Onchium longispiculum, Holovachov, Oleksandr, 2015

Holovachov, Oleksandr, 2015, Swedish Plectida (Nematoda). Part 8. The genus Onchium Cobb, 1920, Zootaxa 3911 (4), pp. 521-546 : 540-542

Onchium longispiculum sp. n.

( Fig 11 ; Table 5 )

Type material. Holotype male (slide # Type –8700) and three male and three female paratypes (slides # Type –8700 – Type –8705) deposited in the invertebrate type collection of the Department of Zoology, Swedish Museum of Natural History, Stockholm, Sweden.

Additional material. One male (slide # 139335) deposited in the general invertebrate collection of the Department of Zoology, Swedish Museum of Natural History, Stockholm, Sweden.

Type 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 (four males and three females).

Additional locality. 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 (one male).

Etymology. The specific epithet " longispiculum " refers to the length of the spicules in males of this species, which is unusual for the genus.

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 far behind base of onchiostyle. Labial region conoid, continuous with body contour; lips fused; ventral side projects slightly more anteriorly than dorsal side. 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 absent. Nerve ring surrounding pharynx at anterior third of its length. Hemizonid not seen. Secretory-excretory system present; renette cell large, 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 anteriorly towards lip region and opens to exterior on ventral side, at base of lips, anterior to cephalic sensilla. Oral opening subterminal. Buccal cavity uniformly tubular: cheilostom barrel-shaped; stegostom tubular, with dorsal onchiostyle. Onchiostyle cylindrical. 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 large coelomocytes are present on each body side along anterior part of intestine. Reproductive system diorchic, both testes outstretched. Spicules paired, usually asymmetrical, right spicule is often longer than left spicule, with arcuate cylindrical shaft and small manubrium; manubrium ventrally inclined. Gubernaculum present, platelike. Alveolar supplements indistinct. Tubular supplements absent. Midventral precloacal sensilla absent. Tail with three-four subventral papilliform sensilla. Tail elongate-conoid, ventrally curved. Three caudal glands present, their nuclei are incaudal. Spinneret conoid, functional.

Female. Body slender, fusiform, tapering anteriorly along 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 is located far behind base of onchiostyle. Labial region conoid, continuous with body contour; lips fused; ventral side projects a bit more anterior than dorsal side. 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 absent. 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 towards lip region and opens to exterior via a pore on ventral side, at basis of lips, anterior to cephalic sensilla. Oral opening subterminal. Buccal cavity uniformly tubular: cheilostom barrelshaped; stegostom tubular, with dorsal onchiostyle. Onchiostyle cylindrical. Pharynx glandular, subcylindrical 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 large coelomocytes present on each body side along anterior part of intestine. Reproductive system didelphic, amphidelphic; ovary branches reflexed antidromously. Anterior genital branch 93–141 µm long (equal to 7.0–9.0% of total body length), located on either right-hand side (n=2) or left-hand side (n=1) of intestine, posterior genital branch 131–152 µm long (equal to 9.1–9.9% of total body length), located on left-hand side (n=2) or right-hand side (n=1) of intestine. Oviduct short. Spermathecae filled with filamentous sperm. Uterus voluminous. Vagina straight, 0.1–0.2 times vulval body diameters long. Epiptygmata, pars refringens vaginae and sensory structures around vulva absent. Rectum 0.9–1.1 anal body diameters long. Tail elongate-conoid, ventrally curved. Three caudal glands present, their nuclei are incaudal. Spinneret conoid, functional.

Diagnosis. Onchium longispiculum sp. n. is particularly characterised by 1.34–1.77 mm long body; anterior end without cephalic capsule; anteriormost somatic sensilla located posterior to onchiostyle base; ocelli absent; excretory pore located at basis of lips; onchiostyle uniformly cylindrical; alveolar supplements indistinct, tubular supplements absent; spicules arcuate and 44–65 µm long.

Relationships. Onchium longispiculum sp. n. can be separated from all other valid species of the genus Onchium by the absence of developed ocelli and very long somewhat asymmetrical spicules. Additional characters separating it from other species of the genus Onchium are summarised in the Table 6 .

Figures

FIGURE 11. Onchium longispiculum sp. n., male (A – C) and female (D – H). A: Male posterior region; B, E: Anterior end, median section; C, F: Anterior end, surface view; D: Pharyngeal region; G: Female tail; H: Female reproductive system (posterior part). Scale bars: A – C, E – F = 20 µm, D, G – H = 50 µm.

Tables

TABLE 5. Morphometrics of Onchium longispiculum sp. n. from the Swedish west coast (all measurements are in µm, except for the ratios a, b, c, c', V, NR).

	Holotype ♂	5 ♂♂ (including holotype)	3 ♀♀
Body length	1693	1686±73 (1565–1765)	1338–1583
a	82.8	84.2±10.3 (68.9–93.9)	32.7–37.4
b	4.9	4.1±0.6 (3.4–4.9)	4.8–6.3
c	13.3	13.6±1.2 (11.8–15)	13.2–15.0
c'	7.3	7.5±1.7 (6–10.3)	6.1–7.3
V(%)	–	–	54.6–57.9
Body diameter	20.5	20.2±2 (18.0–23.0)	41.0–48.0
Pharynx length	343	418±58 (343–503)	245–277
Tail length	127	125±15 (105–144)	98–116
Anal or cloacal body diam.	17.5	17.0±2.1 (14.0–20.0)	16.0–17.0
Labial region diameter	7.0	7.1±0.5 (6.5–8.0)	6.5–8.0
Cephalic setae length	4.0	3.1±0.6 (3.0–4.0)	2.0–3.0
Amphid from ant. end	2.0	1.7±0.4 (1.5–2.0)	2.0–3.0
Amphid width	3.0	2.4±0.4 (2.0–3.0)	2.0
Onchiostyle length	8.0	8.3±1.4 (7.0–11.0)	9.0–10.0
Nerve ring from ant. end	90	95±8 (90–109)	92–107
NR (%)	26	23±2 (22–26)	34–44
Vagina length	–	–	7.0–9.0
Rectum length	–	–	15.0–17.0
Spicule length (right)	61	62±3 (59–68)	–
Spicule length (left)	55	58±8 (44–65)	–