Rhabdias aegyptiaca El-Garhy & Garo, 2006

(Figs. 1–4)

(Syn. Rhabdias africanus of Abdel-Hakeem et al. [2022] nec Kuzmin, 2001)

Host: Sclerophrys regularis (Reuss); Anura: Bufonidae; Common African toad.

Locality: From Abu-Rawash, Kerdasa, north of Giza, Egypt, 30°01’33”N, 31°04’18”E.

Site of infection: Lung cavity and inside the lung alveoli.

Prevalence: 30 of 40 Sclerophrys regularis (75.0% infected).

Intensity: 1–40 worms/host specimen.

Mean intensity: 11.7 (350/30) worms.

Relative density/abundance: 8.8 (350/40) worms.

Deposited specimens: Collector HT, Neotype (AU) 2023.20.Rr01; Paratypes (n = 8) (AU) 2023.20.Rr02–Rr09; Vouchers (n = 11) (AU) 2023.20.Rr10–Rr20.

Representative DNA sequences: cox 1 mtDNA (GenBank accession no. MZ820693); ITS2 rDNA (GenBank accession no. MZ853954); 28S rDNA (GenBank accession no. MZ820429); 28S-ITS rDNA (GenBank accession no. OL998834).

Amended description. [Based on 20 gravid specimens. Comparative critical measurements, morphometric percentages, and morphometric ratios are given in Tables 3 & 4.] Medium-sized, slender nematode with almost parallel margins, range 4,400 –7,700 µm (mean 5,900 ± 200; 16.9%) long. Body cuticle inflated, with transverse striations/folds (Figs. 1A, 3D). Inflation widening from anterior end to post esophagi-intestinal junction by short distance, keeping the same widening towards beginning of last third of the body, then narrowing towards in caudal region; pronounced caudal inflation around anus and rapidly narrowing towards posterior end leaving naked tail tip (Figs. 1F, 1G, 2D, 2E, 3F, 3G). Lateral cuticular-lined pores, small, extending in longitudinal row on each lateral side of body post beginning of esophageal level by short distance till forward slightly from tail tip (Figs. 1B, 2A), regularly distributed at distances of 35–39 µm and surrounded by thickened portion of cuticular inflation (Fig. 3E). Pores connected to hypoderm by short tubular narrow ducts piercing inflated layer of cuticle (Fig. 1B). Anterior end narrow, truncated in lateral view, round in apical view; abruptly widening posteriorwards from level of esophageal apex followed by gradual widening up to first 1/5–1/4 of body length (Figs. 1A–1C, 2A, 2B), keeping the same widening towards beginning of last 1/9–1/8 of the body, then gradually narrowing towards posterior extremity and with more abruptly tapering in caudal region (Figs. 1F, 1G, 2D, 2E, 3F, 3G). Body width at esophago-intestinal junction 82–150 µm (114 ± 5.2; 20.5%) breadth, 1.2%–3.0% (2.0% ± 0.1; 24.1%) of body length; maximum width at vulva region, 147–375 µm (261 ± 14.9; 25.6%) breadth, 2.9%–7.2% (4.4% ± 0.2; 23.5%) of body length; at anus level 44–113 µm (81 ± 3.7; 20.8%) breadth, 0.7%–1.9% (1.4% ± 0.1; 20.4%) of body length.

