Onchium robustum Gerlach, 1965

Holovachov, Oleksandr, 2015, Swedish Plectida (Nematoda). Part 8. The genus Onchium Cobb, 1920, Zootaxa 3911 (4), pp. 521-546 : 536-540

Onchium robustum Gerlach, 1965

( Figs 9–10 ; Table 4 )

Material examined. Seven males and 12 females (slides # 139326, 139329–139334) deposited in the general invertebrate collection of the Department of Zoology, Swedish Museum of Natural History, Stockholm, Sweden.

Habitat and locality. 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 (two males); 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 (three males and three females); algae from 20–40 m deep, Skagerrak off the west coast of Sweden (N 58° 21' 49'', E 11° 04' 42''), 0 9 August 2011, legit O. Holovachov (two males); coarse sediment with algae from 45–55 m deep, Skagerrak off the west coast of Sweden (N 58° 17' 32'', E 11° 11' 24''), 0 9 August 2011, legit O. Holovachov (nine females).

Description. Male. Body slender, 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 in posterior part. 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 at level with or posterior to ocelli. Labial region rounded, continuous with body contour; lips fused. Annulation along anteriormost part of body very weak; cuticle encircling labial region (from basis of labial region to level of middle of onchiostyle) thickened, forming "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. Nerve ring surrounding pharynx at anterior third of its length. Hemizonid not seen. Secretory-excretory system present; renette cell located on the left-hand side of body, along anterior part of intestine; it extends anteriorly along ventral side of pharynx and forms 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 level of onchiostyle base. Oral opening terminal. Buccal cavity uniformly tubular: cheilostom barrel-shaped; stegostom tubular, with large dorsal onchiostyle. Onchiostyle with sharp anterior cone, cylindrical shaft and dorsally expanded base; 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 completely reduced (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 anterior part of intestine, their cell bodies extend anteriorly into narrow ducts that are located laterally. Reproductive system diorchic, both testes outstretched. Spicules paired, symmetrical, with arcuate subcylindrical shaft and small manubrium; manubrium is ventrally inclined. Gubernaculum present, platelike with double caudal projections. Alveolar supplements present along pharyngeal region, small and often poorly discernible, extending from level of ocelli to level of cardia or more posteriorly; over 30 were observed in one individual. Tubular supplements absent. Midventral precloacal sensilla absent. Tail usually with four pairs of papilliform sensilla (rarely with only three): first and third pair (counting from cloaca) are subventral, second and fourth pair are sublateral. Tail elongate-conoid, ventrally curved. Three caudal glands present, their nuclei are incaudal. Spinneret broad conoid, functional.

Female. Body slender, 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; anteriormost somatic sensilla are located at level with ocellus body. Labial region rounded, continuous with body contour; lips fused. Annulation along anteriormost part of body very weak; cuticle encircling labial region (from basis of labial region to level of middle of onchiostyle) strongly thickened, forming a "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; "lens" not visible in some specimens. Nerve ring surrounding pharynx at anterior third of its length. Hemizonid not seen. Secretory-excretory system present; renette cell located on lefthand side of the body, along anterior part of intestine; it extends anteriorly along ventral side of pharynx and forms 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 level between onchiostyle base and ocelli. Oral opening terminal. Buccal cavity uniformly tubular: cheilostom barrel-shaped; stegostom tubular, with large dorsal onchiostyle. Onchiostyle with sharp anterior cone, cylindrical shaft and dorsally expanded base; 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 completely reduced (indistinct) posterior to nerve ring level. Dorsal pharyngeal gland orifice opens at base of onchiostyle; orifices of subventral pharyngeal glands indistinct. In general, onchiostyle and pharynx are more strongly developed in females than in males. Cardia short, embedded in intestine. Several large coelomocytes are present on each body side along posterior part of pharynx and anterior part of intestine, their cell bodies extend anteriorly into narrow ducts that are located laterally. Reproductive system didelphic, amphidelphic; ovary branches reflexed antidromously. Anterior genital branch 140–448 µm long (equal to 6.8–15.1% of total body length), located on right-hand side of intestine (n = 12), posterior genital branch 121–291 µm long (equal to 5.6–14.2% of total body length), located on left-hand side of intestine (n = 11). Oviduct tubular. Spermathecae filled with sperm. Uterus cells with granular cytoplasm. Vagina straight, 0.3–0.5 times vulval body diameters long. Epiptygmata, pars refringens vaginae and sensory structures around vulva absent. Rectum 0.7–1.2 anal body diameters long. Tail elongate-conoid, ventrally curved. Three caudal glands present, their nuclei are incaudal. Spinneret broad conoid, functional.

