Introduction
Blend et al. (Reference Blend, Karar and Dronen2017) established Paraschistorchis Blend, Karar & Dronen, Reference Blend, Karar and Dronen2017 as a separate genus out of Schistorchis Lühe, Reference Lühe1906 (sensu stricto) to include four taxa with a ‘simple’ oral sucker of a muscular nature and ceca ending in separate ani: Paraschistorchis longivesiculurus (Hafeezullah, Reference Hafeezullah1981) Blend, Karar & Dronen, Reference Blend, Karar and Dronen2017, Paraschistorchis seychellesiensis (Toman, Reference Toman1989) Blend, Karar & Dronen, Reference Blend, Karar and Dronen2017, Paraschistorchis stenosoma (Hanson, Reference Hanson1953) Blend, Karar & Dronen, Reference Blend, Karar and Dronen2017 and Paraschistorchis zancli (Hanson, Reference Hanson1953) Blend, Karar & Dronen, Reference Blend, Karar and Dronen2017 (see Blend et al. Reference Blend, Karar and Dronen2017; WoRMS 2025). A key was presented to Paraschistorchis spp. based on differences in the following features: oral sucker shape (rounded to elliptical vs funnel-shaped to subquadrate), seminal vesicle shape (saccular vs tubular), anterior extent of vitellarium (restricted to ovarian level vs extended more anteriorly), testes shape (transversally elongated vs rounded to elliptical), sucker sizes (equally sized vs unequally sized) and egg sizes (see Blend et al. Reference Blend, Karar and Dronen2017).
Based on new materials of some Paraschistorchis spp., Magro et al. (Reference Magro, Cutmore, Carrasson and Cribb2023) re-diagnosed Paraschistorchis, referring to striking observations involving the difficulty of observing the U-shaped partial sphincter embedded within the oral sucker of some taxa and the presence of some glandular components within the musculature of the oral sucker – contrary to the original description by Blend et al. (Reference Blend, Karar and Dronen2017). Additionally, Magro et al. (Reference Magro, Cutmore, Carrasson and Cribb2023) reduced the taxonomic importance of the funnel-shaped oral sucker of P. zancli for differentiation among other species of Paraschistorchis, meanwhile referring to the importance of sucker ratios, the range of the anterior extent of the vitellarium, and host(s) group as differential features.
Toman (Reference Toman1989) described P. seychellesiensis (as Schistorchis [Alloschistorchis] seychellesiensis Toman, Reference Toman1989) from the intestine of the honeycomb filefish, Cantherhines pardalis (Rüppell) (Tetraodontiformes: Monacanthidae), off the Seychelles Islands in the western Indian Ocean. From the same host off Heron and Lizard Islands, Australia, Magro et al. (Reference Magro, Cutmore, Carrasson and Cribb2023) designated some specimens of schistorchiines as belonging to P. seychellesiensis based on general body morphology and similarity of most features with those of the original description. The most striking difference in the Australian population of P. seychellesiensis was the testes arrangement, which was in a cluster initially two testes wide but narrowing progressively to one testis wide posteriorly, and in a long post-testicular region about one-third of body length (see Magro et al. Reference Magro, Cutmore, Carrasson and Cribb2023, Fig. 7B) compared to a median column of one testis wide and a shorter post-testicular region less than one-fifth of body length (see Toman Reference Toman1989, Fig. 2b). Accordingly, Magro et al. (Reference Magro, Cutmore, Carrasson and Cribb2023) raised a question of whether such differences could be considered intra- or interspecific; hence, further sampling and sequencing from the type-location to improve confidence in the aforementioned identification was recommended. Karar et al. (Reference Karar, Blend and Mansour2025) clarified the presence of three constant patterns of testes arrangement in schistorchiines with multiple testes (a median column, two distinct submedian columns and a clustered configuration) without intraspecific variation in testicular arrangement, indicating testes arrangement to be a characteristically and taxonomically informative differential feature as well as constant within each species. Hence, distinct differences in patterns of the testes arrangement, in length of the post-testicular space, in the range of the anterior extension of the vitellarium, in egg sizes, and in the distant localities with different ecosystems that exist between the Indian and Australian specimens of P. seychellesiensis, prompted us to suspect the Australian specimens to be a distinct taxon apart from the original description of P. seychellesiensis.
In light of these suspicions, this manuscript aims at addressing the confusing taxonomic state of Australian specimens of P. cf. seychellesiensis, determining the validity and nature of its relationship with the original description of this species, and establishing criteria for defining taxa recognized within Paraschistorchis via a thorough revision of the genus and detailed morphometrical comparisons coupled with revealing multivariate statistical analyses.
Materials and methods
Parasite investigation
Various museums provided photographs of the oral suckers/whole specimens of species within Paraschistorchis including the Harold W. Manter Laboratory (HWML), University of Nebraska, Lincoln, Nebraska, USA; the Meguro Parasitology Museum (MPM), Tokyo, Japan; the United States National Parasite Collection (USNM), Smithsonian National Museum of Natural History, Washington, DC, USA; Queensland Museum, Collections & Research Centre, Hendra, Queensland, Australia; and the Zoological Survey of India (ZSI), Kolkata, India. The oral suckers/whole worms from type and/or voucher specimens of the following species were examined: P. longivesiculurus (ZSI W7282/1), P. seychellesiensis (MPM 19144), P. stenosoma (USNM 1338141; HWML 798, 1303, 1304, 1305), and P. zancli (USNM 1338142; HWML 1307, 1308). Additionally, whole worms from deposited voucher specimens in the Parasitology Laboratory, Zoology Department, Faculty of Science, South Valley University (SVU), Qena, Egypt, of the following species were re-examined: Plesioschistorchis callyodontis (Yamaguti, Reference Yamaguti1942) Blend, Karar & Dronen, Reference Blend, Karar and Dronen2017 (SVU 2017.12.Pc01), Plesioschistorchis haridis (Nagaty, 1957) Blend, Karar & Dronen, Reference Blend, Karar and Dronen2017 (SVU 2017.15.Ph01–08), and Schistorchis carneus Lühe, Reference Lühe1906 (SVU 2017.17.Sc01–05; SVU 2023.20.Sc01–Sc14).
Schematic drawings were prepared using photographs and a micro-projector. Measurements are in micrometers (μm) and expressed as ranges followed by means ± standard deviation (SD). Comparative measurements were sourced from original species descriptions or redescriptions, unless otherwise stated. Digenean identification is based on Cribb and Bray (Reference Cribb and Bray1999), Cribb (Reference Cribb, Jones, Bray and Gibson2005), Jones et al. (Reference Jones, Bray and Gibson2005), Blend et al. (Reference Blend, Karar and Dronen2017), and Magro et al. (Reference Magro, Cutmore, Carrasson and Cribb2023). Classification and authorities for fish follow FishBase (Froese and Pauly Reference Froese and Pauly2025).
Statistical analyses
For the analytical data interpretation, five statistical analyses were performed using IBM SPSS Statistics (V. 25) (IBM Corp. 2017): coefficient of variation (CV), correlation analysis (r), hierarchical agglomerative clustering (HAC), principal component analysis (PCA), and linear discriminant analysis (LDA).
The CV of each target measurement was calculated as the standard deviation (SD) divided by the mean value and presented as a percentage. Commonly used ranges for CV follow the classification of Pimentel-Gomes (Reference Pimentel-Gomes1985) as follows: CV<10% ‘low variability’; 10% ≤ CV ≤ 20% ‘moderate variability’; CV >20% ‘high variability’.