Oral opening small, circular, with an internal sclerotized cuticle constituting a serrate ring (Figs. 1D, 1E, 3C). Four submedian lips distinct (two dorso-lateral and two ventro-lateral), situated close to oral opening with inner edges overhanging edge of oral opening; each bearing one prominent large elongated internal labial papilla only (Figs. 1D, 1E, 3A–3C). Two lateral lips (pseudolabia) reduced in a small elevation of body-wall, situated farther from oral opening than submedian ones; each possessing one semi-internal small elongated labial papilla located just near their apex (Figs. 1D, 1E, 3A–3C). Amphids positioning on more posterior surface of outer edge of each pseudolabium. Velum not evident. Vestibulum almost barrel-shaped, 8–14 µm (11 ± 0.5; 20.3%) long, lined with thin cuticle. In apical view, entrance of buccal capsule oval to semi-circular (Figs. 1D, 1E, 3C). Buccal capsule, small, cup-shaped with thick sclerotized uniform walls (Figs. 1B, 1C, 2A, 2B), 7–16 µm (13 ± 0.5; 14.5%) depth and maximum width at midlength of its depth level 11–17 µm (15 ± 0.4; 10.7%). Buccal capsule length to width ratio 0.52–0.97 (0.85 ± 0.03; 14.11%). Posterior part of buccal capsule contacting anterior end of esophagus (Figs. 1B, 1C, 2A, 2B).

Esophagus, comparatively narrow, short, club-shaped, with a slightly rounded anterior end and a more rounded posterior one (Fig. 1A), 336–428 µm (377 ± 5.4; 6.5%) long, occupying 4.9%–8.4% (6.5% ± 0.2; 14.3) of total body length. Apex of esophagus encircling almost posterior half of the buccal capsule forming a shoulder-like structure (Figs. 1B, 1C, 2A, 2B). Slight dilatation present just at the first third of esophagus length or directly anterior to it (Figs. 1A, 1B and 2A); posterior bulbar part oval (Fig. 1A). Esophagus width 17–32 µm (25 ± 1; 17.9%) at anterior end, 30–45 µm (34 ± 0.9; 13.0%) at level of nerve ring and 36–58 µm (44 ± 1.3; 12.9%) at middle of muscular part; bulbar part 42–67 µm (55 ± 1.6; 12.8%) wide. Nerve ring small, encircling muscular portion of esophagus just posterior to its dilatation (Fig. 1A), at 103–131 µm (117 ± 2; 7.6%) from anterior end of esophagus, representing 26.8%–35.5% (31.1% ± 0.6; 8.9%) of esophagus length; and at 110–147 µm (130 ± 2.5; 8.6%) from anterior end of body, 0.2%–2.8% (1.9% ± 0.1; 27.3%) of body length. Two lateral cords originating from hypodermal cells, running longitudinally along ventral and dorsal midline of the body (Figs. 4A–4C). Excretory pore located posterior to nerve-ring, somewhat conspicuous, at 170–207 µm (190 ± 2; 4.7%) from anterior end of body or 2.6%–3.9% (3.2% ± 0.2; 28.0%) of body length. Excretory duct thin, tubular, poorly visible in most specimens, straight in both cuticular and hypodermal parts. Excretory glands inconspicuous in gravid individuals; in an early mature individual, excretory glands two, faint, elongated, subequal in size, shorter than esophagus, slightly widening towards the posterior end forming a drop-like rear part, and terminating at the base of esophageal level.

Intestine simple, wide, occupying whole internal body space anterior to level of anterior bend of reproductive system and posterior to its posterior bend; anterior part of intestine wider than esophageal bulb (Fig. 1A), then widening posteriorly reaching maximum width in posterior part of body anterior to anal opening. Wall of intestine composed of a single layer of cylindrical and voluminous cells with spherical nuclei and grainy cytoplasm of an apparent basophilic nature (i.e., dense blue-colored cytoplasm) constituting allmost parts of intestine (Fig. 4A) except for posteriormost part near rectal region that having an acidophilic nature (i.e., red-colored cytoplasm) (Fig. 4B); numerous microvilli constituting intestinal epithelium (Figs. 4A, 4B). Pre-intestinal distance 351–438 µm (390 ± 5.3; 6.1%) long, 5.1%–8.7% (6.7% ± 0.2; 14.5%) of body length. Rectum short, slightly curved ventrally, with thick sclerotized walls and narrow lumen (Figs. 1F, 1G, 2D, 2E). Posterior portion of intestine separated from rectum by a pre-rectal muscular sphincter (Fig. 1F, 1G). Anus pore-like to a transverse slit with a conspicuous post-anal ventral elevation of body wall (Figs. 1F, 1G, 2D, 2E, 3F, 3G). Pre-anal distance 4,060 –7,410 µm (5,710 ± 220; 17.5%) long, 93.2%–97.6% (96.1% ± 230; 1.1%) of body length. Salient large cuticular inflation anterior and posterior to anus in most specimens (Figs. 1F, 1G, 2D, 2E).

