Plectus parvus Bastian, 1865

Tahseen, Qudsia & Mustaqim, Malka, 2012, Descriptions of six known species of Plectus Bastian, 1865 (Nematoda, Plectida, Plectidae) from India with a discussion on the taxonomy of the genus, Zootaxa 3205, pp. 1-25 : 9-12

Plectus parvus Bastian, 1865

( Figs. 5 , 6 )

Measurements: Table 1.

Description. Adult: Body straight to slightly arcuate upon fixation, tapering towards extremities, more towards posterior end. Cuticle 0.8–1.0 µm thick; outer cuticle very finely striated, inner cuticle smooth. Striae relatively prominent in tail region. Hypodermis devoid of glands. Lateral fields with two cuticular alae. Deirids conspicuous, papilliform flanked by cuticular alae. Somatic setae sparse including 2–4 pairs in the cervical region. Lip region continuous with body contour. Lips triangular, amalgamated appearing radially ridged; inner labial sensilla inconspicuous; outer labials papilliform, not discernible in LM. Cephalic sensilla setose, 1.5–2.0 µm long, anteriorly directed. Amphidial fovea small, circular with prominent ductus amphidialis, located at posterior to middle level of stoma. Stoma largely cylindrical; cheilostom with conical or sloping walls, gymnostom wide with arched walls; stegostom cylindrical gradually tapering at base. Pharynx comprising of anterior cylindrical corpus, slightly narrow isthmus and an ovoid basal bulb of 11– 15 x 8–10 µm, dimension with a simple grinder. Post-bulbar extension usually very reduced, often indistinguishable from glandular, conoid cardia. Nerve ring encircling pharynx at ca 54–57% of pharyngeal length. Secretory-excretory pore slightly posterior to nerve ring, at ca 66–70% of pharyngeal length. Secretory-excretory duct cuticularised, undergoing one and a half turns to finally join the renette cell. Intestine granular with wide lumen. Rectum ca 0.7–0.9 times anal body diam. long. Anus a crescent-shaped slit.

Female: Reproductive system didelphic, amphidelphic, compactly built; ovaries dorsally reflexed antidromously with developing oocytes. Anterior ovary on right and posterior on left side of intestine; occasionally crossing each other. Spermatheca absent. Uterus occasionally with two eggs of 42– 46 x 15–18 µm dimension. Sperms absent in the genital tract. Vagina ca 20–25% of corresponding body diameter; usually with epiptygmata and one pair of sphincter muscles. Vulva nearly equatorial representing an ovoid, transverse slit; vulval lips slightly protruded. Vulva-anus distance 133–151 µm. Tail cylindrical, slightly arcuate, regularly tapering to a bluntly rounded terminus. Caudal setae seven including one subdorsal pairs, one lateral and one subventral pair, pointing downward. Terminal seta 6–11 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 parvus was collected from a field in Meerut, Uttar Pradesh, India.

Voucher specimens. Nine females on slide Plectus parvus Bastian, 1865 MUP /1–6 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 with lip region continuous with main body contour; lips with simple, smooth inner ends; cephalic setae forwardly directed making an acute angle with main body axis; amphidial fovea usually ovoid, at posterior to middle level of stoma; secretory-excretory duct conspicuous with turns, pharyngeal bulb without or with a reduced post-bulbar extension and simple grinder, vagina with epiptygmata and one pair of sphincter muscles; tail spinneret simple.

Remarks. P. parvus has been reported mainly from terrestrial habitats and rarely occurs in aquatic habitats. The present population is the first report of the species from India; however, it has been reported widely across the globe ( Andrássy, 2005). The present population shows conformity to the original and subsequently reported populations of P. p a r v u s in all morphological and morphometric characteristics, with some minor differences observed. The present specimens are relatively smaller in size (0.33–0.38 mm vs 0.35–0.55 mm), showing a smaller range of linear measurements and absence of males [vs males described apud Maggenti (1961); Zell (1993) Andrássy (2005)]. The present population of P. p a r v u s differs from P. geophilus in having relatively smaller ‘a’ (15.1–19.0 vs 19.2–34.8) and ‘ċ’ (3.1–4.4 vs 3.9–6.4) values; cephalic sensilla forming an acute (vs right) angle with main body axis and reduced [vs long) post-bulbar extension in P. geophilus apud Zell (1993) ]. The present population differs from P. c o m m u n i s in having smaller body (0.33–0.38 mm vs 0.43–0.74 mm); wider lip region (6 µm vs 8.5–11 µm); cephalic setae parallel (vs at right angle) to body axis; smaller stoma (13–15 µm vs 14.5–21 µm); simple [vs parietinus -like grinder and reduced [vs long post-bulbar extension in P. communis Zell (1993) ]. The present population differs from P. minimus in having smaller ‘a’ (15.1–19.0 vs 24.4–32.4), ‘b’ (3.7–4.0 vs 3.1–3.6) and ‘ċ’ (3.1– 4.4 vs 4.1–5.8) values; grinder located in anterior (vs middle) region of basal bulb; reduced (vs long) post-bulbar extension and tail spinneret simple [vs crooked in P. geophilus apud Zell (1993) and De Ley & Coomans (1994)].

Figures

FIGURE 5. Plectus parvus Bastian, 1865. A: Female, entire. B: Anterior end. C: Pharyngeal region. D: Female reproductive system. E: Caudal region.

FIGURE 6. Plectus parvus Bastian, 1865. A: En face view. B – D: Anterior end. E: Pharyngeal region. F: Lateral fields with deirids. G: Body region showing lateral alae. H: Female reproductive system. I: Vulval region. J, K: Caudal region (Scale bar = 5 µm).