Pearson correlation coefficient (r) was calculated to determine the strength of relationships among the measurements (=variables) of target records and used to disclose the degree of dependency of one variable on the other. We investigated the correlation/relationship between body length and other measurements, given its importance as a growth indicator for the parasite. The strength of correlation was determined according to Lehman (Reference Lehman2005) as follows: r = zero ‘non-correlated’; r <0.20 ‘very weak’; 0.21 ≤r ≤ 0.39 ‘weak’; 0.40 ≤ r ≤ 0.59 ‘moderate’; 0.60 ≤ r ≤ 0.79 ‘strong’; 0.80 ≤ r < 1 ‘very strong’; r = 1 ‘perfect’. A two-tailed test of significance (p = ‘probability value’) was estimated to determine the statistical significance of each measurement relative to others; p <0.05 is considered statistically significant, indicating a strong and reliable relationship. Conversely, a high p-value (p >0.05) suggests the relationship is likely due to coincidence.
Based on the distinctive characteristics of HAC in clustering target data – in particular, the focusing on merging clusters according to overall similarity rather than being heavily influenced by individual extreme values, and its typical illustration by a dendrogram showing relations among resultant clusters (see Ding and He Reference Ding and He2002; Rokach and Maimon Reference Rokach, Maimon, Rokach and Maimon2005) – HAC was applied to the available records/specimens of Paraschistorchis spp. to identify groups of specimens with similar features. Inserted measurements in HAC were expressed as percentages relative to body length and/or ratios in all comparisons among target records (see Table S1). Data standardization was performed to prevent bias from feature scales, ensuring that all features contribute equally to the distance calculations and facilitate observing the relative importance of each feature in the clustering process (see Xu and Wunsch Reference Xu, Wunsch, Xu and Wunsch2008). Ward’s method and squared Euclidean distance were applied to create compact, roughly equal-sized clusters, minimize within-cluster variance, produce balanced cluster sizes, provide efficient computation and clear dendrogram interpretation, and determine statistically significant differences between the averages of study groups (see Everitt et al. Reference Everitt, Landau, Leese and Stahl2011; Murtagh and Legendre Reference Murtagh and Legendre2014).
PCA was applied to standardized data first to reduce dimensionality and remove noise while preserving between-group variance, and LDA was employed to improve species discrimination (see Timm Reference Timm2007; Borade and Adgaonkar Reference Borade and Adgaonkar2011). PCA represents an unsupervised classification method that effectively reduces the dimensionality of high-dimensional datasets, facilitating clearer visualization of group patterns through principal components (see Timm Reference Timm2007; Jackson Reference Jackson2005). By emphasizing components with the greatest variance, PCA suppresses noise that may mask group differences. Additionally, the loadings on principal components (Eigenvalues) identify the variables most influential in separating groups (see Jackson Reference Jackson2005). PCA also serves as a valuable pre-processing step to enhance the performance of supervised classification methods such as LDA (see Borade and Adgaonkar Reference Borade and Adgaonkar2011). LDA analysis is a supervised technique that maximizes the separation between predefined groups by finding linear combinations of variables that maximize between-group variance while minimizing within-group variance; this leads to better class separability compared to unsupervised methods like PCA (see Timm Reference Timm2007; Borade and Adgaonkar Reference Borade and Adgaonkar2011). LDA also reduces dimensionality while preserving class discrimination, improves interpretability by highlighting key discriminative features, handles high-dimensional data effectively, and enhances classification accuracy (see Timm Reference Timm2007; Zhao et al. Reference Zhao, Zhang, Yang, Zhou and Xu2024).
Results
Morphology
Taxonomic position
Superfamily Apocreadioidea Skrjabin, 1942
Family Megaperidae Manter, 1934
(Syns.: Apocreadiidae Skrjabin, 1942; Euryperidae Manter, 1933 nec Pascoe, 1870; Homalometridae Cable & Hunninen, 1942; Schistorchiidae Yamaguti, Reference Yamaguti1942; Sphincterostomatidae Yamaguti, Reference Yamaguti1937; Trematobrienidae Dollfus, 1950)
Subfamily Schistorchiinae Yamaguti, Reference Yamaguti1942
(Syn.: Sphincterostomatinae Yamaguti, 1958)
Genus Paraschistorchis Blend, Karar & Dronen, Reference Blend, Karar and Dronen2017
Paraschistorchis australiensis n. sp.
Micrograph of adult specimens of Paraschistorchis australiensis n. sp. stained in Mayer’s haematoxylin. (a) Holotype specimen (QM G240537), ventral view. (b) Paratype specimen (QM G240532), ventral view. Scale bars: (a, b) 500μm.
Illustrations of Paraschistorchis australiensis n. sp. (a) Adult whole mount, holotype specimen (QM G240537), ventral view. (b) Terminal genitalia, dorsal view. (c) Elliptical eggs. Scale bars: (a) 500μm; (b) 200μm; (c) 100μm.
Illustrations of adult voucher specimens of Paraschistorchis australiensis n. sp. (QM G240528). (a) Ventral view. (b) Dorsal view. Scale bars: (a, b) 500μm.
(Syn. Paraschistorchis seychellesiensis sensu Magro, Cutmore, Carrasson & Cribb, Reference Magro, Cutmore, Carrasson and Cribb2023 nec Toman, Reference Toman1989)
Taxonomic summary (modified after Magro et al. Reference Magro, Cutmore, Carrasson and Cribb2023)
Type-Host: Honeycomb filefish, Cantherhines pardalis (Rüppell) (Tetraodontiformes: Monacanthidae).
Type-Locality: Off Heron Island (23°27′S, 151°55’E), southern GBR; off Lizard Island (14°40′S, 145°27′E), northern GBR, Australia.
Site of infection: Intestine.
LSID: CEC34872-0E35-4D25-8210-CE8B8E228ABA.
Deposited specimens: Queensland Museum, Collections & Research Centre, Hendra, Queensland, Australia; Holotype (QM) G240537; Paratypes (n = 3) (QM) G240530–32; Vouchers (n = 10) (QM) G240528–29, G240533–36 & G240538–41.
Representative DNA sequences: Partial cox1 mtDNA (accession no. OQ445545–46); ITS2 rDNA (accession no. OQ442931–32); 28S rDNA (accession no. OQ442914).
Etymology: The species designation ‘australiensis’ refers here to the host locality ‘Australia’ where the worms were recorded.
Description
[Based on 8 mature specimens; see Table 1.] Body broad, linguiform; maximum width at mid-hindbody. Tegument thin, spinous to near mid-forebody. Anterior extremity broadly rounded; posterior extremity attenuated (n = 4), rounded (n = 2), or somewhat truncated (n = 2). Forebody shorter than hindbody, slightly narrows near anterior end. Hindbody larger, three times forebody length, gradually widens near third quarter of body, then gradually narrows towards posterior extremity (n = 5); or just a slight widening near third quarter of body (n = 3). Eye-spot pigment dispersed in forebody. Pre-oral lobe short, well-developed. Oral sucker spherical, muscular but with some glandular components, ventro-subterminal, of ‘simple’ type, without circumoral lobes. Mouth conspicuous, rounded, directed antero-ventrally; surrounded by strong semi-circular muscle fibres that sometimes cover mouth aperture; well-developed, prominent, partial U-shaped muscular sphincter half-encircles oral aperture postero-ventrally. Ventral sucker pre-equatorial, spherical, sessile, unspecialized, about two-thirds of oral sucker size, at junction of first and second fifths of body. Suckers separated by short distance. Pre-pharynx not evident. Pharynx subquadrate to transversally elliptical, well-developed, not papillate, smaller than ventral sucker, typically partly dorsal to oral sucker. Oesophagus short. Intestinal bifurcation posterior to mid-level of forebody to immediately anterior to ventral sucker, occupies region between suckers. Ceca two, broad, simple, wider anteriorly, slightly arcuate posteriorly, equal in length, extend to lateral margins, terminate slightly anterior to posterior extremity in small, separate ani; posterior margin sometimes indented slightly at each anus; anus lateral, slit-like, narrow, subterminal.