Reproductive system amphidelphic, typical of Rhabdias with approximately similar anterior and posterior limbs in length and shape. Both limbs of reproductive system reflected, bending in the region of oviduct (seminal receptacle) parts (Fig. 4C). Oogonia located on dorsal side, along intestine, and arranged in almost straight columns (Fig. 4D). Mature ovulae arising from division of oogonia, arranged on a loop surrounded by layers of discus progligerus (Figs. 4C, 4E). Male gametes lining the lumen of the oviduct as numerous small globular to semi-polygonal cells with evident nuclei (Fig. 4F). Vulva somewhat post-equatorial, at 2,500 –3,500 µm (2,900 ± 90; 10.7%) from anterior end or 50.2%–58.6% (54.3% ± 0.9; 5.7%) of body length. Vulva rounded opening with salient lips (Figs. 1H, 1I, 2C). A salient large prominent cuticular inflation present at cephalic and caudal sides of the vulval opening in all specimens (Figs. 1H, 1I, 2C). Vagina short, transverse, straight, lined with thin cuticle (Figs. 1H, 1I, and 2C). Uteri amphidelphic, tubular, wide, thin-walled, filled with numerous eggs arranged in 3–4 longitudinal rows (Fig. 1H, 4G); most eggs containing fully developed larvae -particularly those close to vulva- (Fig. 1I, 1J, 2G, 3H, 4G), and some at morula stage (Fig. 2F). Larvated eggs oval, 99–107 µm (104 ± 0.9; 3.1%) × 53–73 µm (65 ± 1.9; 9.7%).

Tail conical, less elongated, gradually tapering from level of anus to posterior end, wholly covered with inflated cuticle except for end tip (Figs. 1F, 1G, 2D, 2E, 3F, 3G). Tail tip medium-long, tubular, thin, needle-like. Tail length 156–317 µm (223 ± 9.8; 19.7%) long or 2.4%–6.8% (3.9% ± 0.2; 27.1%) of body length. Phasmids situated at about midlength of tail.

Variability in adult specimens. Applying the coefficient of variation on the most significant morphometrical features in adult specimens clarified a wide range of variability, ranging from comparatively less variable to mild or more variable: Some characteristics exhibited comparatively less variability (CV <10%) that are functional in differentiation and comparisons. These metrical characters included values of esophagus length, distance from anterior end of esophagus to nerve ring, pre-nerve ring distance, distance from anterior end of body to excretory pore, pre-intestinal distance, and eggs size; in addition to ratio of pre-anal distance in relation to body length, pre-vulva distance in relation to body length and distance from anterior end of esophagus to nerve ring in relation to esophagus length. A mild range of variability (10%≤ CV ≤15%) has been observed in buccal capsule width, buccal capsule length to width ratio, esophageal bulb width, esophagus length as ratio of body length, and esophagus width at the anterior end, level of nerve ring and middle of the muscular part. In contrast, many characteristics were more variable (CV>15%) and included body length, vestibulum depth, buccal capsule depth, esophagus width at anterior end, pre-anal distance, pre-nerve ring distance as ratio of body length, and distance from anterior end of body to excretory pore as ratio of body length in addition to tail length and body width at the esophago-intestinal junction, vulva region, and anus level as values and ratio of body length.

