Onchium ocellatum Cobb, 1920

Holovachov, Oleksandr, 2015, Swedish Plectida (Nematoda). Part 8. The genus Onchium Cobb, 1920, Zootaxa 3911 (4), pp. 521-546 : 522-527

Onchium ocellatum Cobb, 1920

( Figs 1–2 ; Table 1 )

Material examined. Four males and eight females (slides # 139311–139316, 139318, 139320, 139322, 139329) 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 (one male); 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 male and two females); 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 males and two females); algae from 15–55 m deep, Skagerrak off the west coast of Sweden (N 58° 17' 24'', E 11° 10' 48''), 0 9 August 2011, legit O. Holovachov (one female); 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 (three 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 posterior 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 one to three oval pigment bodies of different size; "lens"-like structure absent; number of pigment bodies is different between different individuals and can even be different on opposite body sides in the same specimen; in some specimens ocelli not visible. Nerve ring surrounding pharynx at anterior third of its length. Hemizonid not seen. Secretory-excretory system present; renette cell located on 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 for a short distance and opens to exterior on ventral side, at level with or just anterior to nerve ring. Oral opening terminal. Buccal cavity uniformly tubular: cheilostom barrel–shaped; stegostom tubular, with large dorsal onchiostyle. Onchiostyle with acute tip and subcylindrical body. 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, small, platelike. Alveolar supplements present along pharyngeal region but visible only when specimen is in subventral position, they are very small and located at the bottom of a narrow ventral grove. Tubular supplements absent. Midventral precloacal sensilla absent. Tail usually with four pairs of papilliform sensilla: 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 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 posterior 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 one to three oval pigment bodies of different size; "lens"-like structure absent; number of pigment bodies is different between different individuals and can even be different on different body sides in same specimen. Nerve ring surrounding pharynx at anterior third of its length. Hemizonid not seen. Secretory-excretory system present; renette cell located on 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 for a short distance and opens to exterior on ventral side, at level with or just anterior to nerve ring. Oral opening terminal. Buccal cavity uniformly tubular: cheilostom barrel-shaped; stegostom tubular, with large dorsal onchiostyle. Onchiostyle with acute tip and subcylindrical body. 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 the 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 didelphic, amphidelphic; ovary branches reflexed antidromously. Anterior genital branch 83–197 µm long (equal to 4.7–8.8% of total body length), located on right-hand side of intestine (n = 6), posterior genital branch 90–231 µm long (equal to 6.2–10.4% of total body length), located on left-hand side of intestine (n = 6). Oviduct short. Spermathecae filled with sperm. Uterus voluminous. Vagina straight, 0.2–0.4 times vulval body diameters long. Epiptygmata, pars refringens vaginae and sensory structures around vulva absent. Rectum 0.8–1.4 anal body diameters long. Tail elongate-conoid, ventrally curved. Three caudal glands present, their nuclei are incaudal. Spinneret conoid, functional.

Diagnosis. Onchium ocellatum is particularly characterised by 1.44–2.23 mm long body; anterior end without cephalic capsule; anteriormost somatic sensilla located posterior to ocelli; ocelli with one to three oval pigment bodies of different size; excretory pore located just anterior to nerve ring level; onchiostyle uniformly cylindrical; male with alveolar supplements along pharyngeal region, tubular supplements absent; spicules arcuate and 34–42 µm long.

Remarks. This species was originally described based on a single juvenile specimen, presumably a female. Most of the characters mentioned in the original description of Onchium ocellatum are characteristic for the genus Onchium in general and can not be used for species identification. Present specimens agree with the original description of O. ocellatum in the position of excretory pore, which was clearly described and illustrated to be located just in front of nerve ring. Most of the other presently considered valid species of Onchium have the excretory pore located close to the anterior end, while its position is unknown in O. minutum Kito, 1981 . Therefore, we consider our population identical with O. ocellatum as described by Cobb (1920). For the same reasons (position of excretory pore) both the populations of Onchium ocellatum as described by Blome (1982) and Onchium parocellatum as described by Luc & De Coninck (1959) are considered conspecific with recent specimens.

