Onchium metocellatum (Wieser, 1956) Gerlach & Riemann 1973
Creators: name: Holovachov, Oleksandr
Publisher: name: Zenodo
Year: 2015
Subjects: Biodiversity
Taxonomy
Animalia
Nematoda
Adenophorea
Araeolaimida
Leptolaimidae
Onchium
Onchium metocellatum

description: Onchium metocellatum (Wieser, 1956) Gerlach & Riemann, 1973
(Figs 3–5; Table 2)
Material examined. Four males and ten females (slides # 139317–139324, 139331, 139335) 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 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); 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 (one male), 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); 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 males and five 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 two small opaque round bodies which are often more visible in males than in females. Nerve ring surrounding pharynx at anterior third of its length. Hemizonid not seen. Secretory-excretory system present; renette cell indistinct; excretory ampulla just posterior to nerve ring level. Excretory canal weakly cuticularised, extends from excretory ampulla anteriorly along ventral side of pharynx and opens to exterior at base of labial region. Oral opening terminal. Buccal cavity uniformly tubular: cheilostom barrel-shaped; stegostom tubular, with large dorsal onchiostyle. Onchiostyle with sharp anterior cone, cylindrical shaft and bifurcated 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 indistinct posterior to nerve ring level. Dorsal pharyngeal gland orifice opens at base of onchiostyle; orifices of subventral pharyngeal glands indistinct. Cardia indistinct. A few coelomocytes present on each body side along the intestine. Reproductive system diorchic, both testes outstretched. Spicules paired, symmetrical, with weakly curved subcylindrical shaft and small manubrium; manubrium is ventrally inclined. Gubernaculum present, plate-like. Alveolar supplements recognizable in some specimens along pharyngeal region but their number can not be assessed with certainty. Tubular supplements absent. Midventral precloacal sensilla absent. Tail with two subventral pairs of papilliform sensilla. 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 at level with or posterior to ocelli. Labial region truncated, continuous with body contour; lips fused. Annulation along anteriormost part of body very weak; cuticle encircling labial region does not form 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 two small opaque round bodies. 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 the left-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 along ventral side of pharynx and opens to exterior at base of labial region. Oral opening terminal. Buccal cavity uniformly tubular: cheilostom barrel-shaped; stegostom tubular, with large dorsal onchiostyle. Onchiostyle with sharp anterior cone, cylindrical shaft and bifurcated base; connected to the 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 usually becoming 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. A few large coelomocytes are present on each body side along intestine. Reproductive system didelphic, amphidelphic; ovary branches reflexed antidromously. Anterior genital branch 160–245 µm long (equal to 8.8–13.2% of total body length), located on right-hand side of intestine (n = 9), posterior genital branch 119–260 µm long (equal to 6.2–14.1% of total body length), located on left-hand side of intestine (n = 9). Oviduct narrow tubular. Spermathecae filled with spermatozoa. Uterus composed of large cells. Vagina straight, 0.2–0.4 times vulval body diameters long. Epiptygmata, pars refringens vaginae and sensory structures around vulva absent. Rectum 0.8–1.6 anal body diameters long. Tail elongate-conoid, ventrally curved. Three caudal glands present, their nuclei are incaudal. Spinneret conoid, functional.
Diagnosis. Onchium metocellatum is particularly characterised by having 1.23–2.04 mm long body; anterior end without cephalic capsule; anteriormost somatic sensilla located at level with or posterior to ocelli; each ocellum consists of two small opaque round bodies; excretory pore located at base of lips, anterior to cephalic sensilla; onchiostyle with sharp anterior cone, cylindrical shaft and bifurcated base; male with alveolar supplements along pharyngeal region, tubular supplements absent; spicules arcuate and 17–24 µm long.
Remarks. We base our identification mainly on the size and shape of the onchiostyle and the position of ocelli relative to it: in both the recent population and in the original description provided by Cobb (1920) under the name Onchulella ocellata (later renamed to Onchium metocellatum Wieser, 1956) the onchiostyle is large and solid, and the ocelli are located at its base. Other characters mentioned in the original description of Onchulella ocellata are characteristic for the genus Onchium in general and can not be used for species identification. The only difference between the recent population and the female described by Cobb (1920) is the shape of the ocelli (consisting of two small opaque round bodies in recent specimens vs. yellowish with colourless lens in the original specimen). However, as shown in Onchium ocellatum, the shape of ocelli is variable within one species and can not be used as a sole distinguishing character in the genus Onchium. Camacolaimus conicaudatus Allgén, 1935 is synonymized with Onchium metocellatum on the same basis—shape and size of onchiostyle and position of ocelli are identical in our specimens and in C. conicaudatus as described by Allgén (1935).
description: Published as part of Holovachov, Oleksandr, 2015, Swedish Plectida (Nematoda). Part 8. The genus Onchium Cobb, 1920, pp. 521-546 in Zootaxa 3911 (4) on pages 527-531, DOI: 10.11646/zootaxa.3911.4.3, http://zenodo.org/record/242601