Remarks. The observed morphological characteristics, including excretory system with lateral canals, phasmids, non-stichosome esophagus, eggs without polar plugs, weakly developed ovejector, and protandrous hermaphroditic parasitic females, place our specimens within the Rhabditida (Willmott & Chabaud 2009). Having a short, clavate esophagus without pseudobulb, capsuliform buccal cavity, inflated cuticle, hermaphroditic nature, and parasitic lung habitat in amphibians align with the characteristics of Rhabdiasidae Railliet, 1915 (Anderson & Bain 2009). Based on possessing a spineless, striated, inflated cuticle; a small buccal capsule narrower than the anterior end of the esophagus; four lips and two lateral pseudolabia without modification into spine-like projections; and absence of onchia, our material is identified within Rhabdias (see Yamaguti 1961; Anderson & Bain 2009; Kuzmin 2013; Kuzmin & Tkach 2025). The unique combination of morphological features, supported by morphometric data (see below, and Tables 3, 4), warrants our specimens as R. aegyptiaca.

Our review of the original and only description of R. aegyptiaca, revealed an incomplete diagnosis with the absence of significant information about some characteristic keys that are commonly used for differentiating among different taxa of Rhabdias, particularly, the absence of information on the size and structure of buccal capsule, vestibulum, nerve ring location and positions of cuticular inflation (see El-Garhy & Garo 2006). Furthermore, some notable and critical errors were detected in: (1) doubtful egg size without determining length and width where stated that “eggs measure 66 µm”; (2) confusing dimensions of some esophageal parts where mentioned that “Oesophagus (550 µm long and 55µm wide) club-shaped, with muscular anterior part dilating slightly (55µm) and glandular bulbous posterior part (100µm)”; (3) poor quality of most line drawings, which are rather schematic; (4) a single holotype specimen was not fixed clearly in the text of the original description; and (5) absence of scale bars on photomicrographs (see El-Garhy & Garo 2006, figs. 3–6). Such deficiency in information weakens the validity of metrical comparisons based on such measurements, which in turn, makes the comparison of any taxon of Rhabdias with R. aegyptiaca doubtful, particularly the closely related records of the same locality/biogeographic realm (i.e., recorded of R. africanus [Abdel-Hakeem et al. 2022], Rhabdias sp. [Saad et al. 2009], and R. cf. bufonis [Moravec et al. 1987; Morsy et al. 2018]).

Even though the partially deteriorated state of the type specimen of R. aegyptiaca (see Fig. 5) and the resultant challenge to distinguish internal structures clearly, our investigation referred to morphological differences in some characteristics compared to the original description of El-Garhy & Garo (2006), besides the aforementioned morphometric measurement errors. These morphological errors incorporate a bluntly rounded anterior end in lateral view (see El-Garhy & Garo 2006, fig. 3) and having six lips of a uniform shape and size, positioned at the same distance from oral opening (see El-Garhy & Garo 2006, fig. 8) compared to a truncated anterior end and four submedian lips close to oral opening with two pseudolabia situated farther from oral opening than submedian ones in the type specimen. Comparison of the type-specimen with present specimens shows an identical morphology except for absences of a pronounced caudal inflation around the anus and having non-salient vulval lips (see Fig. 5). Metrically, the type specimen exhibited larger values in measurements of pre-nerve ring distance, distance from esophagus anterior end to nerve ring, pre-excretory pore distance, and pre-vulval distance; however, their corresponding morphometric percentage relative to body length falls within the range of our specimens (see Table 3). Based on the condition of the type specimen and the impossibility of its detailed studies limited by the permanent mount (see Fig. 5), we establish the neotype, followed by eight paratypes.

Comparison. According to the aforementioned problematic status of the original description of R. aegyptiaca, we found that further comparison with closely related taxa of Rhabdias should be investigated to resolve/remove any confusion/overlap with the presently known Rhabdias spp.