Measurements, morphometric percentages, and morphometric ratios of Paraschistorchis australiensis n. sp. compared to other species of Paraschistorchis Blend, Karar & Dronen, Reference Blend, Karar and Dronen2017
--- = Neither given in original description nor available from published illustration(s).
[ ] Values outside the minimum and maximum values reported in the original description.
() Mean values of the target measurement.
a Sandwich isle file, Cantherhines sandwichiensis (Quoy & Gaimard) (misidentified as Cantherhines pardalis [Rüppell] according to Magro et al. [Reference Magro, Cutmore, Carrasson and Cribb2023]).
b Honeycomb filefish, Cantherhines pardalis (Rüppell).
c Large-scaled leatherjacket, Cantheschenia grandisquamis Hutchins.
d Moorish idol, Zanclus cornutus (Linnaeus).
e White-spotted spinefoot, Siganus canaliculatus (Park) (Syn. Siganus oramin [Bloch & Schneider]).
f Proportion of body length; range values are given first, followed by the mean values.
g Proportion of body width.
h Calculated from measurements given in the original description.
i Calculated from the illustrations of the original description (Hanson Reference Hanson1953, figs. 1a, 2a; Hafeezullah Reference Hafeezullah1981, fig. 1; Toman Reference Toman1989, fig 2b; Lo et al. Reference Lo, Morgan, Galzin and Cribb2001, figs. 1A, 1B; Magro et al. Reference Magro, Cutmore, Carrasson and Cribb2023, fig. 7A).
j Measurements of an immature specimen.
k CV, coefficient of variation; r, Pearson correlation coefficient.
l Significant correlation with body length at level <0.05 (2-tailed) (i.e., probability value <0.05).
Testes 10 (n = 2)–11 (n = 6), smooth, subglobular to ellipsoidal or pyriform, subequal in size, contiguous. Testes arranged in a cluster initially of two testes wide then narrows progressively and eventually to one testis wide posteriorly (n = 5); in early mature specimens, testicular chain exhibited as pairs of adjacent testes arranged in an oblique zigzag pattern and terminates in a single testis posteriorly (n = 3). Testicular region occupies second third of body; each of pre- and post-testicular spaces occupies approximately one-third of body length. Seminal vesicle saccate, large, transversally sub-elliptical, unipartite, median to slightly dextral, postero-dorsal to ventral sucker. Pars prostatica tubular, short, thin-walled, extends dorso-laterally over sinistral margin of ventral sucker, forms short hermaphroditic duct; duct itself cylindrical, very small, extends antero-dorsally to ventral sucker. Prostatic cells few, free in parenchyma, surrounds pars prostatica and hermaphroditic duct. Genital pore ventro-submedian, slightly sinistral, anterior to ventral sucker in area between post-bifurcation and anterior margin of ventral sucker.
Ovary rounded in outline, smooth, pre-equatorial, at end of anterior third of body, dextral, typically slightly separated from ventral sucker anteriorly and anteriormost testis posteriorly. Canalicular seminal receptacle saccular, antero-dorsal to ovary. Oötype complex not observed. Right and left transverse vitelline ducts short, unite anterior to anteriormost testes. Vitellarium follicular; extends from about level of anterior margin (n = 6) or occasionally mid-level of ventral sucker (n = 2) to close to posterior extremity; anteriorly principally lateral to intestinal ceca, posteriorly occupying most available space around and posterior to testes, unevenly ventrally confluent posterior to testes, separated into relatively distinct columns by intestinal ceca, testes, and excretory vesicle. Uterus para-ovarian, short, saccular, intercecal, restricted to area lateral and anterior to ovary; dorsally overlaps left margin and extends antero-sinistrally around left side of ventral sucker into metraterm. Post-uterine distance very long, about last two-thirds of body. Metraterm tubular, short, thin, runs short distance parallel to left and anterior margins of ventral sucker, opens into hermaphroditic duct at post-bifurcal level. Eggs relatively numerous, operculated, oval, thin-shelled, non-filamented.
Excretory vesicle Y-shaped; consists of median, wholly post-testicular stem that extends to near anterior end of last third of body, close to level of posterior testis, and wider anteriorly (with or without anterior swelling); stem bifurcates into two very long, thin excretory arms parallel to ceca, each extends into forebody close to anterior end of oral sucker. Excretory pore terminal.
Morphometric variability in adult specimens
Applying the coefficient of variation (CV) to morphometric features of the present specimens reveals wide variability, with some features being less variable while others are more variable (see Table S2). Nine metric characters exhibit low CV with non-significant correlation to the body length (r ≤0.41, p >0.05), including pre-bifurcal distance, cecal length, post-uterine distance, post-ovarian distance, pre-testicular distance, testes width, and testicular zone length as percentages of body length in addition to dimensions of eggs. Such metric features suggest less spread/dispersion, a high degree of consistency/uniformity, and relative stability; hence, they are functional in differentiation and comparisons among related species.
Among several morphometric characters with moderate CV, pharynx width as % of body length and pharynx width/oral sucker width ratio only show a significant correlation with body length (r ≥0.74, p <0.05). Five metric characters of moderate CV show a coefficient of variation very close to the range of low coefficient of variation (CV ≤11.6), incorporating ratios of oral sucker length, oral sucker width, ventral sucker length, pre-ovarian distance, and post-testicular distance as % of body length. These features exhibit a weak, non-significant correlation to body length (r ≤0.46, p >0.05). Aforementioned measurements also can be used to compare and distinguish between morphologically similar taxa, but with caution, provided that there is notable variation within the target measurement between close taxa.
Concerning morphometric characters with high CV, dimensions of the seminal vesicle, distance between suckers, and distance from intestinal bifurcation to ventral sucker as well as the ratios of post-vitelline region length and dimensions of the seminal vesicle relative to body length are notably the largest (CV >50). Except for body width, pre-vitelline and post-vitelline distances, all measurements and dimensions with a high coefficient of variation show significant correlation with body length (r ≥0.78, p ≤0.05). Of all the metric characters expressed either as ratios or percentages of body length, seminal vesicle length, and distance from the intestinal bifurcation to the ventral sucker exhibit significant correlation with body length (r ≥0.75, p ≤0.05). These metric features indicate a larger amount of variation or dispersion, a low level of consistency or uniformity, and considerable instability; therefore, they are less useful or ineffective for distinguishing and comparing related species.
