Acrobeloides tricornis ( Thorne, 1925 ) Thorne, 1937

Kim, Taeho & Park, Yucheol Lee and Joong-Ki, 2021, First report and morphological description of two Acrobeloides species (Nematoda: Rhabditida: Cephalobidae) in South Korea, Journal of Species Research 10 (4), pp. 405-411 : 408-409

Acrobeloides tricornis ( Thorne, 1925) Thorne, 1937

( Table 1, Fig. 2 )

Ȓ각ẘfflḏẃflệ (ljḍ)

Acrobeles tricornis : Thorne, 1925: 189, figs. 13. Acrobeloides setosus : Brzeski, 1962: 337, figs. 7-9. Acrobeloides uberrinus : Anderson, 1965: 232, fig. 1. Acrobeloides syrtisus : Yeates, 1967: 530, fig. 2. Acrobeloides tricornis : Throne, 1937: 11.

Material examined. 1$, Mechuri Island, Daenam-ro, Danwon-gu, Ansan-si, Gyeonggi-do, South Korea (GPS

coordinates: 37°12′02.9″N, 126°32′24.6″E), extracted by sieving and the Baermann funnel method from overgrown field soil. The specimen (slide No. ZCIVIV0000003207) is deposited at the National Institute of Biological Resources, South Korea .

Measurements. See Table 1.

Description. Female: Body cylindrical, length 332.0 μm long, ventrally curved after fixation ( Fig. 2A ). Cuticle annulated; annuli 1.4 μm wide and 0.7 μm thick at mid-body. Lateral field occupying 23.9% of width of body at mid-body. Lateral incisures varying in number along body length: three incisures before deirid, branching off from deirid into five incisures ( Fig. 2B ); two incisures fading out at anus level; three incisures extending to tail terminus ( Fig. 2C ). Head region continuous with neck. Lip region 6.1 μm in diameter, with triradiate symmetry with 6 + 4 papillae ( Fig. 2D ). Three pairs of asymmetrical lips; pairs of lips separated by U-shaped primary axils. Secondary axils shallow. Guarding process absent. Cephalic probolae absent. Three labial probolae high, with conical basal part and acuate distal part. Stoma cephaloboid, length 1.8 times lip region diameter ( Fig. 2B ). Cheilorhabdions oval-shaped, strongly cuticularized. Pharyngeal corpus fusiform with swollen metacorpus, 2.7 times isthmus length. Isthmus narrower than corpus, distinctly demarcated from metacorpus. Basal bulb oval-shaped with well-developed valves; 1.2 times as long as its width. Cardia surrounded by intestinal tissue. Nerve ring located at anterior isthmus, at 70.9% of pharynx length. Excretory pore position at posterior isthmus, at 79.7% of pharynx length. Position of deirids in lateral field at basal bulb level, at 86.9% of total neck length. Reproductive system monodelphic-prodelphic ( Fig. 2E ). Vulva lips not protruding. Vagina length 0.3 times body diameter. Post-uterine sac inconspicuous. Uterus length 1.8 times body diameter. Spermatheca 0.9 times body width. Oviduct short. Ovary straight. Rectum length 1.2 times anal body diameter ( Fig. 2C ). Tail conoid, with truncated terminus. Phasmids located middle of tail, at 46.3% of tail length.

Male: Unknown.

Distribution. Austria, Bulgaria, Canada, France, Georgia, Germany ( Andrássy, 1984), Hungary ( Andrássy, 2005), Kazakhstan, Kyrgyzstan, Lithuania, The Netherlands ( Andrássy, 1984), New Zealand ( Yeates, 1967; Boström and Holovachov, 2010), Norway (Spitzbergen) ( Loof, 1971; Boström, 1987), Poland ( Brzeski, 1962), Russia ( Andrássy, 1984), Senegal ( De Ley et al., 1990; Boström and Holovachov, 2010), South Korea (this study), Spain ( Abolafia and Peña-Santiago, 2003), USA ( Thorne, 1925; Anderson, 1965), Tajikistan ( Andrássy, 1984), Turkey ( Boström, 1993), Turkmenistan, and Uzbekistan ( Andrássy, 1984).

Habitat. Soil from an overgrown field.

Remarks. The specimen here described generally matches with morphological features that previously studied for A. tricornis ( Thorne, 1925; Loof, 1971; Boström, 1993). Acrobeloides setosus Brezeski, 1962 , A. syrtisus Yeates, 1967 and A. uberrinus Anderson, 1965 were treated as synonyms of A. tricornis by Boström and Holovachov (2010). The examined specimen in this study is morpho- logically very similar to A. uberrinus , a junior synonym of A. tricornis , in the longitudinal incisures in the lateral field with three incisures extending to tail terminus ( Anderson, 1965; De Ley et al., 1990) ( Fig. 2C ). Further examination of multiple nematode individuals is needed to provide a species delimitation of their morphological variations.

The two newly reported A. bodenheimeri and A. tricornis species described herein are distinguished by shape of labial probolae (low and rounded vs high with acuate distal part), post-uterine sac (distinct vs inconspicuous) and some morphometric characters (such as body length [529.5 vs 332.0] and the ratio of body length to body width [a = 27.2 vs 17.5]). In this study, we were not able to find male specimens of both species and species identification of nematodes was conducted based purely on morphological characters examined from a single female individual, which is very common in soil nematode taxonomy. More- over, morphological characters from male representatives in many Acrobeloides species (such as A. apiculatus , A. nanus and A. varius ) have not yet been documented. Although a single female specimen was examined from each of the two species, their morphological characters described herein generally agree with those of A. bodenheimeri and A. tricornis .

Figures

Fig. 2. Acrobeloides tricornis (Thorne, 1925) Thorne, 1937. A, Entire female; B, Female neck region; C, Female posterior region; D, Female head region; E, Female reproductive system. am, amphid; an, anus; bb, basal bulb; ca, cardia; co, corpus; cpa, cephalic papilla; de, deirid; ep, excretory pore; in, intestine; is, isthmus; lf, lateral field; lpa, labial papilla; lpr, labial probolae; nr, nerve ring; ovi, oviduct; ova, ovary; pa, primary axil; ph, phasmid; re, rectum; sa, secondary axil; spe, spermatheca; st, stoma; ut, uterus; va, vagina; vu, vulva.