Acrobeles undulatus Loof, 1964

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 : 331-334

Acrobeles undulatus Loof, 1964

( Figs 1–6 , Table 1 )

Material examined. Twenty-seven females and three males on slides # SMNH-149098–SMNH-149101, SMNH- 149103, SMNH-Type-8437 ( Nothacrobeles istvani ) 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’ and N 34 53.754’, W 115 42.248’), 28 March 2010, legit O. Holovachov & P. De Ley.

Description. Adult. Body variably arcuate when killed by heat, male posterior end generally strongly arcuate ventrad. Cuticle coarsely annulated, annuli 2.4–3.4 µm wide at midbody and 2.0–3.0 µm in pharyngeal region. Annuli with incised margins visible, which can be interpreted as two rows of punctations. Longitudinal striation absent. Lateral field with three incisures (two wings), the outer crenate; areolate; extending almost to tail terminus in both sexes; occupying about one-fifth to one-fourth 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 12–13 tines along its margin: 4–5 short tines and one longer apical tine facing the primary axil, 5 short tines and one long tine in the middle facing the secondary axil. Three labial probolae 12.0– 14.5 µm high, deeply bifurcate into two prongs having an outer margin with 10 tines, an inner margin with 8 tines and two longer apical tines directed forward; bases joined by tangential ridges. Small basal knob present. Stoma length about one time lip region diameter. Stomatal parts less well discernible. Cheilorhabdia oval in median section; metastegostom with a prominent dorsal denticle. Pharyngeal corpus cylindrical-fusiform with wide lumen; isthmus narrow, often wrinkled, demarcated from corpus by a break in muscular tissue; bulb oval with valves. Cardia enveloped by intestinal tissue. Nerve ring and excretory pore at level of metacorpus to metacorpus-isthmus junction. Deirids at level of metacorpus to isthmus.

Female. Reproductive system monodelphic, prodelphic, in a dextral position in relation to intestine. Ovary reflexed posteriorly at oviduct, straight or with a double flexure posterior to vulva. Spermatheca and postvulval uterine sac well developed. Vulval lips slightly protruding. Phasmids located at about 15 to 30% of tail length. Tail conoid with 12–16 ventral annuli, tail terminus minutely rounded.

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 30–33 µm and at 61–63 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 pointed, minutely rounded terminus.

Diagnosis (supplemented with literature data). Acrobeles undulatus is characterised by a 400–650 µm long body in females and 435–459 µm in males; cuticle coarsely annulated; annuli with incised margins; lateral field with three incisures (two wings), the outer crenate, areolate, 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 12–13 tines along its margin: 4–5 short tines and one longer apical tine facing the primary axil, 5 short tines and one long tine in the middle facing the secondary axil; three labial probolae, deeply bifurcate into two prongs having an outer margin with 10 tines, an inner margin with 8 tines and two longer apical tines directed forward; pharyngeal corpus 3.0–4.4 times isthmus length; nerve ring and excretory pore at level of metacorpus to metacorpus-isthmus junction; deirids at level of metacorpus to isthmus; spermatheca 21–42 µm long; postvulval uterine sac 24–63 µm long; spicules 23–24 µm long.

Remarks. The population collected from sand in the Kelso Dunes area, Mojave Desert, California and described here, agrees in many respects with the type specimens of Acrobeles undulatus described from Venezuela by Loof (1964). One major difference is that no males were found in the type population. Another difference is that the cuticle is described as divided into blocks by longitudinal striae in the type specimens. However, the brevity of the original description and poor state of type material has already caused much speculation about the exact morphology of cuticle in A. undulatus ( Rashid et al. 1990a, Mundo-Ocampo et al. 2003). Due to the absence of any other diagnostic differences between recent specimens and the original description of A. undulatus , and until new material from the type locality can be obtained, we consider both populations conspecific.

A female population of A. undulatus was also described from Iran by Amirzadi et al. (2013). These specimens have a shorter body than the type specimens and those from Kelso Dunes, other differences are: cuticle slightly tessellated, this more apparent in neck region, annules narrower (1–2 µm vs 2.4–3.4 µm), non-areolated lateral field as evidenced in drawings and photographs (vs distinctly areolated), more posterior phasmid position in females (15–33% of tail length vs 32–40% of tail length). All these differences suggest that the population from Iran represents a different species and it is therefore not included in the diagnosis of A. undulatus .

Figures

FIGURE 1. Acrobeles undulatus Loof, 1964.A. Pharyngeal region. B. Female gonad. C. Female tail. D. Male tail. Scale bar=20 µm.

FIGURE 2. Acrobeles undulatus Loof, 1964, SEM micrographs. A. Anterior end, lateral view (ventral right). B. Anterior end, lateral view (ventral left). C. Female tail (arrow indicates phasmid). D. Lateral field (arrow indicates deirid). E. Vulva and lateral field, subventral view. Scale bars=5 µm.

FIGURE 3. Acrobeles undulatus Loof, 1964, photomicrographs. A-B. Labial region and anterior cuticle of a male at different focus levels. B-C. Labial region and anterior cuticle of a female at different focus levels. E-G.Anterior cuticle and the beginning of lateral field of a female at different focus levels. Scale bars=20 µm.

FIGURE 4. Acrobeles undulatus Loof, 1964, photomicrographs. A-C. Variation in the lateral field morphology along pharyngeal region in male (A-B) and female (C). B. Deirid. D. Lateral field at mid body level of a female. Scale bars=20 µm.

FIGURE 5. Acrobeles undulatus Loof, 1964, photomicrographs.A-C. Female vulval region. D. Female tail, median section. E. Female tail, surface view (arrow indicates phasmid). Scale bars=20 µm.

FIGURE 6. Acrobeles undulatus Loof, 1964, photomicrographs.A-C. Male spicules and gubernaculum at different focus levels (arrow indicates velum in C). D. Male genital papillae (arrows). Scale bars=20 µm.