excretory pore behind nerve ring, 11% of total body length from anterior end.

Testes paired. Spicules long, 4.5 anal body diameters, slightly curved with rounded head. Distal end of spicules with hooks. Gubernaculum long, without 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 diameters. Three terminal setae 15 µm long.

Females. Similar to males. Body slightly longer than that of males. Amphideal fovea smaller than that in males, 10 µm in diameter or 12.5% of corresponding body diameter. Reproductive system monodelphic, prodelphic, with single anterior ovary located to the right of intestine; no spermatheca was found. Vulva situated far posterior to mid–body (V=78–84%); vaginal glands present on either side of proximal portion of vagina. Tail conico-cylindrical without setae. Tail equal to 10% of total body in length or 3.5–3.7 anal body diameters with three terminal setae 15 µm long.

Relationships. Sphaerolaimus callisto sp. nov. belongs to the group of species with long spicules (> 2 anal body diameters in length) that includes S. cuenatus , S. kleini , S. macrocirculoides , S. macrocirculus , and S. penicillus . It is more similar to S. cuneatus in the shape of buccal cavity and indexes a and c. However, new species can be differentiated from S. cuneatus by longer body (2350–2690 µm vs 1980–1990 µm), index b (5.1–5.6 vs 3.9–4.1), absence of precolacal supplements, and longer spicules (321–356 µm vs 270 µm). Sphaerolaimus callisto sp. nov. differs from S. penicillus by longer body, shorter cephalic and subcephalic setae and longer spicules. The main dif- ference between S. callisto and the group of three remaining species, S. kleini , S. macrocirculoides and S. macrocirculus , is the relative spicule length: newly described species is characterized by the spicules longer than tail whilst other species are characterized by the spicules shorter than tail.

Figures

FIGURE 1. Sphaerolaimus callisto 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. Spicules and gebernaculum. Abbreviation: vs—ventral side. Scale bars: A, B, E—100 µm; C, D—20 µm; F—50 µm.

FIGURE 2. Sphaerolaimus callisto 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. Copulatory apparatus. F. Anterior end of female. G. Anterior end of female. Abbreviation: vs—ventral side. Scale bars: A, B—100 µm; C, D, F, G—20 µm; E—50 µm.

FIGURE 3. Sphaerolaimus callisto sp. nov. SEM. A. General view of male. B. Posterior end of male. C. Head end of male, apical view. D. Head end of male, lateral view. E. Anterior end of female, dorsal view.Abbreviations: a—amphid; cs—cephalic setae; ils—inner labial sensilla; ols—outer labial sensilla; scs—subcephalic setae; vs—ventral side. Scale bars: A—100 µm; B—20 µm; C, D, E—10 µm.