Microlaimus capitatus, Gagarin, 2020

Gagarin, Vladimir G., 2020, Microlaimus capitatus sp. n. and Dichromadora simplex Timm, 1961 (Nematoda, Chromadorea) from the coast of Vietnam, Zootaxa 4732 (2), pp. 323-331 : 324-325

Microlaimus capitatus sp. n.

( Figs 1 , 2 ; Table 1 )

Type material. Holotype male, slide reference number HC1.1.18, deposited in the collection of the Vietnam National Museum (Hanoi, Vietnam).

Paratypes. Seven males and four females on slides deposited in the nematode collection of the Department of Nematology , Institute of Ecology and Biological Resources (Hanoi, Vietnam) .

Type locality. South China Sea, littoral zone of coastal Vietnam. Latitude: 21º26ʹ13ʹʹ–21º26ʹ54ʹʹ. Longitude: 107º45ʹ47ʹʹ–107º45ʹ59ʹʹ. Depth 0.5–1.0 m, sand, salinity 13–14‰. Collected in March 2016.

Etymology. The specific epithet means “headed”, “of head”.

Description. Male. Body of average size and slender. Cuticle finely annulated, without dots or other punctations, 1.0– 1.5 µm thick at mid-body. Cuticular pores not visible. Head smooth, swollen, set off just anterior to amphidial fovea. Six inner and six outer papilliform labial sensillae. Four cephalic sensillae in the shape of thin setae 3–5 µm long, 35–45% of head diameter. Monospiral amphidial fovea occupying 45–50% of the corresponding body diameter and situated 7–8 µm from the anterior body end. Cheilostom small with longitudinal ribs. Pharyngostom narrow, with weakly sclerotized walls. Dorsal tooth small, weakly developed; two smaller subventral teeth situated slightly below dorsal tooth. Pharynx muscular, enlarged into almost spherical cardiac bulb posteriorly. Lumen of terminal bulb slightly dilated, its cuticle thickened or weakly sclerotized. Cardia small. Renette placed ventrally to the posterior part of cardiac bulb. Excretory pore posterior to nerve ring.

Gonads diorchic; testes opposite and outstretched. Anterior testis situated to left of intestine, posterior testis to right of intestine. Spicules paired, equal in length, curved, 1.4–1.6 times as long as cloacal body diameter. Spicules proximally cephalated and arcuate, distally acute. Gubernacula paired, 16–19 µm long, in the shape of a “scoop”, lying parallel to the spicules. Precloacal submedian supplements absent. Tail slender, elongate-conical, curved ventrally. Caudal glands poorly developed, with terminal spinneret. Terminal setae present on tail.

Female. Similar to male in general characteristics. Structure of cuticle and anterior body end as in male. Head smooth, swollen, offset just anterior to amphidial fovea. Inner and outer labial sensillae in the shape of papillae. Cephalic sensillae in the shape of thin setae. Amphidial fovea in the form of a spiral in one coil, located just behind head. Cheilostom small, with longitudinal ribs. Pharyngostom thin, poorly developed, with dorsal tooth and two smaller subventral teeth. Pharynx muscular, with large spherical cardiac bulb. Renette and excretory pore present.

Reproductive system didelphic, amphidelphic; ovaries homodromous, comparatively short.Anterior ovary situated to right side of intestine, posterior ovary to left side of intestine. Both uteri filled with numerous spermatozoa. Spermatheca not seen. Vagina short, extending to one quarter of the corresponding body diameter. Vulva a transverse slit, postequatorial. Vulva lips not sclerotized, not protruding outside the body contour. Tail elongate-conical, with caudal glands and spinneret.

Diagnosis. Body of average size and slender (L = 648–754 µm, a = 21–28). Cuticle finely annulated. Head smooth, swollen, set off just anterior to amphidial fovea. Amphidial fovea in the form of a spiral in one coil, located immediately behind head, occupying 45–55% of the corresponding body diameter. Inner and outer labial sensillae in the shape of papillae. Four cephalic sensillae in the shape of thin setae 3–5 µm long, 35–45% of head width. Pharyngostom thin, poorly developed, with dorsal tooth and two smaller subventral teeth. Pharynx muscular, with large spherical cardiac bulb. Paired spicules, equal in length, curved, 26–29 µm long; gubernacula paired, “scoop”- shaped, lying parallel to the spicules. Tail slender, elongate-conical. Terminal setae present on tail.

Differential diagnosis. The new species is morphologically similar to M. affinis Gerlach, 1958 and M. amphidius Kamran et al., 2009 . It differs from both of these species by the thin, poorly developed pharyngostom, more posterior vulva (V = 53.3–59.5% vs V = 50% in M. affinis and V = 50.7–51.6% in M. amphidius ) and presence of terminal setae on tail (absent in both other species). In addition, the new species differs from M. affinis in the longer spicules and gubernacula (26–29 µm and 16–19 µm long vs 24 µm and 11 µm long in M. affinis ), while from M. amphidius , it differs in the shorter cephalic setae (3–5 µm long vs 9 µm long in M. amphidius ) ( Gerlach 1958, Kamran et al. 2009).

Figures

FIGURE 1. Microlaimus capitatus sp. n. Holotype male and paratype female. A: male head; B: anterior body end of male; C: male tail; D: vulva region; E: female tail. Scale bars: A—7 µm; B, C—20 µm; D, E—30µm

FIGURE 2. Microlaimus capitatus sp. n. Holotype male and paratype female. A: male, entire body; B: female, entire body; C, D: male head; E, F: female head; G: male, anterior body end; H: female, vulva region; I: male, cloaca region; J: male tail; K: female tail; L: male, tail terminus. Scale bars: A, B—50 µm; G, H, J, K—10 µm; C–F, I, L—5 µm