Towards resolving the problematic status of the digenean genus Astiotrema Looss, 1900: Taxa excluded from Astiotrema (sensu stricto) with special reference to plagiorchioid genera closely related to the restricted concept of Astiotrema.

The restricted concept of Astiotrema Looss, 1900 has been revised to include only eight species and various representative synonyms. However, several remaining taxa of Astiotrema (sensu lato) still need more inspection and scrutiny to determine their correct taxonomic position. Following a comprehensive review, four new genera are erected to accommodate some taxa excluded from Astiotrema (sensu stricto), three of which are closely related to this restricted concept of Astiotrema. Plesioastiotrema n. gen. is erected to accommodate Plesioastiotrema monticellii (Stossich, 1904) n. comb. (syn. Astiotrema monticellii Stossich, 1904) as the type-species and Plesioastiotrema magniovum (Fischthal & Kuntz, 1965) n. comb. (syn. Astiotrema magniovum Fischthal & Kuntz, 1965). Homeoastiotrema n. gen. is established for its type-species, Homeoastiotrema turneri (Bray, Van Oosterhout, Blais & Cable, 2006) n. comb., to accommodate Astiotrema turneri Bray, Van Oosterhout, Blais & Cable, 2006. Ichthyastiotrema n. gen. is erected with its type-species, Ichthyastiotrema fotedari (Dhar, 1977) n. comb. (syn. Astiotrema fotedari Dhar, 1977). A distinct morphologically and taxonomically distant taxon from Astiotrema (sensu stricto) is proposed in its own genus, Alloastiotrema n. gen. with its type-species, Alloastiotrema birmanii (Khan, Gul-E-Lala, Ghazi, Khatoon, Waheed & Khan, 2021) n. comb., to accommodate Astiotrema birmanii Khan, Gul-E-Lala, Ghazi, Khatoon, Waheed & Khan, 2021 and positioned distant from Astiotrema (sensu stricto). Astiotrema erinaceum (Poirier, 1886) Stossich, 1904, Astiotrema trituri Grabda, 1959 and Astiotrema (Biguetrema) tananarivense Deblock & Capron, 1962 are adopted synonyms of Galactosomum erinaceum (Poirier, 1886) Bittner & Sprehn, 1928, Neoastiotrema trituri (Grabda, 1959) Tkach, 2008 and Laiogonimus tananarivensis (Deblock & Capron, 1962) Fischthal & Thomas, 1968, respectively. Astiotrema lazeri El-Naffar, Saoud & Hassan, 1984 and Astiotrema gangeticus Gupta & Singh, 1985 nec Harshey, 1932 are synonymized with Glossidium pedatum Looss, 1899 and Orientocreadium batrachoides Tubangui, 1931, respectively. Based on its contradiction to the diagnosis of members of the Orientocreadiidae Yamaguti, 1958, we declare Orientocreadium lucknowensis Nigam, Chandra, Johri & Saxena, 2015 as incertae sedis. Longigula Qiu, Zhang & Li, 1983 and Kalipharynx Boeger & Thatcher, 1983 are morphologically closest to Astiotrema (sensu stricto) compared to members of the Plagiorchiidae Lühe, 1901 based on both genera possessing a cirrus-pouch with a unipartite, saccular seminal vesicle. The problematic status of Pseudoparamacroderoides Gupta & Agrawal, 1968 (sensu lato), the closest related genus to Astiotrema (sensu stricto), is discussed through evaluating the differential characteristics among listed species to indicate the extent of their validity and identifying the genuine species within this genus to re-evaluate the confusing and overlapping species to help understand their relationships with closely related plagiorchioids. Accordingly, only three species are recognized within Pseudoparamacroderoides Gupta & Agrawal, 1968 (sensu stricto): Pseudoparamacroderoides dongthapensis Truong, Curran & Bullard in Truong, Curran, Dutton & Bullard, 2021; Pseudoparamacroderoides pseudobagri (Wang in Wang, Zhao, Chen & Tao, 1983) n. comb. (syns. Astiotrema pseudobagri [Wang in Wang, Zhao, Chen & Tao, 1983] Karar, Blend, Dronen & Adel, 2021; Gauhatiana pseudobagri Wang in Wang, Zhao, Chen & Tao, 1983); and Pseudoparamacroderoides seenghali Gupta & Agrawal, 1968 (Syn. Pseudoparamacroderoides vittati Kakaji, 1969 n. syn.). Pseudoparamacroderoides raychaudhurii Agarwal & Kumar, 1983 is re-evaluated as Alloglossidium raychaudhurii (Agarwal & Kumar, 1983) n. comb. Anomalomacroderoiditrema n. gen. is erected for the type-species, Anomalomacroderoiditrema keni (Agarwal & Agarwal, 1984) n. comb., to accommodate specimens of Pseudoparamacroderoides keni Agarwal & Agarwal, 1984. Although the morphological convergence of Gauhatiana Gupta, 1953 within the Plagiorchioidea Lühe, 1901 has been suggested, it is neither a plagiorchiid nor a macroderoidid and does not appear closely related to Astiotrema (sensu stricto); it evidently is a member of the Monorchiidae Odhner, 1911. Alloglyptus Byrd, 1950 is taxonomically positioned as a gorgoderoid in the Allocreadiidae Looss, 1902, neither a plagiorchioid taxon nor closely related to Astiotrema (sensu stricto). The ambiguity of the seminal receptacle in some taxa of Astiotrema is discussed.

