Acrobeles ornatus Thorne, 1925

Boström, Sven & Holovachov, Oleksandr, 2019, Descriptions of species of Acrobeles von Linstow, 1877 (Nematoda, Rhabditida, Cephalobidae) from Kelso Dunes, Mojave National Preserve, California, USA, Zootaxa 4651 (2), pp. 330-350 : 336-340

Acrobeles ornatus Thorne, 1925

( Figs 7–10 , Table 2 )

Material examined. Five females and two males on slide # SMNH-149104, deposited in the invertebrate collection of the Department of Zoology, Swedish Museum of Natural History, Stockholm, Sweden.

Locality and habitat: USA, California, Mojave National Preserve, Kelso Dunes , soil around roots of desert plants (N 34 53.698’, W 115 42.155’), 28 March 2010, legit O. Holovachov & P. De Ley.

Description. Adult. Body arcuate ventrad when killed by heat, male posterior end generally strongly arcuate ventrad. Cuticle coarsely annulated, annuli 2.7–3.4 µm wide at midbody and 3.0–4.0 µm in pharyngeal region. Annuli with two rows of fine punctations closely adjacent to anterior and posterior margins of each annule, and with irregularly arranged large dots. Lateral field with three incisures (two wings), the outer crenate, extending almost to tail terminus in both sexes; occupying about one-fifth of midbody diameter. Lip region slightly offset, carrying six labial and four cephalic papillae and two small, rounded amphid openings. Six triangular lips arranged in three pairs, one dorsal and two ventrolateral. Pairs of lips separated by primary axils with two triangular-elongate guarding processes originating from the incomplete first annulus. Secondary axils separating the lips of the pair also with two triangular-elongate guarding processes but separate from the first annulus. Each lip asymmetrically triangular, with about 14–16 tines along its margin: 7–8 short tines facing the primary axil, 7–8 short tines facing the secondary axil and two longer apical tines. Three labial probolae 12.0– 14.5 µm high, deeply bifurcate into two prongs having an outer margin with 6 tines, an inner margin with 7 tines and two long, slender apical tines directed outwards with almost touching ends. Stoma length about one time lip region diameter. Stomatal parts less well discernible. Cheilorhabdia rounded in median section; metastegostom with a dorsal denticle. Pharyngeal corpus cylindricalfusiform with wide lumen; isthmus narrow, demarcated from corpus by a break in muscular tissue; bulb oval with valves. Cardia enveloped by intestinal tissue. Nerve ring at level of metacorpus-isthmus junction. Excretory pore and deirids at level of isthmus.

Female. Reproductive system monodelphic, prodelphic, in a dextral position in relation to intestine. Ovary reflexed posteriorly at oviduct, straight posterior to vulva. Spermatheca and postvulval uterine sac well developed. Vulval lips slightly protruding. Phasmids located at about one-third to half of tail length. Tail conoid with 18–20 ventral annuli, tail terminus pointed.

Male. Reproductive system monorchic, dextral in position; testis reflexed ventrad anteriorly. Spicules paired and symmetrical, curved ventrad; with oval manubrium and subcylindrical, gradually narrowing shaft. Gubernaculum plate-like. Genital papillae distributed as follows: one ventrosublateral pair at level of cloaca, and two ventrosublateral precloacal pairs at 49 µm and at 93–99 µm anterior to cloaca; a single midventral papilla on anterior cloacal lip; two pairs (one subventral and one lateral pair) at mid-tail; three pairs (one subdorsal, one subventral and one lateral pair) near tail terminus. Phasmids located at about two-fifths of tail length. Tail strongly curved ventrad, conoid, with a 10–12 µm long, pointed, terminal spike.

Diagnosis (supplemented with literature data). Acrobeles ornatus is characterised by a 789–903 µm long body in females and 730–797 µm in males; cuticle coarsely annulated; lateral field with three incisures (two wings), the outer crenate, extending almost to tail terminus in both sexes; six triangular lips arranged in three pairs; pairs of lips separated by primary axils with two triangular-elongate guarding processes; secondary axils separating the lips of the pair with two triangular-elongate guarding processes; each lip asymmetrically triangular, with about 14–16 tines along its margin: 7–8 short tines facing the primary axil, 7–8 short tines facing the secondary axil and two longer apical tines; three labial probolae, deeply bifurcate into two prongs having an outer margin with 6 tines, an inner margin with 7 tines and two long, slender apical tines directed outwards with almost touching ends; pharyngeal corpus 2.6–3.4 times isthmus length; nerve ring at level of metacorpus-isthmus junction; excretory pore and deirids at level of isthmus; spermatheca 36–60 µm long; postvulval uterine sac 30–41 µm long; spicules 47–49 µm long.

Remarks. The population of Acrobeles ornatus collected from the Kelso Dunes area and described here agrees in many respects with the type specimens described from Colorado by Thorne (1925). Some major differences are the tail length in females (65–72 µm vs about 46–49 µm in the type population) and the number of incisures in the lateral field (3 vs 2 µm in the type population, although Thorne’s Fig. 39d–e seem to indicate 3 incisures).

* Pharynx is often contracted and folded, thus these measurements and ratios are provisional; ** Number of annuli from anterior end to nerve ring, excretory pore and deirid, respectively; # Calculated from data in the original description; - indicates that data is not available or not applicable.

Figures

FIGURE 7. Acrobeles ornatus Thorne, 1925.A. Pharyngeal region. B. Female gonad. C. Female tail. D. Male tail. Scale bar=20 µm.

FIGURE 8. Acrobeles ornatus Thorne, 1925, photomicrographs. A-C. Labial region of a female at different focus levels. D. Anterior cuticle of a female. E-F. Variation of lateral field morphology at its anterior end (E), deirid (arrow) level (F) and midbody (G). Scale bar=20 µm.

FIGURE 9. Acrobeles ornatus Thorne, 1925 females, photomicrographs. A-B. Vulval region, median section, postvulval uter- ine sac. C. Posterior part of pharyngeal region (arrow indicates excretory pore). D. Tail, median section. E. Tail (arrow indicates phasmid). F. Tail, surface view. G. Scale bar=20 µm.

FIGURE 10. Acrobeles ornatus Thorne, 1925 males, photomicrographs.A. Tail, median section. B-E. Cloacal region, spicules and gubernaculum at different focus levels (arrow indicates phasmid in B, arrows indicate papillae near tail terminus in E). Scale bar=20 µm.