Halicephalobus similigaster ( Andrássy, 1952 ) Andrássy, 1974

Mahboob, M. & Tahseen, Q., 2022, Description of a new and a known insect-associated species of the genus Halicephalobus Timm, 1956 (Panagrolaimidae: Rhabditida) along with intrageneric relationship, Journal of Natural History 55 (39 - 40), pp. 2549-2567 : 2556-2563

Halicephalobus similigaster ( Andrássy, 1952) Andrássy, 1974

= Cephalobus (Tricephalobus) similigaster Andrássy, 1952

= Micronema similigaster (1952) Rühm, 1956

= Halicephalobus minutus ( Körner, 1954) Andrássy, 1974

= Halicephalobus parvus ( Körner, 1954) Andrássy, 1974

( Figures 5 (a-e), 6 (a-h), 7, 8)

Measurements

See Table 1 .

(Continued)

Description

Female

Body small, narrowing at both extremities and slightly curved at ventral side upon fixation from the vulva. Cuticle smooth, 0.4–0.5 μm thick at mid-body. Lateral field with two ridges extending from metacorpus up to mid of the tail. Lip region narrow, continuing into the adjoining body. Lips small, six in number, each with minute labial sensilla. Amphids small opening on lateral lips. Stoma panagrolaimoid type with anterior wide and posterior narrower parts, 5 times longer than wide or ca 9.8–10.8% of total pharyngeal length. Cheilostom constituting a short tube, about as long as wide with weakly cuticularised parallel walls. Gymnostom with prominent and cuticularised parallel walls. Stegostom covering half of the stoma length, gradually narrowing. Metastegostom anisomorphic, a pointed tooth on dorsal wall. Pharynx muscular, covering 23.5–24.2% of total body length, comprising 36–42 μm long corpus, well-developed metacorpus of 12–15 μm × 10–12 μm in dimension, narrower, 17–23 μm long isthmus and a muscular, ovoid basal bulb of 14–17 μm × 12–15 μm in dimension with well-developed grinder and haustrulum. Cardia conoid, 3– 4 μm in length, embedded into the intestine. Nerve ring surrounding isthmus at mid-level or at ca 70.6–74.0% of total pharyngeal length. Secretory–excretory duct located closely posterior to nerve ring or ca 81.5–82.7% of the pharyngeal length. Deirids not discernible. Intestinal wall thin with indistinct cells and nuclei. Intestinal lumen wide anteriorly forming a bacterial pouch. Rectum as long as anal body diameter. Reproductive system monodelphic. Ovary with double flexures. Anterior flexure 62–94 μm long and posterior flexure 13–57 μm long with distal tip of ovary not extending up to anterior level of vulva. Oocytes arranged in two rows distally continuing into a single row proximally. Oviduct not distinct. Spermatheca small, filled with numerous minute spermatids. Uterus about twice corresponding body diameter long. Post-uterine sac absent. Vagina obliquely oriented with thick wall, constituting 40–47% of corresponding body diameter. Vulva a short slit, vulval lips not protruded. Tail elongate-conoid, usually dorsally bent with a minute terminal mucro. Some specimens possessed faintly visible cells in caudal region resembling caudal glands but without a discernible duct or opening. Phasmids located at mid-tail.

Male

Unknown.

Dauer/phoretic juveniles

Body much smaller,ventrally arcuate upon fixation.Cuticle smooth,thin ca 0.1–0.2 μm. Lateral line(s) inconspicuous. Lip region twice wider than high, continuous with body contour. Stoma ca 5–7 times longer than wide.Pharynx well developed, covering ca 31–32% of the total body length, differentiated into 27–32 μm long slender procorpus, ca 8–9 × 5–7 μm metacorpus; 13–14 μm long, narrower isthmus, expanded posteriorly to an oval basal bulb of about 8–10 × 7–8 μm in dimension.Basal bulb with faint haustrulum and grinder. Nerve ring encircling midregion of isthmus. Secretory–excretory duct inconspicuous, occasionally seen posterior to nerve ring. Cardia short, conoid. Bacterial chamber not observed. Intestine thick-walled; rectum about as long as anal body diameter. Tail elongate-conoid.

The dauer larvae were isolated from the head of excised adult Rhinotermes lucifugus and Odontotermes sp. , whereas other stages and adults were found in the populations collected from rotting bark and rotting wood. The nematodes were cultured on NGM seeded with bacteria. No males could be found in the culture.