Towards resolving the problematic status of the digenean genus Astiotrema Looss, 1900: An updated concept and revision of species composition for Astiotrema (sensu stricto).

Species of Astiotrema Looss, 1900 (sensu lato) infect a wide range of fishes, amphibians and reptilians. They also possess a considerably wide spectrum of morphological features. Several species were recognized for variable, confusing, overlapping and unspecialized morphological characters rather than for unique distinguishing features, causing continuing dispute around the validity of several species. Following comprehensive review, a revised restricted concept of Astiotrema is proposed including a morphologically strict definition. Both Tremiorchis Mehra Negi, 1926 and Astioglossimetra Bilqees, Khatoon Khan, 2002 are synonymized with Astiotrema (sensu stricto). Several nominal species are synonymized, others are excluded and characters for each recognized species are presented and explained. Only eight species are recognized: Astiotrema cyclemysi Siddiqi, 1965, Astiotrema emydis Ejsmont, 1930, Astiotrema fotedari Dhar, 1977, Astiotrema impletum (Looss, 1899) Looss, 1900, Astiotrema karachiensis (Bilqees, Khatoon Khan, 2002) n. comb., Astiotrema odhneri Bhalerao, 1936, Astiotrema ranarum (Mehra Negi, 1926) Fotedar, 1971 and Astiotrema reniferum (Looss, 1898) Looss, 1900. A key to the species of Astiotrema (sensu stricto) is presented, a comprehensive list of all host-locality records is included and host-parasite specificity is elucidated.

A new genus and species of Anchitrematidae Mehra, 1935 (Digenea: Gorgoderoidea) in a freshwater fish from the River Nile at Qena, Egypt with amendation of the family.

Museum specimens of a previously unknown species of the Anchitrematidae from a freshwater fish collected from the River Nile at Qena, Egypt were examined. This species is somewhat similar to species of Anchitrema, but was found to have a thick, continuous, shelf-like rim around the periphery of the ventral aspect of the distome, the lateral aspect of the forebody was composed of large internal patches of lightly vacuolated, somewhat glandular cells that formed pad-like structures (pelops-like structures), and it possessed a uterine seminal receptacle; characteristics that previously have not been found in any species in the family. Species currently assigned to Anchitrema or Mujibia (the only two genera in the family) are known only from reptiles and mammals and no species has previously been reported from a freshwater fish. We feel that these differences are sufficient enough to warrant the erection of Piscianchitrema n. gen. and amendation of the family. We support the synonymies of Anchitrema congolense with Anchitrema latum, and Anchitrema lucknowensis with Anchitrema indicum. The current status of species previously described in the family is discussed, and keys to the genera and species of Anchitrema are provided.

Unusual opecoelids from Red Sea triggerfishes with special reference to characteristic concepts of the Opistholebetinae Fukui, 1929 (Digenea: Opecoelidae).