Remarks
Present specimens are identified within the Megaperidae based on parasitizing the alimentary canal of marine fishes, possessing a mouth at the anterior extremity, the presence of both suckers and a pharynx, a hermaphroditic duct, evidence of a pharynx, the absence of a hermaphroditic sac, the majority of the tegument is unarmed, a Y-shaped excretory vesicle and testes in the hindbody (Blend et al. Reference Blend, Karar and Dronen2017, Reference Blend, Karar and Dronen2024; Bray Reference Bray, Jones, Bray and Gibson2005a, Reference Bray, Jones, Bray and Gibson2005b, Reference Bray, Bray, Gibson and Jones2008; Cribb Reference Cribb, Jones, Bray and Gibson2005; Cribb and Bray Reference Cribb and Bray1999; Jones Reference Jones, Jones, Bray and Gibson2005). Possessing a genital pore anterior to the ventral sucker and an oral sucker with a partial U-shaped sphincter half encircling the oral aperture postero-ventrally, our specimens are consistent with the Schistorchiinae (Blend et al. Reference Blend, Karar and Dronen2017; Cribb Reference Cribb, Jones, Bray and Gibson2005; Cribb and Bray Reference Cribb and Bray1999). Our material keyed out to Paraschistorchis on the basis of having the following combination of morphological features: multiple testes, ceca open independently via marginal or submarginal separate ani, and a normally/not strongly glandular oral sucker (Blend et al. Reference Blend, Karar and Dronen2017; Magro et al. Reference Magro, Cutmore, Carrasson and Cribb2023). With the unique combination of morphological features, supported by morphometric percentages, ratios (see below; Tables 1, S1), and multivariate classification analyses, the present specimens are identified as a unique species of Paraschistorchis.
Our review of the original description of P. seychellesiensis combined with a photograph examination of the type-specimen revealed several morphological differences relative to the Australian record of Magro et al. (Reference Magro, Cutmore, Carrasson and Cribb2023). Minor differences included an elongated unspined body of almost equal width, absence of an oesophagus, a muscular oral sucker, an oval ventral sucker with a large rhomboid aperture, an ovary contiguous with the anteriormost testis posteriorly, and an unequal anterior extent of the dextral and sinistral vitelline fields (see Toman Reference Toman1989; Blend et al. Reference Blend, Karar and Dronen2017, fig. 9) vs a broad linguiform body with maximum breadth at mid-hindbody, a spinous tegument to mid-forebody, presence of a short oesophagus, a muscular oral sucker but with notable glandular components, a spherical ventral sucker with a small aperture, a slight separation of the ovary from the anteriormost testis posteriorly, and an equal anterior extent of the dextral and sinistral vitelline fields (see Magro et al. Reference Magro, Cutmore, Carrasson and Cribb2023; Karar et al. Reference Karar, Blend and Mansour2025; Present study). The most significant differences are represented in that (i) the original description of P. seychellesiensis exhibits transversally elongated testes arranged in a single median column vs roughly ovoid testes, in a cluster initially two testes wide but narrowing progressively to one posteriorly in present specimens; and a smaller post-testicular region in the former occupying the last one-sixth of body length compared to a larger post-testicular region of about one-third body length. (ii) The anterior extension of the vitellarium in P. seychellesiensis ranges from about the level of the posterior margin of the ventral sucker or slightly posterior to it without confluence of vitelline follicles in the post-testicular region compared to an anterior extent of the vitellarium in present specimens from about the level of the anterior margin of the ventral sucker with confluence of vitelline follicles in the post-testicular region. (iii) Morphometrically, the original description of P. seychellesiensis is characterized by a distinctive anterior testis/ovary length ratio (1:0.69 vs 1: 0.94–1.58 [1:1.22]), anterior testis/ovary width ratio (1:1.95 vs 1: 0.85–1.69 [1:1.24]), and smaller eggs (43–48 × 35–40 μm vs 53–69 [59] × 33–42 [36] μm) (see Toman Reference Toman1989; Blend et al. Reference Blend, Karar and Dronen2017, Fig. 9; Magro et al. Reference Magro, Cutmore, Carrasson and Cribb2023; Table 1). (iv) Despite being reported from the same host species, C. pardalis, the apparent distant locality and distinctive ecology of the GBR compared to that of the Seychelles Islands within the western Indian Ocean can be considered as an interesting outcome of the recognition of separate species in light of the plausibility of the existence of associated/closely related digeneans within the same host species as well as evidence mounting regarding the presence of species complexes within some schistorchiine taxa (see Magro et al. Reference Magro, Cutmore, Carrasson and Cribb2023; Mansour et al. Reference Mansour, Karar, Hassan, Mohamadain and Abdel-Hakeem2025; Karar et al. Reference Karar, Blend and Mansour2025; Present study). Toman (Reference Toman1989) described P. seychellesiensis from the Indian Ocean using only two specimens. We feel this is insufficient to either ascertain the full range of morphological/morphometrical variation or identify intra-specific changes. However, the combination of several major and minor morphological differences, coupled with the apparent distant/distinct localities and increased likelihood of the existence of associated/closely related schistorchiine species within the same host species, confirms the Australian population of P. cf. seychellesiensis as a different and distinct taxon.
In comparison with the other Paraschistorchis spp. (i.e., P. longivesiculurus, P. stenosoma, and P. zancli), the Australian specimens of P. cf. seychellesiensis exhibit distinctive morphological and morphometrical features incorporating (i) a broad linguiform body vs an elongate one; (ii) a larger body width around one-third of body length vs less than one-fourth of body length; (iii) a ventral sucker noticeably smaller than oral sucker; (iv) a subquadrate pharynx, typically partly dorsal to the oral sucker; (v) and testes initially arranged in a cluster two testes wide but narrowing progressively to one posteriorly vs a single median column.
In addition to the aforementioned differential features, Australian specimens of P. cf. seychellesiensis are differentiated from P. longivesiculurus by a larger post-testicular region of about one-third of body length, and a short saccular seminal vesicle in a postero-dorsal position to the ventral sucker (see Magro et al. Reference Magro, Cutmore, Carrasson and Cribb2023; Present study) compared to a smaller post-testicular region less than one-tenth of body length, and a long, winding, tubular seminal vesicle extending well into the hindbody (see Hafeezullah Reference Hafeezullah1981). The record of the Australian specimens of P. cf. seychellesiensis from a tetraodontiform fish within the Monacanthidae Nardo vs from an acanthuriform one within the Siganidae Richardson coupled with the distant locality and distinctive ecology of the GBR compared to the Gulf of Mannar and Coromandel Coast, India, represents another reliably diagnostic character (see Hafeezullah Reference Hafeezullah1981; Magro et al. Reference Magro, Cutmore, Carrasson and Cribb2023; Present study). Paraschistorchis stenosoma is characterized by the anterior extent of the vitellarium located posterior to the ovary, which is itself far from the ventral sucker, and a relatively long uterus that extends slightly posterior to the ovary compared in Australian specimens of P. cf. seychellesiensis to an anterior extent of the vitellarium to about the level of the anterior margin of the ventral sucker and a shorter uterus restricted to the area lateral and anterior to the ovary (see Hanson Reference Hanson1953; Magro et al. Reference Magro, Cutmore, Carrasson and Cribb2023; Present study). Additional features distinguishing P. zancli from the Australian specimens of P. cf. seychellesiensis include a smaller post-testicular region less than one-fifth of body length and records from an acanthuriform fish restricted to the Zanclidae Bleeker, compared to a larger post-testicular region of about one-third of body length and a record from a tetraodontiform fish within the Monacanthidae (see Hanson Reference Hanson1953; Toman and Kamegai Reference Toman and Kamegai1974; Magro et al. Reference Magro, Cutmore, Carrasson and Cribb2023). In light of the aforementioned distinctive body outline and significant morphometric variation as well as taking into consideration criteria proposed by Bray et al. (Reference Bray, Cutmore and Cribb2022) and Cribb et al. (Reference Cribb, Barton, Blair, Bott, Bray, Corner, Cutmore, De Silva, Duong, Faltýnková, Gonchar, Hechinger, Herrmann, Huston, Johnson, Kremnev, Kuchta, Louvard, Luus-Powell, Martin, Miller, PérezPonce de León, Smit, Tkach, Truter, Waki, Vermaak, Wee, Yong and Achatz2025) for trematode species recognition, we conclude that the Australian specimens of P. cf. seychellesiensis are not conspecific with any taxon of Paraschistorchis.
