Onchium parocellatum ( Allgén, 1940 ) Gerlach, 1962

Holovachov, Oleksandr, 2015, Swedish Plectida (Nematoda). Part 8. The genus Onchium Cobb, 1920, Zootaxa 3911 (4), pp. 521-546 : 531-536

Onchium parocellatum ( Allgén, 1940) Gerlach, 1962

( Figs 6–8 ; Table 3 )

Type material. Lectotype female (slide # Type –8699) from the collection of C. Allgén.

Additional material. Four females (slides # 139325–139328) deposited in the general invertebrate collection of the Department of Zoology, Swedish Museum of Natural History, Stockholm, Sweden.

Type locality. Norway, Vega.

Additional localities. Sand and shells from 15–22 m deep, Bonden island in Skagerrak off the west coast of Sweden (N 58 12' 37'', E 11 18' 53''), 0 9 August 2011, legit O. Holovachov (one female); gravel, mud and algae from 30–70 m deep, Skagerrak off the west coast of Sweden (N 58° 22' 14'', E 11° 05' 00''), 0 9 August 2011, legit O. Holovachov (one female); coarse gravel from 30–50 m deep, Skagerrak off the west coast of Sweden (N 58° 17' 58'', E 11° 10' 05''), 0 9 August 2011, legit O. Holovachov (two females).

Description. Male. Not found.

Female. Body slender, weakly fusiform or cylindrical over most of its length, tapering anteriorly in anterior half of pharyngeal region and posteriorly on tail; strongly ventrally curved upon fixation, especially so along tail. Cuticle finely annulated; annules without ornamentation. Lateral field absent. Crystalloids absent. Body pores and epidermal glands absent. Somatic sensilla present, most prominent along pharyngeal region and on tail; anteriormost somatic sensilla are located anterior to ocelli. Labial region rounded, continuous with body contour; lips fused. Annulation along anteriormost part of body very weak; cuticle encircling labial region does not form "cephalic capsule". Inner and outer labial sensilla indistinct. Cephalic sensilla setiform. Subcephalic and cervical sensilla absent. Amphidial fovea ventrally-unispiral, located anterior to cephalic sensilla bases. Ocelli present, each ocellum consists of anteriorly located small opaque "lens" and posteriorly located large pigment body; two such bodies are present on left body side in lectotype specimen. Nerve ring surrounding pharynx at anterior third of its length. Hemizonid not seen. Secretory-excretory system present; renette cell visible only in some specimens, located on right-hand side of body, along anterior part of intestine; it extends anteriorly along ventral side of pharynx and forms an excretory ampulla just posterior to nerve ring level. Excretory canal weakly cuticularised, extends from excretory ampulla anteriorly towards lip region and opens to exterior on ventral side, at base of lips, anterior to cephalic sensilla. Oral opening terminal. Buccal cavity uniformly tubular: cheilostom barrel-shaped; stegostom tubular, with large dorsal onchiostyle. Onchiostyle uniformly cylindrical, bluntly rounded; connected to dorsal body side by oblique muscles. Pharynx glandular, cylindrical anteriorly, expanding posteriorly; not subdivided into sections; without bulbs; valvular apparatus absent. Dorsal pharyngeal sector strongly developed over entire pharynx length, more so in its posterior part. Subventral pharyngeal sectors weakly developed along anterior part (anterior to nerve ring) and indistinct posterior to nerve ring level. Dorsal pharyngeal gland orifice opens at base of onchiostyle; orifices of subventral pharyngeal glands indistinct. Cardia short, embedded in intestine. Several large coelomocytes are present on each body side along posterior part of pharynx and entire intestine, their cell bodies extend anteriorly into narrow ducts that are located laterally. Reproductive system didelphic, amphidelphic; ovary branches reflexed antidromously. Anterior genital branch 183–224 µm long (equal to 6.9–9.9% of total body length), located on right-hand side of intestine (n = 4), posterior genital branch 179–335 µm long (equal to 6.8–12.7% of total body length), located on left-hand side of intestine (n = 4). Oviduct narrow, tubular. Spermathecae filled with sperm. Uterus composed of large cells. Vagina straight, 0.1–0.3 times vulval body diameters long. Epiptygmata, pars refringens vaginae and sensitive structures around vulva absent. Rectum 0.6–1.2 anal body diameters long. Tail elongate-conoid, ventrally curved. Three caudal glands present, their nuclei are incaudal. Spinneret broad conoid, functional.