Diagnosis. Onchium robustum is particularly characterised by 1.39–2.96 mm long body; anterior end with prominent cephalic capsule; anteriormost somatic sensilla located at level with or posterior to ocelli; ocelli with small opaque "lens" and large pigment body; excretory pore located at level between onchiostyle base and ocelli; onchiostyle with sharp anterior cone, cylindrical shaft and dorsally expanded base; male with alveolar supplements along pharyngeal region, tubular supplements absent; spicules arcuate and 32–42 µm long.

Remarks. The Swedish specimens agree with the original description in all major diagnostic characters: body size and other measurements, shape of anterior body and presence of cephalic capsule, shape and size of onchiostyle and position of excretory pore.

Figures

FIGURE 9. Onchium robustum Gerlach, 1965, female (A – D, G, I) and male (E – F, H). A – B: Pharyngeal region (anterior and posterior part); C, E: Anterior end, median section; D, F: Anterior end, surface view; G: Female reproductive system (anterior part); H: Male posterior region; I: Female tail. Scale bars: A – B, G – I = 50 µm, C – F = 20 µm.

FIGURE 10. Onchium robustum Gerlach, 1965, female (A, D) and male (B – C, E – F). A – C: Anterior end showing variability in the shape of ocelli; D – E: Anterior end showing excretory pore (arrows); F: Anterior end showing two anteriormost alveolar supplements (arrows). Scale bars: A – F = 20 µm.

Tables

TABLE 4. Morphometrics of Onchium robustum Gerlach, 1965 from the Swedish west coast (all measurements are in µm, except for the ratios a, b, c, c', V, NR).

	7 ♂♂	12 ♀♀
Body length	1605±208 (1385–1971)	2293±359 (1717–2963)
a	80.8±3.6 (77.3–86.3)	65.5±8.1 (54.7–83.5)
b	5.3±0.4 (4.8–5.8)	4.6±0.7 (3.4–6)
c	14.3±1.5 (12.8–16.3)	18.2±2 (14.6–22)
c'	5.9±0.6 (4.7–6.5)	5.4±0.6 (4.6–6.6)
V(%)	–	55.3±3.2 (46.7–59.0)
Body diameter	20.1±2.1 (18.0–23.5)	35.2±5.4 (28.5–47.0)
Pharynx length	302±31 (260–343)	508±79 (355–624)
Tail length	112±9 (97–122)	125±8 (114–136)
Anal or cloacal body diam.	19.0±1.6 (17.0–21.5)	23.6±3.3 (19.0–29.0)
Labial region diameter	8.0±1.1 (7.0–10.0)	11.4±1.3 (9.0–13.5)
Cephalic setae length	3.4±0.3 (3.0–3.5)	4.3±0.5 (3.5–5.0)
Amphid from ant. end	0.9±0.3 (0.5–1.5)	1.6±0.4 (1.0–2.0)
Amphid width	2.0±0.5 (1.5–3.0)	2.7±0.4 (2.0–3.5)
Onchiostyle length	11.9±1.7 (10.0–14.0)	15.9±1.6 (13.0–18.5)
Ocellum "lens"	24.5±1.9 (22.0–27.0)	30.9±2.8 (28.0–33.5)
Ocellum body	28.3±2.0 (26.0–31.5)	33.3±2.7 (28.0–37.0)
Anteriormost body pore	35.6±3.5 (30.0–41.5)	34.0±3.8 (29.0–40.0)
Nerve ring from ant. end	107±10 (93–119)	133±11 (119–150)
NR (%)	36±2 (34–39)	27±4 (22–36)
Excretory pore from ant. end	9.7±1.6 (8.0–12.0)	25.0±5.9 (17.0–36.0)
EP (%)	3.2±0.3 (2.6–3.7)	5±1.4 (3.5–7.4)
Vagina length	–	13.4±1.5 (11.0–16.0)
Rectum length	–	24.0±4.9 (19.0–36.0)
Spicule length	36.7±3.5 (32.0–41.5)	–