The honeycomb filefish, C. pardalis, is widely distributed within the range of the Eastern Atlantic Ocean (Gulf of Guinea, Annobon Islands to the south coast of Africa) and the Indo-West Pacific Ocean (South Africa to the Red Sea and east to southern Japan and southeastern Oceania) (Froese & Pauly Reference Froese and Pauly2025). The Seychelles marine environment is tropical, island-based, and shaped by nutrient dynamics in the West Indian Ocean; it includes shallow shelf waters, coral and granitic reefs, and unique endemic species supported by steep gradients and varied environments with diverse habitats including mangroves, seagrass beds, and lagoons, all influenced by equatorial currents and upwellings (Fassbender et al. Reference Fassbender, Stefanoudis, Filander, Gendron, Mah, Mattio, Mortimer, Moura, Samaai, Samimi-Namin, Wagner, Walton and Woodall2021; Lourie et al. Reference Lourie, Zora, Servant, Searle, Vizeu-Pinheiro, Borges, Court and Rosa2025; Nevill Reference Nevill2014). The GBR is a continuous barrier reef along the northeast Australian coast with varied temperate to tropical habitats containing extensive coral reefs, seagrass beds, mangroves, and islands, all influenced by the seasonal monsoons, currents of the Pacific Ocean, and cyclones (Bensted-Smith and Kirkman Reference Bensted-Smith and Kirkman2010; Butler et al. Reference Butler, Rees, Beesley and Bax2010; Richards and Day Reference Richards and Day2018). Hence, it is possible for a single host species within two different environments to be infected by a variety of associated/closely related digenean parasites (see Pérez-Ponce de León et al. Reference Pérez-Ponce de León, Solórzano-García, Huston, Mendoza-Garfias, Cabañas-Granillo, Cutmore and Cribb2024; Cribb et al. Reference Cribb, Barton, Blair, Bott, Bray, Corner, Cutmore, De Silva, Duong, Faltýnková, Gonchar, Hechinger, Herrmann, Huston, Johnson, Kremnev, Kuchta, Louvard, Luus-Powell, Martin, Miller, PérezPonce de León, Smit, Tkach, Truter, Waki, Vermaak, Wee, Yong and Achatz2025). We see increasing records of two or more species of schistorchiines from the same host – both belonging to the same genus and similar morphologically – distributed over different marine ecosystems and even within the same ecosystem of nearby/same localities. The sandwich isle file, Cantherhines sandwichiensis (Quoy & Gaimard) (Tetraodontiformes: Monacanthidae), was recorded harbouring Pa. stenosoma (see Hanson Reference Hanson1953; Skrjabin Reference Skrjabin1959; Pritchard Reference Pritchard1963; Yamaguti Reference Yamaguti1970) and Pa. zancli off the Hawaiian Islands (Yamaguti Reference Yamaguti1970). Blend et al. (Reference Blend, Karar and Dronen2017) recorded Pl. callyodontis and Pl. haridis from the common parrotfish, Scarus psittacus Forsskål (Perciformes: Scaridae), from nearby localities in the northern Red Sea proximate to Hurghada, Egypt. From the same host, C. pardalis, Pa. seychellesiensis was recorded off the Seychelles Islands (see Toman Reference Toman1989), and Pa. stenosoma was reported from a comparatively distant locality, off Lizard and Heron Islands, Australia (see Magro et al. Reference Magro, Cutmore, Carrasson and Cribb2023). Magro et al. (Reference Magro, Cutmore, Carrasson and Cribb2023) described both Blendiella tridecimtestis Magro, Cutmore, Carrasson & Cribb, Reference Magro, Cutmore, Carrasson and Cribb2023 and Blendiella trigintatestis Magro, Cutmore, Carrasson & Cribb, Reference Magro, Cutmore, Carrasson and Cribb2023 from the same hosts, the orange-lined triggerfish, Balistapus undulatus (Park), and the halfmoon triggerfish, Sufflamen chrysopterum (Bloch & Schneider) (Tetraodontiformes: Balistidae), within the same locality, off Lizard Island, Australia. Additionally, the yellowmargin triggerfish, Pseudobalistes flavimarginatus (Rüppell) (Tetraodontiformes: Balistidae), has been reported harbouring Lobatotrema aniferum Manter, Reference Manter1963 off Lizard Island, Australia (see Magro et al. Reference Magro, Cutmore, Carrasson and Cribb2023) as well as Lobatotrema droneni Karar, Blend & Mansour, Reference Karar, Blend and Mansour2025, the latter from a relatively distant locality off Safaga, Northern Red Sea, Egypt (see Karar et al. Reference Karar, Blend and Mansour2025).
According to the distinguishing morphological characteristics of testes configuration/arrangement, anterior extent of the vitellarium, significant morphometric variations present, several consistent statistical analyses (see below), comparatively distant geographic localities (Seychelles Islands vs GBR, Australia), and in light of criteria proposed by Bray et al. (Reference Bray, Cutmore and Cribb2022) and Cribb et al. (Reference Cribb, Barton, Blair, Bott, Bray, Corner, Cutmore, De Silva, Duong, Faltýnková, Gonchar, Hechinger, Herrmann, Huston, Johnson, Kremnev, Kuchta, Louvard, Luus-Powell, Martin, Miller, PérezPonce de León, Smit, Tkach, Truter, Waki, Vermaak, Wee, Yong and Achatz2025) regarding challenges in the recognition of trematode species, we concluded that these differences are evidence for considering the Australian population of P. cf. seychellesiensis to represent a free-standing species. As this requires a new name, we have established Paraschistorchis australiensis n. sp. to accommodate specimens of Paraschistorchis seychellesiensis sensu Magro, Cutmore, Carrasson & Cribb, Reference Magro, Cutmore, Carrasson and Cribb2023.
Multivariate classification analyses
Hierarchical agglomerative clustering analysis
The resultant dendrogram of HAC primarily divides target records of Paraschistorchis into two groups, each of which is subdivided into two sister clusters (see Fig. 4). Thus, four main clusters are observed and are arranged from the closest to the most distant record: Cluster (A) consists of records of P. seychellesiensis and the maximum value of records of P. stenosoma ex. C. pardalis; Cluster (B) comprises records of P. australiensis n. sp.; Cluster (C) contains records of P. longivesiculurus; and Cluster (D), which represents a 3-way polytomy consisting of a branch of remaining records of P. stenosoma and two branches representing all records of P. zancli – a branch of specimens of Hanson (Reference Hanson1953) and a branch of records of both Lo et al. (Reference Lo, Morgan, Galzin and Cribb2001) and Magro et al. (Reference Magro, Cutmore, Carrasson and Cribb2023). Except for the maximum value of records of P. stenosoma ex. C. pardalis, specimens within each taxon aggregate closely together, which supports their distinctiveness based on the measured data (see Fig. 4).