Based on the high restriction of presence and distribution of rhabdiasid species to a specific world ecozone (biogeographic realm) with scarcity occurrence of the same species in two different geographical ecozones (Kuzmin 2013; Tkach et al. 2014a; Müller et al. 2018, 2023); in addition to the absence of any species record parasitizing in hosts from different classes and orders (Kuzmin 2013; Tkach et al. 2014a), comparisons of our specimens with taxa of Rhabdias have been initially limited to all those recorded in the Palaearctic ecozone (where formerly the Saharo-Arabian realm was considered a part of the west Palaearctic ecozone) from anuran amphibian hosts only. Our specimens exhibit a great affinity with R. bufonis through the key to Palaearctic species by Kuzmin (2013) according to the following combined characteristics: possessing a cephalic end with 6 lips, parasitizing in anurans, absence of spherical swelling on the cephalic end; normal developing of anus and rectum without atrophy in gravid specimens, having a wide cup-like buccal capsule of a uniform wall without segmentation with a diameter less than 20 µm, and egg width larger than 50 µm. Although the recorded locality of our specimens (Egypt) belongs to the Saharo-Arabian ecozone, their geographical position in the north-eastern corner of the African continent (i.e., Egypt locates in the African subtropical region) makes it highly adjacent and environmentally connected to the Afrotropic ecozone. In addition, the wide distribution range of the common African toad, S. regularis in most African tropical/ subtropical countries (see AmphibiaWeb 2025) and with unclear information about whether or not the distribution of a taxon of Rhabdias coincides with the distribution areas of its host(s), prompts us a comparison of our species with those of Rhabdias taxa that parasitize in Afrotropical Anurans. According to the identification key to taxa of Rhabdias from Afrotropical anurans (Kuzmin et al. 2022), our specimens exhibit close relation with R. africanus depending on the combination of features as follows: anterior end without dilatation; body gradually widening from apex posteriorly; buccal capsule comparatively thin-walled, cup-shaped, less than 25 μm wide, about 11–17 (15) µm with a uniform wall without segmentation, its length to width ratio 0.52–0.97 (0.85); prominent cephalic lips with one prominent large internal labial papilla on each lip; positioning cephalic papillae into four submedian and two lateral; and onchia absent. Furthermore, S. regularis in some nearby localities within the same country range (i.e., Egypt) has been reported to harbor four species of Rhabdias, which in turn need to be compared to our specimens (see Discussion below). These records include one record of R. africanus (Abdel-Hakeem et al. 2022) and an undetermined Rhabdias sp. (Saad et al. 2009) in addition to two records of R. cf. bufonis (Moravec et al. 1987; Morsy et al. 2018). Two African species of Sclerophrys Tschudi are reported for infection with Rhabdias aside from S. regularis; (i) Guttural toad, Sclerophrys gutturalis (Power) (syns. Amietophrynus gutturalis [Power]; Bufo gutturalis Power) harbors Rhabdias picardiae Junker, Lhermitte-Vallarino et Bain, 2010 (see Junker et al. 2010) as well as (ii) both flat-backed toad, Sclerophrys maculata (Hallowell) and Garman’s toad, Sclerophrys garmani (Meek) harbors R. africanus (see Kuzmin et al. 2022). Due to the widespread occurrence of these hosts in the African continent in the same and/or nearby localities (see AmphibiaWeb 2025), records of their Rhadias taxa are included for comparison to our specimens.