The identity of specimens described as (or synonymised with) O. ocellatum by Allgén (1933, 1935, 1938), Bresslau & Stekhoven (1940), Gerlach (1962), Timm (1952) and Wieser (1956a, b) is impossible to verify without being able to study actual specimens. Their respective descriptions are rather incomplete and do not include all the important morphological data. At present, we will consider all these specimens to represent O. ocellatum for the sake of taxonomic stability.

Figures

FIGURE 1. Onchium ocellatum Cobb, 1920, female (A, E – H) and male (B – D). A: Pharyngeal region; B, E: Anterior end, median section; C, F: Anterior end, surface view; D: Male posterior region; G: Female reproductive system (anterior part); H: Female tail. Scale bars: A, G – H = 50 µm, B – F = 20 µm.

FIGURE 2. Onchium ocellatum Cobb, 1920, female (A, C – D) and male (B). A: Anterior end showing onchiostyle; B: Anterior end (left ventrosublateral view) showing the anteriormost alveole (arrow); C: Excretory ampulla and pore (arrow); D: Female and eggs inside the possible host. Scale bars: A – C = 20 µm, D = 200 µm.

Tables

TABLE 1. Morphometrics of Onchium ocellatum Cobb, 1920 from the Swedish west coast (all measurements are in µm, except for the ratios a, b, c, c', V, NR).

	4 ♂♂	8 ♀♀
Body length	1639±273 (1457–2042)	1781±238 (1435–2228)
a	76.5±12.6 (69.3–95.3)	50.5±6.1 (41.5–56.5)
b	5.4±0.3 (5.1–5.7)	4.4±0.4 (3.9–5.1)
c	11.7±1.0 (10.9–13.2)	14.1±1.2 (11.7–15.4)
c'	7.2±1.0 (6.6–8.7)	6.2±0.7 (5.6–7.9)
V(%)	–	53.5±2.1 (49.6–56.5)
Body diameter	21.4±0.6 (21.0–22.0)	36.0±8.2 (26.5–52.0)
Pharynx length	301±42 (259–359)	401±36 (366–453)
Tail length	1406±11 (131–155)	126±12 (109–145)
Anal or cloacal body diam.	19.5±1.1 (18.0–20.0)	20.7±3.2 (15.5–26.0)
Labial region diameter	9.8±0.9 (8.5–11.1)	11.5±1 (10.0–13.0)
Cephalic setae length	5.4±0.9 (4.0–6.5)	6.2±0.5 (6.0–7.0)
Amphid from ant. end	1.8±0.4 (1.5–2.0)	1.5±0.3 (1.5–2.0)
Amphid width	2.7±0.4 (2.0–3.0)	2.9±0.5 (2.0–3.5)
Onchiostyle length	13.0±0.4 (13.0–13.5)	14.4±1.3 (12.0–16.0)
Ocellum body (anterior)	24.8±1.2 (23.5–26.5)	26.6±1.9 (24.0–30.0)
Ocellum body (posterior)	27.0±1.2 (26.0–28.5)	29.9±1.4 (28.5–32.0)
Anteriormost body pore	33.1±1.1 (32.0–34.0)	35.0±4.8 (31.5–45.0)
Nerve ring from ant. end	97±8 (86–103)	114±12 (95–128)
NR (%)	33±7 (24–40)	28±2 (26–31)
Excretory pore from ant. end	94±4 (91–99)	107±10 (97–122)
EP (%)	32±45 (28–36)	27±2 (24–30)
Vagina length	–	9.6±2.0 (6.5–12.0)
Rectum length	–	21.1±3.4 (17.0–27.5)
Spicule length	38.9±3.4 (34.0–42.0)	–