Habitat and locality

Four populations of H. similigaster ( Andrássy, 1952) Andrássy, 1974 were isolated from the heads of Odontotermes sp. , collected from a rotten log of Dulbergia sissoo, and Rhinotermes lucifugus , collected from the hollow of a Pongamia pinnata (Fabaceae) tree and extracted from rotting wood and bark, collected from Uttar Pradesh, India, in the districts of Siddharthnagar (at coordinates 27.0820°N 82.3801°E) and Aligarh (at coordinates 27.5534°N 78.0429°E).

Voucher material

Forty-three females of H. similigaster , on slides NIT/Q3/1-3, Q4/1-4, Q5/1-3 and Q6/1-4, were deposited in the Nematodes Collection, Department of Zoology, Aligarh Muslim University, Aligarh, Uttar Pradesh, India.

Emended diagnosis

Halicephalobus similigaster is characterised by females having lateral fields with two ridges; narrow and continuous lip region with small lips having minute labial sensilla; stegostom narrower than gymnostostom, covering half of stoma length; metastegostom anisomorphic with a conical denticle at dorsal wall; nerve ring surrounding middle part of isthmus; secretory–excretory pore located just posterior to nerve ring; rectum as long as anal body diameter; ovary with double flexures with distal tip of ovary usually not extending anterior to vulva; post-uterine sac absent; vulval lips not protruded; and tail elongate-conoid ending in small mucro.

Remarks

The genus Halicephalobus is represented by species having parthenogenetic females. The species of the genus have by and large very few discriminating morphological traits and can be grossly classified into long-tailed (e.g. H. minutus , H. parvus , H. similigaster and H. persicus ) and short-tailed species (e.g. H. brevicauda ). Some other conspicuous features ( Figure 7 ) of the species include a broad tail with distinct inner sclerotisation at the tip (e.g. H. laticauda ), and reproductive system not dorsally reflexed (e.g. H. mephisto ). The other species, H. gingivalis , H. intermedius , H. limuli , and H. palmaris , do not show any conspicuous distinctive features but have been differentiated largely based on morphometric characteristics. The new species H. termitis is a small, plump species with a unique character among congeners (i.e. a far posterior vulva); it also possesses a short conoid tail (c = 6.6–8.1), similar to H. brevicauda (c = 11–14); however, the latter is distinct from other species in having a larger stoma. Another species showing distinctness is H. limuli , whose eggs measured 52 µm in length, as compared to H. palustris (41–46 µm), H. intermedius (32–35 µm) and H. gingivalis (42 µm × 20 µm), and whose ovary is without distal flexure. The present populations of H. similigaster conformed well with the original population of H. similigaster ( Andrássy, 1952) Andrássy, 1974 in most morphometric and morphological characters; however, some minor differences were observed in stoma length (5 times vs 6 times longer than wide); position of nerve ring (encircling mid vs posterior region of isthmus) and secretory–excretory pore (located anterior to vs at level of basal bulb).

The present population of Halicephalobus similigaster ( Andrássy, 1952) Andrassy, 1974 differs from Halicephalobus limuli Timm, 1956 in its relatively longer body size (233–390 µm vs 420–460 µm); having relatively wider stoma (6 vs 8 times longer than wide); stegostom narrower than gymnostom (vs as wide as gymnostom); metastegostom with a conoid process (vs without conoid process) at dorsal wall; rectum length equal to (vs shorter than) anal body diameter; egg dimensions (52 µm vs 23–44 × 8–13 µm) and tail configuration (ventrally curved vs dorsally bent); and H. similigaster collected from the head of Odontotermes sp. , Rhinotermes lucifugus , rotten bark and wood [vs around the appendages of horseshoe crab ( Limulus sp. ) apud Timm (1956)]. Earlier, the species H. minutus and H. parvus were synonymised ( Köhler 2011) with H. similigaster due to overlapping morphometrical characters.

Figures

Figure 5. Halicephalobus similigaster (Andrássy, 1952) Andrássy, 1974 female. (a) Entire body; (b) anterior end; (c) pharyngeal region; (d) reproductive system; (e) tail region.

Figure 7. Comparison of stoma, reproductive system and tail of species of the genus Halicephalobus Timm, 1956. Scale bar = 10 µm.