Three digeneans belonging to the Opecoelidae are reported and described from triggerfishes (Tetraodontiformes: Balistidae) collected in the northern Red Sea off Egypt. Both Macvicaria longicirrata (Manter, 1963) Aken-Ova, Cribb Bray, 2008 and Neopycnadena tendu (Bray Justine, 2007) n. comb. were recovered from the intestine of the titan triggerfish, Balistoides viridescens (Bloch Schneider)-each represents a new host record-and Gaevskajatrema balistes n. sp. was found parasitizing the lower intestine of the Picasso triggerfish, Rhinecanthus assasi (Forsskål). We continue to support synonymy of Gaevskajatrema ponticum (Koval, 1966) Machkevsky, 1990 with Gaevskajatrema perezi (Mathias, 1926) Gibson Bray, 1982, not as a differentiated species. We adopt the restricted posterior extension of the ceca and vitellarium to the testicular zone, without extension of either into the post-testicular space, as diagnostic in distinguishing Gaevskajatrema. Gaevskajatrema balistes n. sp. differs from G. perezi based on its substantially smaller body size with fewer eggs, a longer cirrus-pouch reaching ovarian level and it parasitizes a distinct host group from a structurally and ecologically different ecosystem. Neopycnadena n. gen. is erected for Pseudopycnadena tendu Bray Justine 2007 based on its possessing a large broadly oval cirrus-pouch with a massive field of prostatic cells occupying the entire volume of the cirrus-pouch, a wide, cup-shaped and thick-walled ejaculatory duct, distinct dorsal position of the excretory pore, the bifurcal dextral position of the genital pore, its report from a distinct host group and distant locality and its phylogenetic uniqueness compared with Pseudopycnadena fischthali Saad-Fares Maillard 1986. Neopycnadena n. gen. is ecologically similar to opistholebetines in their life-cycles and morphology; however, phylogenetically separate from opistholebetines as well as from the Polypipapiliotrematinae Martin, Cutmore Cribb in Martin, Sasal, Cutmore, Ward, Aeby Cribb, 2018 and members of Clade [C] of Martin and colleagues, thus we conclude that Neopycnadena n. gen. is unique. Neopycnadeninae n. subfam. is proposed to accommodate Neopycnadena n. gen. We consider that the probable characterization of tetraodontiform specialist taxa (as indicated by the presence of a muscular post-oral ring) and the specificity of the Opistholebetinae Fukui, 1929 sensu stricto with a tetraodontiform host are no longer reliable characters differentiating Gaevskajatrema and Macvicaria Gibson Bray, 1982. The nature of the post-oral structure is discussed and it is adopted to be a diagnostic feature at the generic level among taxa of the Opistholebetinae sensu latu. It is concluded that the expanded concept of the Opistholebetinae is more supported than the restricted one, Birendralebes Srivastava Ghosh, 1972 remains incertae sedis within the Opecoelidae Ozaki, 1925 rather than in the Opistholebetinae, and we provide a generic key to the Opistholebetinae.

Re-examination of the phylogenetic relationships within the Gyliauchenidae Fukui, 1929 (Digenea) based on morphological and molecular evidence with a proposal for Paragyliaucheninae n. subfam. and a description of Flagellotrema convolutum Ozaki, 1936.

Flagellotrema convolutum Ozaki, 1936 was found parasitising the intestine of two new host fish species, the Indian sail-fin surgeonfish, Zebrasoma desjardinii (Bennett) (Acanthuridae), and the Picasso triggerfish, Rhinecanthus assasi (Forsskål) (Balistidae), from the northern Red Sea off Egypt. Another description of this species is provided with detailed morphological observations made of the genital systems. Using newly acquired molecular data from the D1-D3 regions of 28S rDNA, the phylogenetic relationships of subfamilies and genera within the Gyliauchenidae Fukui, 1929 are elucidated with morphological support. The Petalocotylinae Ozaki, 1934 and the Robphildollfusiinae Paggi & Orecchia, 1963 are recognized as valid subfamilies within the Gyliauchenidae. The Apharyngogyliaucheninae Yamaguti, 1942 and the Ichthyotreminae Caballero & Bravo-Hollis, 1952 remain junior synonyms of the Gyliaucheninae Fukui, 1929. Based on its unique position relative to all gyliauchenid subfamilies and its distinct separation from all other gyliauchenine genera, the Paragyliaucheninae n. subfam. is erected to contain Paragyliauchen Yamaguti, 1934. Paragyliauchen differs from all other gyliauchenine genera by having a pharynx differentiated into two, well-developed muscular regions: an anterior region composed of a ring with indented projections anteriorly and a posterior region that is ellipsoidal or barrel-shaped. Modified and/or new keys to the four subfamilies we recognize within the Gyliauchenidae as well as the genera within each subfamily are presented, and we discuss the evolutionary development and etymology of the unique anatomy of the anterior of gyliauchenids.