Dendrogram illustrating the hierarchical clustering of analyzed records of species of Paraschistorchis Blend, Karar & Dronen, Reference Blend, Karar and Dronen2017 generated using Ward’s method and squared Euclidean distance. Abbreviations: ave, average recorded values of the target taxon; max, maximum recorded values of the target taxon; min, minimum recorded values of the target taxon; Sa, sample.
All entries for P. australiensis n. sp. cluster tightly together and are separated clearly from other taxa by a large rescaled distance (RD>10), suggesting it forms a coherent and well-defined group that represents a separate taxonomic unit and is genetically/morphologically cohesive. Additionally, specimens of P. australiensis n. sp. exhibit clear subclusters, thus showing intraspecific or population-level variation within the taxon. Minimum and maximum values indicate the range of variation within P. australiensis n. sp., but even these extremes remain closer to each other than to taxa outside the group. The nearest clusters to P. australiensis n. sp. are Cluster (A) and Cluster (C), but the large distance separating them indicates clear differentiation. The closest taxon outside P. australiensis n. sp. by branch length and clustering is primarily P. longivesiculurus, but the maximum value of records of P. stenosoma ex. C. pardalis is also very close, slightly more distant than P. longivesiculurus. Regarding P. seychellesiensis, this species is clustered distinctly apart from P. australiensis n. sp. by a long branch and the positioning at the very bottom of the dendogram, indicating that it is a separate taxonomic entity, quite distant from the latter morphologically, and might imply it is more genetically distinct or geographically separated. Interestingly, the clustering of P. seychellesiensis and the maximum value of records of P. stenosoma ex. C. pardalis in a very small cluster (Cluster A) reflects high/wide intraspecific variation within the population of P. stenosoma ex. C. pardalis collected by Magro et al. (Reference Magro, Cutmore, Carrasson and Cribb2023) to the extent that its morphometric measurements approach the value of P. seychellesiensis. However, both species are morphologically distinctive.
Principal component analysis
Ten PCAs (=Factors) with eigenvalues greater than one were extracted, which contribute 92.3% of the variation across all characters (see Table S2). The first three axes of the PCA explain 28.5%, 17.4%, and 11.6% of the variation, respectively (see Table S2). The most important contributors to PC1 are dimensions of the oral sucker [OS], pharynx [Ph] and ovary [Ov] as % of body length in addition to ratio of body width [BW] relative to body length; of PC2 are ratios of the width [W] of internal features relative to body width including, OSW/BW, PhW/BW, ventral sucker width [VSW]/BW, OvW/BW and anterior testis width [ATW]/BW; of PC3 are ratios of PhW/OSW, OSW/VSW and OS length [OSL]/VSL as well as vitellarium length as % of body length (see Tables S2 and S3).
Visualisation of the grouping and separation of taxa of Paraschistorchis in the three-dimensional multivariate space (see Fig. 5) illustrates that: (i) the cluster of P. australiensis n. sp. is distinctly in the upper region of the plot, physically separated from other taxa in the plot with visible gaps and boundaries, no intrusion into the space occupied by another cluster, and specimens of the cluster are tightly grouped together showing high within-cluster similarity. Such a clear spatial separation from other groups, with no point of interspersion, separation in several dimensions, and cohesive grouping of specimens within the cluster, all indicate a unique multivariate profile suggesting strong differentiation and supporting the recognition of P. australiensis n. sp. as a distinct taxon. (ii) Likewise, P. longivesiculurus and P. seychellesiensis are isolated from all other taxa points, indicating a high degree of differentiation from all other taxa. (iii) In contrast, clusters of P. zancli and P. stenosoma are positioned close together, the clusters’ edges encroach upon each other with some points from the two clusters lying within or close to the other cluster’s spatial extent, and an overlap occurs along one or more of the factor score dimensions. This spatial proximity, shared boundaries, mixed/interspersed points, and overlap on multiple axes show more similarity between these two species than between others, which may reflect closer morphological relationships between both clusters.
Visualization through a 3D-Scatter plot of the grouping and separation of taxa of Paraschistorchis Blend, Karar & Dronen, Reference Blend, Karar and Dronen2017 in the three-dimensional multivariate space based on the first three REGR (regression) factor scores. Each point represents an individual specimen, with lines connecting each specimen to its respective group centroid.
Linear discriminant analysis
Based on ten factor scores (=principal components [PCs]), the structure matrix with results of standardized canonical discriminant function coefficients (see Table S4) clarifies that the strongest contributors to group separation within both first and second functions are the PC4 and PC3 in function 1 (coefficient = 0.156 and –0.103, respectively), and PC8, PC1, PC3, and PC10 in function 2 (coefficient = –0.321, 0.288, 0.159, and 0.118, respectively). Hence, the constituting effective morphometric features of the aforementioned PCs represent the strong discriminative power in separating the five groups. These features include dimensions of OS, Ph and Ov as % of body length; ratios of ATL, BW, egg width, forebody length, pre-vitelline distance, post-vitelline distance, and vitellarium length as % body length; and ratios of forebody length/BW, OSL/VSL, OSW/VSW, PhL/OSL, PhW/OSW, and PhL/PhW (see Tables S2 and S3).
The visualization of the canonical discriminant function analysis (DFA) (Fig. 6) shows that each taxon forms a distinct spatial cluster without overlap with others. The centroid of each group is well separated from those of other taxa, indicating clear differentiation in the canonical space, presenting good separation between groups based on the measured PCs, and validating the discriminatory power of these canonical functions. Concerning P. australiensis n. sp., this taxon is well-differentiated with a clearly separated centroid from those of other taxa; its specimens cluster distinctly and separate sharply from other related taxa along the two canonical axes, highlighting its taxonomic uniqueness based on the set of measured variables used in the analysis. The position of P. seychellesiensis as a single isolated point might reflect a limited sample size and/or distinct differentiation; this taxon is represented by only one specimen, however, it is metrically (see Tables 1 and S1) and morphologically distinct (see Toman Reference Toman1989; Blend et al. Reference Blend, Karar and Dronen2017).
Scatter plot of a Canonical Discriminant Function Analysis (DFA) visualizing the extent of the grouping and separation of taxa of Paraschistorchis Blend, Karar & Dronen, Reference Blend, Karar and Dronen2017 from each other based on ten factor scores extracted from the principal component analysis (PCA).
The classification results (see Table S5) under the original categorization clarify that all specimens of each taxon are correctly assigned into their own groups, yielding a perfect classification accuracy of 100%. Nearly all specimens remain correctly classified under cross-validation showing robust predictive power. A minor misclassification was detected, as one specimen originally from P. stenosoma (out of nine total) was incorrectly assigned as P. australiensis n. sp. (11.1%), causing an overall cross-validation accuracy of about 94.4%; however, this value remains high, indicating strong differentiation based on the variables used. The slight cross-validation error between P. stenosoma and P. australiensis n. sp. might reflect some overlap or similarity in those specimens, but it is minimal. Accordingly, we find that the classification table provides strong evidence that the discriminant functions effectively separate the taxa. The perfect original categorization and high cross-validated accuracy demonstrate that these diagnostic variables reliably distinguish P. australiensis n. sp. and related taxa. Additionally, the slight misclassification in cross-validation is minor and does not undermine the overall discrimination strength.