Rhabdias aegyptiaca is differentiated from R. bufonis by a combination of the following features: a barrel-shaped vestibulum, a circumoral lip with one prominent large internal labial papilla, a wide cup-shaped buccal capsule with a small length-to-width ratio, 0.52–0.97 (0.85), and salient lips vs an elongated vestibulum, two types of labial papillae on each circumoral lip, a narrower funnel/tear-shaped buccal capsule with a more considerable length to width ratio, 1.00–1.25 and indistinct vulval lips (see Kuzmin 2013). Besides, R. bufonis is characterized from R. aegyptiaca by the apex of esophagus surrounding buccal capsule entirely (see Kuzmin 2013, figs. 17A, 17B), excretory glands approximately of the same length as esophagus and exceed the esophagi-intestinal junction by a well distance posteriorly, absence of post-anal ventral elevation of body wall, and a tail wholly covered with an inflated cuticle without a naked needle-like tail tip (see Kuzmin 2013, fig. 17D). In addition, R. bufonis has a longer body length, a smaller ratio of maximum body width in relation to body length, a longer esophagus, and a nerve ring positioned at a longer distance from anterior end of esophagus compared to R. aegyptiaca (see Table 3).

Rhabdias aegyptiaca is the morphologically most similar to R. africanus except for a few morphological characters as follows: a truncate anterior end, salient vulval lips with large prominent vulval inflation, a post-equatorial vulva situated at 46.8%–49.7% (47.8%) of body length, a barrel-shaped vestibulum, and a smaller buccal capsule (7–16 [13] µm × 11–17 [15] µm vs 15–20 [17.7] µm × 20–23 [21.5] µm [see Kuzmin 2001, figs. 17A– 17C]). Besides, R. africanus has a body length approximately three times that of R. aegyptiaca with larger values in esophagus length, pre-nerve ring distance, distance from esophagus anterior end to nerve ring, pre-intestinal distance, distance to vulva, and tail length, as well as a higher width measurement at vulva region with a lower ratio in relation to body length in contrast to observed values in R. aegyptiaca (see Table 4). Both species are reported from two different biogeographic realms (R. aegyptiaca is a Saharo-Arabian species, whereas R. africanus is an Afrotropic species [Kuzmin 2001]) and collected from different hosts within the same genus, Sclerophrys (R. aegyptiaca from S. regularis vs R. africanus from S. maculata and S. garmani [Kuzmin 2001]).

In comparison of R. aegyptiaca against records of R. picardiae (Junker et al. 2010; Svitin et al. 2018), it exhibits the following differences: one prominent large internal labial papilla on each submedian lip, a barrel-shaped vestibulum without velum, with a smaller width of 11–17 (15) µm and a larger length to width ratio of 0.64–0.94 (0.79), presence of slight dilatation presents just anterior to esophagus mid-length, salient vulval lips with large prominent vulval cuticular inflation and a thin needle-like naked tail tip. In contrast, R. picardiae is characterized by two types of labial papillae on each lip (see Svitin et al. 2018), an elongated vestibulum (see Svitin et al. 2018) with a distinct velum (see Junker et al. 2010), a triangular buccal capsule with a larger width (23–25 µm [Junker et al. 2010]; 18–25 [21] µm [Svitin et al. 2018]) and smaller length to width ratio (0.30–0.40 [Junker et al. 2010]; 0.4–0.6 [0.54] [Svitin et al. 2018]), absence of cuticular dilation at the region in front of the mid-length of the esophagus, an inconspicuous vulva without salient lips, and a tail wholly covered with an inflated cuticle (see Junker et al. 2010; Svitin et al. 2018). Besides, R. picardiae is distinctly longer with obvious higher values in esophagus length, pre-nerve ring distance, distance from esophagus anterior end to nerve ring, pre-excretory pore distance, pre-intestinal distance, distance to vulva and tail length as dimensions and ratios of body length in addition to tail length as dimensions only (see Table 4). Rhabdias aegyptiaca is a Saharo-Arabian species from Egypt, northeast Africa, and collected from S. regularis (Bufonidae) whereas R. picardiae is an Afrotropic species from South Africa, Southern Africa, and recorded from S. gutturalis (Bufonidae) (syn. Amietophrynus gutturalis [Power]) (Junker et al. 2010) as well as the Delaland’s River Frog, Amietia delalandii (Duméril and Bibron) (Pyxicephalidae) (Svitin et al. 2018).