Plectus minimus Cobb, 1893

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 : 12-15

Plectus minimus Cobb, 1893

( Figs. 7 , 8 )

Measurements: Table 1

Description. Adult: Body small to medium-sized, slender, almost straight or slightly curved ventrally after fixation, tapering at both extremities, more towards posterior region. Outer cuticle with fine, transverse striations, inner cuticle smooth. Somatic setae fine and sparse, usually 3 pairs present in the cervical region. Lateral fields with weak cuticular alae. Lip region continuous with adjoining body contour. Lips amalgamated. Outer and inner labial sensilla papilloid, inconspicuous in LM; cephalic sensilla setose, 2.0–2.5 µm long, anteriorly directed, located at one lip diam. from anterior end. Amphids oval-shaped, situated at middle level of stoma. Stoma plectoidtype, tubular, thin-walled, weakly cuticularised, about three lip diam. long. Cheilostom rod-shaped slightly cuticularised with refringent dot-like anterior part; gymnostom cuticularised, parallel-walled; stegostom uniformly wide with abrupt narrowing at base. Pharynx comprising of a cylindrical corpus with indistinguishable isthmus and an elongate to ovoid muscular, basal bulb of 22– 33 x 11–13 µm dimension, gradually tapering into a post-bulbar part; anterior part of basal bulb having a simple grinder. Nerve ring located at about 48–51% of pharyngeal length. Secretory-excretory pore posterior to nerve ring or at about 54–58% of pharyngeal length with the duct forming a loop before joining the renette cell. Body at pharyngeal end 3.0–3.5 lip diameters wide. Cardia 4–7 µm long with narrow lumen. Intestine transparent, with wide lumen. Rectum thin-walled, ca 0.8–1.0 times anal body diam. long. Anus a crescent-shaped slit.

Female: Reproductive system didelphic-amphidelphic. Ovaries dorsally reflexed. Anterior ovary on right and posterior on left side of intestine. Oocytes arranged in double rows at distal end of ovary followed by a single tile. Vagina at right angle to longitudinal body axis, 1/3 of corresponding body diameter without a distinct epiptygmata and with a pair of sphincter muscles. Vulva a transverse slit, almost equatorial. Tail ventrally arcuate, gradually tapering to a narrow tip provided with a crooked terminal spinneret. Caudal setae including two subdorsal pairs, one lateral pair and one subventral pair, pointing downwards. Terminal seta 8–10 µm anterior to tail terminus. Caudal glands linearly arranged with prominent nuclei.

Male: Not found

Locality and habitat. A sample containing Plectus minimus Cobb, 1893 was collected from a small ditch (alluvium soil) at the outskirts of Aligarh, Uttar Pradesh, India.

Voucher specimens. Ten females on slide Plectus minimus Cobb, 1893 NOSR / 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 to adjoining body; lips with simple, smooth inner ends; cephalic setae forwardly directed making an acute angle with main body axis; amphidial fovea usually oval, occasionally circular, at middle level of stoma; secretory-excretory duct inconspicuous proximally, pharyngeal bulb weak with simple grinder, vagina simple without folded walls, provided with one pair of sphincter muscles, tail terminus having a crooked spinneret..

Remarks. Plectus minimus , although reported by various workers, namely Geraert (1962); Zell (1993); De Ley & Coomans (1994) and Khan and Araki (2001b), is recorded for the first time from the Indian subcontinent. The present population conforms well to the original population described by Cobb in 1893 in most morphological characteristics but differs in having larger females (0.4–0.5 mm vs 0.2–0.3 mm) with greater vulva-anus distance (4.1–5.6 vs 2–4) in terms of tail length. The present population also resembles the P. minimus population described by De Ley & Coomans (1994) in most morphometric and morphological details including crooked spinneret; however, it differs from latter in having larger females (0.4–0.5 mm vs 0.2–0.3 mm) and greater ‘c’ value (10.8–14.2 vs 7.4–9.7). The present population of P. m i n i m u s shows some minor differences from previously described populations of the species in having relatively larger body (0.43–0.52 mm vs 0.24–0.35 mm); greater ‘c’ value (10.8–14.2 vs 8.5–10.5); relatively larger stoma (15–17 µm vs 8.5–11.5 µm); weak (vs strong) basal bulb; relatively longer tail (32–45 µm vs 28–38 µm) and terminal seta placed farther from tail terminus [8–10 vs 7 µm in P. minimus apud Zell (1993) ]. The present population of P. minimus closely resembles P. pusillus Cobb, 1893 , but shows some minor differences from the latter species in having a relatively larger body (0.43–0.52 mm vs 0.35–0.45 mm); greater ‘c’ value (10.8–14.2 vs 8.4–10.0); and more anteriorly placed amphids (8–11 µm vs 13.5 µm). The most conspicuous difference can be observed in the crooked vs simple spinneret in P. pusillus apud Zell (1993) . Though the present population of P. minimus resembles P. geophilus in some respects, some differences could be observed viz., the relatively greater ‘c’ (10.8–14.2 vs 8.2–12.9) and smaller ‘c'’ (2.6–4.0 vs 3.9–6.4) values; wider lip region (6–7 µm vs 4.5–5.5 µm) and above all a crooked [vs simple spinneret in P. geophilus apud Zell (1993) ].

Figures

FIGURE 7. Plectus minimus Cobb, 1893. A: Female, entire. B, C: Anterior end. D: Pharyngeal region. E: Female reproductive system. F: Caudal region.

FIGURE 8. Plectus minimus Cobb, 1893. A – C: Anterior end. D: Body region showing posterior pharynx. E: Female reproductive system. F: Caudal region. G – I: Tail terminal region (Scale bar = 5 µm).