Plectus geophilus

Plectus geophilus de Man, 1880

(Figs. 1, 2)

Measurements. Table 1.

Description. Adult: Body slightly arcuate upon fixation, regularly tapering towards extremities, more towards posterior end. Cuticle 1.0–1.5 µm thick; outer cuticle very finely striated, inner cuticle smooth; inter-striae distance ca 0.6–0.8 µm; tail striae relatively discernible in LM. Hypodermis devoid of glands. Lateral fields with two slightly-spaced cuticular alae. Somatic setae very few projecting perpendicular to body surface, usually inconspicuous. Lip region continuous with adjoining body contour, low flattened to truncate. Lips triangular with tapering inner ends appearing bifid in SEM. Inner labial sensilla around oral opening and outer labial sensilla located in depressions. Cephalic sensilla setose, 1.0–1.5 µm long, outwardly directed usually perpendicular to longitudinal body axis. Amphidial fovea open circular, 2–3 µm wide, located at middle or posterior to middle level of stoma. Stoma plectoid, tubular; cheilostom not cuticularised, with walls diverging posteriorly; gymnostom cuticularised continuing into equally wider stegostom; posterior part of stegostom tapering towards base. Pharynx comprising of anterior cylindrical corpus continuing into a nearly indistinguishable isthmus and a round to ovoid basal bulb of 10– 16 x 8–10 µm dimension with a simple grinder (= valvular apparatus); post-bulbar extension of variable length ranging from 4–8 µm. Cardia surrounding post-bulbar extension. Nerve ring encircling pharynx at ca 50–60% of its length. Secretory-excretory pore slightly posterior to nerve ring, at ca 65–70% of pharyngeal length; secretoryexcretory duct inconspicuous, making two turns in one specimen (Fig.1 C). A pair of pseudocoelomocytes present between cardia and anterior reflexed ovary measuring ca 1.2–1.6 corresponding body diameter in length. Intestine granular with wide lumen. Rectum 0.8–0.9 times anal body diameter in length. Anus a crescent-shaped slit.

Female: Reproductive system didelphic, amphidelphic, compactly built; ovaries reflexed antidromously with alternately arranged developing oocytes. Spermatheca absent. Crustaformeria not differentiated. Uterus occasionally with smooth-shelled intra-uterine eggs of 40– 53 x 14–18 µm dimension; sperms absent in genital tract. Vagina 20–25% of the corresponding body diameter; provided occasionally with weak epiptygmata (vulva forming folds with inner vaginal wall) and a pair of sphincter muscles. Vulva nearly equatorial, a transverse slit; vulval lips usually slightly protruded. Tail cylindrical, slightly arcuate, regularly tapering to a bluntly rounded terminus. Caudal setae 5 including one subventral and one lateral pair pointing downward. Terminal seta 6–12 µm anterior to tail terminus. Three linearly arranged caudal glands opening to exterior through 1–2 µm long spinneret.

Male: Not found.

Locality and habitat. A sample containing Plectus geophilus was collected from a small ditch (alluvium soil) at Keoladeo National Park, Bharatpur, Rajasthan, India.

Voucher specimens. Seven females on slide Plectus geophilus de Man, 1880 KNP/ 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. [One female was used for SEM study].

Salient characters. A small species having lip region continuous with adjoining body; lips amalgamated with bifid inner ends; cephalic setae perpendicular to main body axis, amphidial fovea circular, located at middle or posterior to middle level of stoma; secretory-excretory duct usually inconspicuous, pharyngeal bulb weak with simple grinder, vagina with weak epiptygmata and one pair of sphincter muscles; tail spinneret simple.

Remarks. P. geophilus, also described by Schneider (1937), Novikova and Gagarin (1971), Zell (1993) and Andrássy (2005), has been reported for the first time from the Indian subcontinent. It has so far been reported from all continents except Antarctica (Andrássy, 2005). The present population shows conformity to P. geophilus in most morphological and morphometric characteristics. However, some very minor differences were observed. The present specimens are relatively smaller in size (0.31–0.35 mm vs 0.34–0.52 mm) showing a smaller range of linear values besides having a relatively anterior vulva (V= 44.2–50.7 vs 49.7–55.6). The present population differs from closely related P. intorticaudatus Truskova, 1976 in having a relatively greater ‘a’ value (15.5–19.9 vs 15.5–17.0) that links the range of values of P. intorticaudatus with P. geophilus; a relatively larger tail (35–45 µm vs 32–39 µm); and greater distance of terminal seta from tail tip [6–12 µm vs 3.5–5 µm in P. intorticaudatus apud Zell (1993)]. The ‘c’ value (10.0–11.6) of latter, as indicated in the original description (Truskova, 1976), is much greater than that (7.9–9.1) estimated in the present population. On critically examining the allometric ratios and characteristics, P. intorticaudatus very much fits in P. geophilus except for the ‘a’ value and the position of terminal seta.

The present specimens differ from P. minimus Cobb, 1893 in having smaller ‘a’ (15.5–19.9 vs 24.4–32.1); ‘b’ (3.5– 5.9 vs 3.1–3.6) and ‘V’ (44.2–50.7 vs 50.0–53.4) values; posterior amphid position, greater stoma length (14–20 µm vs 8.5–11.5 µm) and relatively greater tail length (35–45 µm vs 28–38 µm) and a simple [vs crooked spinneret in P. minimus apud Zell (1993) and De Ley & Coomans (1994 )]. The present specimens of P. geophilus also differ from P. communis Bütschli, 1873 in having a smaller body (0.31–0.35 mm vs 0.43–0.74 mm) and consequently a smaller tail (35–45 µm vs 50–87 µm); continuous (vs setoff lip region); longer cephalic setae (1.0–1.5 µm vs 3–5 µm) and basal bulb with simple (vs parietinus - type) grinder in P. communis apud Zell (1993). The present population differs from P. s i m i l i s Zell, 1993 in having smaller body (0.31–0.35 mm vs 0.44–0.64 mm); smaller ‘a’ value (15.5–19.9 vs 24.4–32.4); longer cephalic setae (1.0–1.5 µm vs 2.5–4 µm); continuous (vs setoff lip region) and a smaller tail (35–45 µm vs 46–75 µm) in P. communis apud Zell (1993).