Diagnosis. Onchium parocellatum is particularly characterised by the 2.27–2.92 mm long body; anterior end without cephalic capsule; anteriormost somatic sensilla located anterior to ocelli; ocelli with small opaque "lens" and large pigment body; excretory pore located at base of lips, anterior to cephalic sensilla; onchiostyle uniformly cylindrical; male unknown.

Remarks. Allgén's slide includes two specimens, one complete female drawn by Allgén (1940, Figs 4 a–b) here designated as lectotype of Onchium parocellatum , and one partial specimen belonging to Onchium robustum Gerlach, 1965 . Although strongly flattened ( Fig. 7 ), the lectotype still clearly shows major diagnostic features of this species, including the shape of the anterior end, shape and size of onchiostyle, size of ocelli and position of excretory pore. This species can be easily distinguished from closely related O. ocellatum by position of the excretory pore (at base of lips vs. at level of nerve ring in O. ocellatum ) and from O. robustum by shape of the anterior body end (without cephalic capsule vs. with cephalic capsule in O. robustum ), shape of onchiostyle (cylindrical vs. expanding posteriorly in O. robustum ) and position of excretory pore (at base of lips vs. at level between onchiostyle base and ocellus in O. ocellatum ).

Figures

FIGURE 4. Onchium metocellatum Wieser, 1956, male (A – B, G) and female (C – F, H). A, C: Anterior end, median section; B, D: Anterior end, surface view; E: Female tail; F: Female reproductive system (anterior part); G: Male posterior region; H: Pharyngeal region, median section. Scale bars: A – D, G = 20 µm, E – F, H = 50 µm.

FIGURE 6. Onchium parocellatum (Allgén, 1940) Gerlach, 1962, female. A: Anterior end, median section; B: Anterior end, surface view; C – D: Pharyngeal region (anterior and posterior part); E: Tail; F: Reproductive system (anterior part). Scale bars: A – B = 20 µm, C – F = 50 µm.

FIGURE 7. Onchium parocellatum (Allgén, 1940) Gerlach, 1962, female lectotype. A: Anterior end showing ocelli (arrow); B: Anterior end showing onchiostyle (arrow); C: Anterior end showing excretory pore (arrow); D: Entire specimen. Scale bars: A – C = 20 µm, D = 200 µm.

FIGURE 8. Onchium parocellatum (Allgén, 1940) Gerlach, 1962 female. A: Anterior end showing ocelli; B: Anterior end showing onchiostyle and excretory pore (arrow); C: Coelomocytes; D: Entire specimen (arrows point at coelomocytes). Scale bars: A – B = 20 µm, C = 100 µm, D = 200 µm.

Tables

TABLE 3. Morphometrics of Onchium parocellatum (Allgén, 1940) Gerlach, 1962 from the Swedish west coast (all measurements are in µm, except for the ratios a, b, c, c', V, N R).

	Lectotype ♀	4 ♀♀
Body length	2271	2656±200 (2435–2920)
a	22.1	52.1±12.4 (37.6–62.7)
b	6.4	6.0±0.4 (5.4–6.4)
c	18	19.5±2.1 (17.2–22.1)
c'	2.9	5.1±1.0 (3.8–6.4)
V(%)	53.8	53.0±3.2 (48.6–55.5)
Body diameter	103	53.0±12.1 (42.0–70.0)
Pharynx length	353	447±36 (410–493)
Tail length	126	137±12 (119–143)
Anal or cloacal body diam.	43.0	27.2±3.6 (22.5–31.0)
Labial region diameter	11.0	11.2±0.4 (11.0–11.5)
Cephalic setae length	4.0	3.6±0.6 (3.0–4.0)
Amphid from ant. end	1.5	1.4±0.6 (1.0–2.0)
Amphid width	3.5	2.5±0.4 (2.0–3.0)
Onchiostyle length	17.0	15.2±1.4 (14.0–17.0)
Ocellum "lens"	24.0	23.4±0.7 (23.0–24.0)
Ocellum body	28.5	28.4±0.4 (28.0–28.5)
Anteriormost body pore	?	15.9±3.9 (10.0–18.0)
Nerve ring from ant. end	110	133±18 (114–148)
NR (%)	31	29.0±2.0 (26.6–30.2)
Excretory pore from ant. end	5.0	2.5±0.7 (2.0–3.5)
EP (%)	1.4	0.6±0.1 (0.5–0.7)
Vagina length	13.5	11.1±1.5 (9.0–13.0)
Rectum length	27.5	24.6±1.7 (22.5–26.0)