Discussion
On nature of excretory vesicle within schistorchiines
One of the most interesting and confusing aspects within schistorchiines is the nature of the excretory vesicle; either an I-shaped vesicle without any anteriolateral extensions or with two blind-ending nubs, or a Y-shaped vesicle with two excretory arms of differing anterior extent. Such patterns were observed among taxa of the same genus and/or within the same species across different records. Within Paraschistorchis, Pa. stenosoma and Pa. zancli were described as possessing an I-shaped vesicle extending to the posterior edge of the posterior testis, followed by a principal pair of collecting tubules crossing the ceca ventrally up to the pharyngeal level (Hanson Reference Hanson1953, figs. 1a, 2a). The same pattern was observed in Pa. seychellesiensis (see Toman Reference Toman1989, fig. 2b; Blend et al. Reference Blend, Karar and Dronen2017, fig. 9). Redescriptions of Pa. stenosoma and Pa. zancli illustrated either an I-shaped excretory vesicle with two blind-ending nubs ‘not arms’ (see Magro et al. Reference Magro, Cutmore, Carrasson and Cribb2023, fig. 7A) or a Y-shaped excretory vesicle with the branches reaching to the intestinal bifurcation (Lo et al. Reference Lo, Morgan, Galzin and Cribb2001, fig. 1A), respectively. Additionally, Pa. australiensis n. sp., exhibits a Y-shaped excretory vesicle with two long excretory arms which differs from the originally examined specimens of Magro et al. (Reference Magro, Cutmore, Carrasson and Cribb2023, Fig. 7B) that illustrated an I-shaped excretory vesicle with two blind-ending ‘nubs’ and not arms. Within Plesioschistorchis, Yamaguti (Reference Yamaguti1942) described the excretory vesicle in Pl. callyodontis and Pl. sigani as a Y-shaped structure with a median stem bifurcating into two arms at the equatorial region or just anterior and proximate to the anteriormost testis. In contrast, Blend et al. (Reference Blend, Karar and Dronen2017) clarified the presence of an I-shaped excretory vesicle in Pl. callyodontis and Pl. haridis extending anteriorly into an area within the anterior half of the testes and which continued to two primary excretory ducts that extended anteriorly into the forebody. Within Schistorchis, Lühe (Reference Lühe1906, figs. 11, 12) clarified the presence of a Y-shaped excretory vesicle in S. carneus with a long median stem and long paired branches terminating anteriorly in the forebody. Johnston (Reference Johnston1913, Figs. 11, 38) referred to S. carneus as having a long excretory vesicle that divides into two comparatively narrow vessels at the level of the seminal receptacle, terminating blindly near the anterior extremity of the body. Redescriptions by Blend et al. (Reference Blend, Karar and Dronen2017) and Mansour et al. (Reference Mansour, Karar, Hassan, Mohamadain and Abdel-Hakeem2025) referred to S. carneus as possessing an I-shaped excretory vesicle that extends close behind to or at the level of the anteriormost testes, then bifurcates into two excretory ducts running as far as the oral sucker. Within Lobatotrema Manter, Reference Manter1963, the excretory vesicle of L. aniferum was described as ‘probably I-shaped, receiving two collecting tubes at the posterior edge of the posterior testis’, and extending forward almost to the mid-level of the oral sucker giving a Y-shaped appearance to the excretory vesicle (see Manter Reference Manter1963). Magro et al. (Reference Magro, Cutmore, Carrasson and Cribb2023) briefly described the excretory vesicle of L. aniferum as I-shaped, reaching to the posterior testis.
Our investigations on deposited specimens of taxa within Paraschistorchis and voucher specimens of Pl. callyodontis and Pl. haridis clarify that the true nature of the excretory vesicle is a Y-shaped structure composed of a median, wide stem, with or without anterior swelling(s) and/or various anterior extension(s) that do not exceed the level of the anteriormost testis. This stem bifurcates anteriorly into two thinner excretory arms dorsal to the ceca, each extending well into the forebody. Sometimes the arms are obvious, as in young adult specimens, or they are obscure/invisible due to the overlapping of the lateral margins of the testes and vitelline follicles. Hence, we believe the I-shaped excretory vesicle reported in previous records was based on the observed shape of the excretory vesicle stem in concert with the excretory arms, either not observed and/or interpreted as collecting tubules, and thus, not a part of the main excretory vesicle.
Concerning the anterior extension of the excretory vesicle stem within the testicular zone, we find remarkably constant patterns for some genera of schistorchiines. The taxa of Lobatotrema, Paraschistorchis, and Sphincteristomum Oshmarin, Mamaev & Parukhin, Reference Oshmarin, Mamaev and Parukhin1961 exhibit an anterior extension terminating at the level of the posterior margin of the most posterior testis (see Hanson Reference Hanson1953; Manter Reference Manter1963; Oshmarin et al. Reference Oshmarin, Mamaev and Parukhin1961; Toman Reference Toman1989; Blend et al. Reference Blend, Karar and Dronen2017; Magro et al. Reference Magro, Cutmore, Carrasson and Cribb2023; Karar et al. Reference Karar, Blend and Mansour2025; Present study). The excretory vesicle stem in Neomegacreadium Machida & Kuramochi, Reference Machida and Kuramochi1999, Plesioschistorchis, and Sphincterostoma Yamaguti, Reference Yamaguti1937 terminates in the anterior zone of the testes (see Yamaguti Reference Yamaguti1937, Reference Yamaguti1942; Machida and Kuramochi Reference Machida and Kuramochi1999; Blend et al. Reference Blend, Karar and Dronen2017; Present re-examination). Taxa of Blendiella Magro, Cutmore, Carrasson & Cribb, Reference Magro, Cutmore, Carrasson and Cribb2023 show a relatively short excretory vesicle stem terminating well short of the posterior testis (see Magro et al. Reference Magro, Cutmore, Carrasson and Cribb2023). Schistorchis reveals diverse patterns, either the anterior extension of the excretory vesicle stem terminates at the level of the posterior margin of the most posterior testis, as in Schistorchis skrjabini Parukhin, 1963 (see Magro et al. Reference Magro, Cutmore, Carrasson and Cribb2023), or it extends well into the testicular zone, as in S. carneus (see Lühe Reference Lühe1906; Johnston Reference Johnston1913; Blend et al. Reference Blend, Karar and Dronen2017; Mansour et al. Reference Mansour, Karar, Hassan, Mohamadain and Abdel-Hakeem2025; Present re-examination). In light of this review, we find that each schistorchiine species shows a constant pattern in the anterior extent of the excretory vesicle stem, even if the species belongs to a genus with diverse patterns in the anterior extent of the excretory vesicle stem.
