Plectus aquatilis Andrassy 1985

Plectus aquatilis Andrássy, 1985

(Figs. 3, 4)

Measurements. Table 1.

Description. Adult: Body straight, slightly arcuate to ‘C’-shaped upon fixation, regularly tapering towards extremities, more towards posterior end. Cuticle 1.5–2.0 µm thick; outer cuticle smooth, inner cuticle finely striated. Lateral fields with two closely-placed cuticular alae. Hypodermis lacking glands. Somatic setae very sparse and small, 2–3 pairs of cervical setae observed in few specimens. Lip region continuous with adjoining body. Lips triangular with attenuated, cuticularised inner margins surrounding the oral aperture; inner labial sensilla inconspicuous, outer labials papilliform, often indistinct in SEM due to shrunk lips. Cephalic sensilla setose, 3–4 µm long, anteriorly directed. Amphidial fovea round with prominent, cuticularised ductus amphidialis, located middle or anterior to middle of stoma. Stoma cylindrical; cheilostom slightly arched with anterior refractive pieces, gymnostom thickly cuticularised, wide, cylindrical; stegostom slightly narrower, differentiated into anterior wider and posterior narrower region tapering at base. Pharynx comprising of anterior cylindrical corpus, a relatively undifferentiated isthmus and an ovoid, relatively weak basal bulb of 26– 35 x 17–25 µm dimension with a parietinus -like grinder having 8–10 pairs of denticulate ridges. Post-bulbar extension about 12–15 µm long. Cardia obscured by post-bulbar prolongation. Nerve ring encircling pharynx at ca 47–51% of its length. Secretory-excretory pore slightly posterior to nerve ring, at ca 54–56% of pharyngeal length. Secretory-excretory duct cuticularised forming one and a half loop before finally connecting with renette cell. Intestine granular with wide lumen. One pair of pseudocoelomocytes present in the anterior region of intestine measuring less than corresponding body diameter in length. Rectum ca 0.7–0.9 anal body diameter long with exceptionally wide lumen and three rectal glands.

Female: Reproductive system didelphic, amphidelphic compactly built; ovaries reflexed antidromously with developing oocytes. Anterior ovary on right and posterior on left side of intestine. Spermatheca absent. Uterus with smooth-shelled intra-uterine eggs of 46– 49 x 28–34 µm dimension. Vulva nearly equatorial representing an ovoid transverse slit; vulval lips protruded; anterior vulval lip with advulval papilla. Vagina 30–35% of the corresponding body diameter; with weak epiptygmata; two pairs of sphincter muscles appearing as elliptical pieces in cross section. Vulva-anus distance 264–491 µm. Tail cylindrical, slightly arcuate, regularly tapering to a bluntly rounded terminus. Caudal setae five including a lateroventral pair and a lateral pair pointing downward. Terminal seta 18– 25 µm anterior to tail terminus. Three linearly arranged caudal glands opening to exterior through 2 µm long spinneret.

Male: Not found.

Locality and habitat. A sample containing P. aquatilis was collected from a pool at Nainital, Uttaranchal, India

Voucher specimens. Eight females on slide P. aquatilis Andrássy, 1985 NU/ 1–5 deposited in the Nematode Collection, Department of Zoology, Aligarh Muslim University, Aligarh, Uttar Pradesh, India. One female deposited at USDANC, Beltsville, MD, USA. [Two females were used for SEM study].

Salient characters. A medium- to large-sized species with lip region continuous to adjoining body; lips with attenuated and cuticularised inner ends; cephalic setae forwardly directed making an acute angle with main body axis; amphidial fovea small, circular, at middle or anterior to middle level of stoma; secretory-excretory duct conspicuous with turns, pharyngeal bulb weak with parietinus -like grinder, vagina with weak epiptygmata and two pairs of sphincter muscles; rectum with conspicuously dilated lumen and tail spinneret simple.

Remarks. P. aquatilis is a relatively common species that has been reported (Andrássy, 1990; 1992; 1997; 2002; 2005) primarily from aquatic habitats and occasionally from moist soil. The species has been widely reported (Andrassy, 2005) from all the continents except Antarctica and due to adaptive qualities, it has even been reported from the Driefontein Mine at a depth of 0.9 km (Borgonie et al., 2011). The present population, the first report of the species from India, shows conformity to P. aquatilis in most morphological and morphometric characteristics. However, some minor differences were observed. The present specimens are relatively smaller in size (0.7–1.2 mm vs 0.8–1.4 mm) with a smaller stoma (20–22 µm vs 24–26 µm). The males could not be found in the present population as also stated in the original description (Andrássy, 1985), however, males have been described and illustrated later (Zell, 1993) with three conspicuous supplements. P. aquatilis most closely resembles P. i n d i c u s Khera, 1972, among the species placed under the ‘ aquatilis ’ group by Zell (1993), in most morphometric and morphological characteristics. However, one major difference observed between the two is the absence of supplements in males of P. indicus. Some other differences from P. indicus, largely observed in the present population (comprised of females only), are the thickly cuticularised and curved cheilostomal walls; gymnostom with parallel walls; vulva a relatively shorter ovoid slit and considerably dilated rectum [vs cheilostomal walls moderately cuticularised and straight; gymnostom with arched walls; vulval slit large, narrow and rectum not dilated in P. indicus apud Zell (1993) and Zullini, Loof & Bongers (2002)]. The present population of P. aquatilis differs from P. communis in having relatively larger body (0.70–1.23 mm vs 0.43–0.74 mm); greater ‘c’ value (7.7–9.6 vs 4.2–6.4); forwardly directed (vs anteriorly directed) cephalic setae; relatively larger stoma (20–22 µm vs 14.5–21.0 µm); weak (vs strong) basal bulb; relatively longer tail (73–160 µm vs 50–87 µm); dilated (vs normal) lumen of rectum and farther placed terminal seta from tail terminus [18–25 µm vs 8–15 µm in P. c o m m u n i s apud Zell (1993)]. The present population of P. aquatilis further differs from P. magadani Kuzmin, 1979 in having relatively larger body (0.70–1.23 mm vs 0.51–0.81 mm); wider (11–14 µm vs 8.5–10.5 µm) and continuous (vs offset) lip region; relatively larger stoma (20–22 µm vs 15–21.5 µm); weak (vs strong) basal bulb; dilated (vs normal) lumen of rectum; relatively longer tail (73–160 µm vs 54–92 µm) and farther placed terminal seta from tail terminus (18–25 µm vs 8–14 µm in P. magadani apud Zell (1993).