Revision of the Megaperidae Manter, 1934 n. comb. (Syn. Apocreadiidae Skrjabin, 1942) including a reorganization of the Schistorchiinae Yamaguti, 1942.

Modified and/or new keys to the four subfamilies now recognized within the Megaperidae Manter, 1934 n. comb. (Syn. Apocreadiidae Skrjabin, 1942) as well as the genera within each subfamily are presented. Two new genera, Paraschistorchis n. gen. and Plesioschistorchis n. gen., both within the Schistorchiinae Yamaguti, 1942, are erected and keys are provided to the species considered in both new genera-distinguished by possessing caeca that end either in separate ani or blindly. Plesioschistorchis callyodontis (Yamaguti, 1942) n. comb. and Plesioschistorchis haridis (Nagaty, 1957) n. comb. are re-described from new material collected from the common parrotfish, Scarus psittacus Forsskål (Perciformes: Scaridae), inhabiting the Red Sea off Egypt; S. psittacus represents a new host record for both species. The taxonomic status of Schistorchis sensu stricto Lühe, 1906 is examined and revised, a key to the four species we consider in this genus offered, and the monotypic genus Megacreadium Nagaty, 1956 declared a junior synonym of Schistorchis. Members of Schistorchis sensu stricto possess a unique "complex" (i.e. highly cellular/glandular) instead of "simple" (i.e. entirely muscular) type of oral sucker that is quite large in relation to body size; an elongate, somewhat sub-rectangular-shaped body; 5+ testes arranged in at least two rows; caeca that open via separate ani; a long post-testicular region; a median genital pore either at the anterior margin of or just anterior to the ventral sucker; and species of Schistorchis sensu stricto parasitize the intestine of marine fish within the Order Tetraodontiformes Berg. With the revision of this genus, we re-describe Schistorchis carneus Lühe, 1906 from the lower and mid-intestine of the white-spotted puffer, Arothron hispidus (Linnaeus) (Tetraodontiformes: Tetraodontidae), collected in the Red Sea off Egypt. Finally, a plea is made for further study of the Megaperidae n. comb. focusing, in particular, on the following: (1) obtaining new type/voucher materials of Plesioschistorchis manteri (Gupta & Tandon, 1984) n. comb. and Schistorchis paruchini Kurochkin, 1974; (2) elucidating the life histories (i.e. intermediate hosts) of members of the Postporinae Yamaguti, 1958 and Schistorchiinae; and (3) generating DNA sequence data for more species of megaperids to help future workers produce increasingly accurate taxonomic classifications that better reflect phylogenetic relationships within this ecologically diverse group of digeneans.

Pelopscreadium aegyptense n. gen., n. sp. and Pelopscreadium spongiosum (Bray & Cribb, 1998) n. comb., (Digenea: Lepocreadiidae), each from disjunct populations of the Yellow boxfish, Ostracion cubicus Linnaeus (Ostraciidae).

Bianium spongiosum Bray & Cribb, 1998 (Lepocreadiidae), described from the yellow boxfish, Ostracion cubicus Linnaeus (Ostraciidae), off Lizard Island, Queensland, Australia, possesses a combination of the following three morphological features which distinguishes it from all the other species currently assigned to the genus: (1) large internal patches of large cells forming sponge-like pads we have termed "pelops"("pelop" sing.) laterally in the forebody extending from near the anterior extremity to about the level of the intestinal bifurcation rather than possessing a scoop; (2) ceca that reach to near the posterior extremity where they end blindly without ani; and (3) a vitellarium which is present laterally but not dorsal to the ceca. Based on this we propose the erection of Pelopscreadium n. gen. (Lepocreadiidae) with the assignment of B. spongiosum to this new genus as the type-species, Pelopscreadium spongiosum (Bray & Cribb, 1998) n. comb. Pelopscreadium aegyptense n. sp., also from the yellow boxfish but from the Red Sea off Sharm El-Naga, Egypt, is described as the second member of the new genus because it shares these three characteristics with P. spongiosum.