Notes on key features of Paraschistorchis
Magro et al. (Reference Magro, Cutmore, Carrasson and Cribb2023) re-diagnosed Paraschistorchis, referring to some novel observations compared to the original diagnosis of Blend et al. (Reference Blend, Karar and Dronen2017). First, Magro et al. (Reference Magro, Cutmore, Carrasson and Cribb2023) revealed a difference in the detectability of the partial sphincter embedded in the oral sucker; it was either a well-developed and prominent one, as in the Australian population of P. cf. seychellesiensis (=P. australiensis n. sp.), or it was exceptionally hard to detect, as in P. stenosoma and P. zancli. Our observations on the oral sucker images of taxa of Paraschistorchis refer to the same aforementioned pattern, where the partial sphincter embedded in the oral sucker is exceptionally difficult to discern in P. longivesiculurus, P. stenosoma, and P. zancli compared to P. australiensis n. sp. and P. seychellesiensis. Second, they noted the presence or absence of glandular components in the oral sucker of muscular nature, where P. stenosoma and the Australian population of P. cf. seychellesiensis (=P. australiensis n. sp.) exhibited a development of gland cells in the oral sucker, in contrast to the absence of such glandular cells in P. zancli. Our investigations are consistent with the observations of Magro et al. (Reference Magro, Cutmore, Carrasson and Cribb2023) and reveal the presence of some glandular cells in the oral sucker of P. seychellesiensis, with the absence of such a structure in P. longivesiculurus. Third, Magro et al. (Reference Magro, Cutmore, Carrasson and Cribb2023) remarked on the insignificance of the funnel-shaped oral sucker of P. zancli as a differential feature (see couplet 1 of the key to species of Paraschistorchis by Blend et al. [Reference Blend, Karar and Dronen2017]). They considered the importance of sucker ratios and the anterior extent of the vitellarium to be more telling characteristics, particularly between P. stenosoma and P. zancli. Fourth, the reliable diagnosis of the host(s) group as a characteristic for each species within Paraschistorchis was discussed: siganid fish for P. longivesiculurus, zanclid fish for P. zancli, and monacanthid fishes for both P. seychellesiensis and P. stenosoma. Our review of host-parasite data of Paraschistorchis spp. shows that P. australiensis n. sp., P. longivesiculurus, P. seychellesiensis, and P. zancli represent oioxenic species (each restricted to a single host species), whereas P. stenosoma represents a stenoxenic species as it was collected from phylogenetically related hosts belonging to tetraodontiform fishes within the Monacanthidae and the Balistidae Rafinesque. Furthermore, Karar et al. (Reference Karar, Blend and Mansour2025) indicated that the change in testes arrangement (i.e., a median column of one testis wide, or two distinct submedian elongate columns each of one testis wide, or a cluster initially of 2−4 testes wide but narrowing progressively to 1−2 testes wide posteriorly) represents a diagnostic characteristic in schistorchiines with multiple testes.
One of the most striking and controversial aspects of the taxa of Paraschistorchis is the difference in the nature of the seminal vesicle, either unipartite or bipartite across records of the same taxon. The seminal vesicle in P. seychellesiensis and P. stenosoma was originally described as bipartite, composed of a saccular posterior portion confined between the level of the ventral sucker and the ovary, joined by a narrow constriction to a much smaller, thinner, claviform anterior portion of variable size. This anterior portion is loosely surrounded by prostatic cells and opens in turn into a short hermaphroditic duct with the uterus (see Hanson Reference Hanson1953; Toman Reference Toman1989). In contrast, the redescription of P. stenosoma by Magro et al. (Reference Magro, Cutmore, Carrasson and Cribb2023) reported a unipartite saccular seminal vesicle. Our examination of type and/or voucher specimens of taxa within Paraschistorchis suggests a unipartite saccular or tubular seminal vesicle, mainly postero-dorsal to the ventral sucker. This vesicle is connected to a short, thin walled, either tubular (as in P. longivesiculurus and P. zancli) or claviform/dilated (as in P. australiensis n. sp., P. stenosoma, and P. seychellesiensis) pars prostatica. This pars prostatica extends dorso-laterally over the sinistral margin of the ventral sucker opening into a short hermaphroditic duct. A few prostatic cells surround the pars prostatica and the hermaphroditic duct. This reveals a misidentification by Hanson (Reference Hanson1953) and Toman (Reference Toman1989) in interpreting the dilated part positioned anterior to the seminal vesicle, which led to considering it as a second part of the seminal vesicle (i.e., bipartite) rather than the pars prostatica.
Given this detailed review and the newly added data provided herein, including a description of a new species within Paraschistorchis, we have amended the key to the recognized species of this genus established by Blend et al. (Reference Blend, Karar and Dronen2017).
Key to species of Paraschistorchis Blend, Karar & Dronen, Reference Blend, Karar and Dronen2017 (based on Blend et al. Reference Blend, Karar and Dronen2017)
1a. Anterior extent of vitellarium does not exceed level of ovary … 2.
1b. Anterior extent of vitellarium distinctly exceeds level of ovary to level of ventral sucker or more anteriorly … 3.
2a. Seminal vesicle long, winding, tubular, extending far beyond ventral sucker; post-testicular distance <10% of body length; suckers about equal in size; parasite of siganids … Paraschistorchis longivesiculurus (Hafeezullah, Reference Hafeezullah1981) Blend, Karar & Dronen, Reference Blend, Karar and Dronen2017.
2b. Seminal vesicle short, saccular, extending relatively short distance posterior to ventral sucker; post-testicular distance >20% of body length; oral sucker distinctly larger than ventral sucker; parasite of monacanthids and balistids … Paraschistorchis stenosoma (Hanson, Reference Hanson1953) Blend, Karar & Dronen, Reference Blend, Karar and Dronen2017.
3a. Testes arranged into a median column of one testis wide; anterior extent of vitellarium terminates at or near level of base of ventral sucker; post-testicular distance <20% of body length … 4.
3b. Testes arranged into a cluster initially of two testes wide, narrowing progressively to one testis wide posteriorly; anterior extent of vitellarium exceeds mid-level of ventral sucker; post-testicular distance about 1/3 of body length … Paraschistorchis australiensis n. sp.
4a. Testes transversely elongated (testis width 2 times its length or more); egg 43–48 (46) × 35–40 (38); parasite of monacanthids … Paraschistorchis seychellesiensis (Toman, Reference Toman1989) Blend, Karar & Dronen, Reference Blend, Karar and Dronen2017.
4b. Testes subglobular to ellipsoidal; egg 45–67 (56) × 32–43 (36); parasite of zanclids … Paraschistorchis zancli (Hanson, Reference Hanson1953) Blend, Karar & Dronen, Reference Blend, Karar and Dronen2017.
Supplementary material
The supplementary material for this article can be found at http://doi.org/10.1017/S0022149X26101266.
Authors contribution
Yasser F.M. Karar: Conceptualization, methodology, formal analysis, investigation, writing – original draft, writing, review and editing, visualization. Charles K. Blend: Conceptualization, writing – review and editing, visualization, supervision. Ali Mansour: Writing – review and editing, validation.
Availability of data and materials
All the data generated from this study are included in the paper and can be accessed from the corresponding author upon request for approval.
Use of artificial intelligence (AI) tools
The authors declare that there is no use of AI-assisted technologies in text/images generation or to analyse or extract insights from data or other material.
Acknowledgements
The authors thank and offer continued appreciation to Dr. Thomas Cribb, Honorary Research Fellow, Biodiversity and Geosciences, Queensland Museum, Collections and Research Centre, Queensland, Australia, for his help and for facilitating the microscopic examination of several deposited specimens on which the present study is based. This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
Financial support
None.
Competing interests
The authors declare that they have no conflict of interest.
Ethical standard
All applicable international, national, and/or institutional guidelines for the care and use of museum-deposited specimens were followed. South Valley University ethics approval no REC-FScSVU:03/12/23.
