Discovery of a New Genus and Species of Allocreadiidae (Trematoda) in Mexico: Mesoamericatrema Magnisacculus n. Gen. n. Sp.

The trematodes of Allocreadiidae are mainly parasites of freshwater fish and the family contains 18 valid genera. Since 2004, 4 genera have been described in the Americas: Paracreptotrema, Paracreptotrematoides, Pseudoparacreptotrema and Auriculostoma, of which the latter was recently synonymized with Creptotrema by Franceschini et al., (2021). During a study in 2018 on fish parasites of Lacantún River, Chiapas, Mexico, allocreadiids were collected from the intestine of Atherinella alvarezi and morphological differences with other genera of the family were immediately noted. These differences were corroborated with molecular data and phylogenetic analyses, which indicated that it was a new genus and species. The objective of this study is to describe the new taxon, which we name Mesoamericatrema magnisacculus n. gen., n. sp. as well as to clarify its phylogenetic relationships, host specificity, and geographical distribution. For this, we carried out a meticulous study of the morphology and compared the new taxon with its congeners. We also obtained sequences of the 28S and ITS from the ribosomal RNA region that were used in generating molecular phylogenies and in calculating genetic distances with sequences of other species of Allocreadiidae available in the Genbank. The new genus is mainly characterized by having a very large cirrus sac in relation to the length of its body and nests in a clade that groups Creptotrema, Creptotrematina, Paracreptotrema, Paracreptotrematoides, Pseudoparacreptotrema and Wallinia, all of which have a Neotropical affinity in terms of their geographic and host distribution. Mesoamericatrema n. gen. is the fifth genus of Allocreadiidae to be described in the Americas since 2004, the third discovered in Mexico since 2016, and the first to be described as parasitizing Atheriniformes, which indicates that Middle America is a region where important speciation events have occurred in freshwater fish parasites.

New species of Cacatuocotyle (Monogenoidea, Dactylogyridae) parasitizing the anus and the gill lamellae of Astyanax aeneus (Pisces, Ostariophysi: Characidae) from the Rio Lacantún basin in the Biosphere Reserve of Montes Azules, Chiapas, Mexico.

As part of a biological inventory of the Rio Lacantún basin in the Biosphere Reserve of Montes Azules from Chiapas State (southeastern, Mexico), the following monogenoid ectoparasites infecting the external surface of the anal opening and the gill lamellae of the freshwater fish Astyanax aeneus (Characidae) in ten streams were found: Cacatuocotyle chajuli sp. nov. (anus), Cacatuocotyle exiguum sp. nov., and Cacatuocotyle sp. (gill lamellae). C. chajuli is differentiated from its single congener, Cacatuocotyle paranaensis (Boeger et al. Syst Parasitol 36:75-78, 1997), from the gills of Characidium lanei (Characidae) from Paraná Brazil, in having a noticeably V-shaped haptoral bar and larger hooks and anchors. C. exiguum differs from these two latter species in the size of its anchors. Illustrations and data on morphological and biometric variability of individual specimens of C. chajuli and C. exiguum from different streams are provided. The present data support the statement about three species of Astyanax, which harbor the richest monogenoidean diversity in the Neotropics with a total of 18 species reported up to now. Occurrences of species of Cacatuocotyle on different sites of infection on three distant host species [including a Neotropical cichlid (Cichlidae) species] suggest that these monogenoideans switching to new hosts can result in the availability of potential hosts within the same habitat instead of showing signs of preferential switching between closely related hosts or on their respective microhabitats.

Lueheia inscripta (Westrumb, 1821) (Acanthocephala: Plagiorhynchidae) in anurans (Leptodactylidae: Bufonidae) from Mexico.

Juveniles of Lueheia inscripta (Westrumb, 1821 Travassos, 1919 (Acanthocephala: Plagiorhynchidae), an acanthocephalan with six lemnisci, are reported and described from mesenteries of frogs Leptodactylus fragilis Brochi, 1877 and a toad Bufo marinus (Linnaeus, 1758) from Morelos state, Mexico. These are new host records extending the known geographical distribution of this species from Brazil and Puerto Rico to Mexico.

Two new species of rhabdochonids (Nematoda: Rhabdochonidae) from freshwater fishes in Mexico, with a description of a new genus.

Two new nematode species, Beaninema nayaritense n. gen., n. sp. and Rhabdochona xiphophori n. sp., are described on the basis of the specimens recovered from the gall bladder and intestine of 2 fishes, Cichlasoma beani (Jordan) (Cichlidae, Perciformes) and Xiphophorus sp. (Poeciliidae, Cyprinodontiformes), respectively, from Mexico. The monotypic genus Beaninema differs from other rhabdochonid genera mainly in the presence of large conical teeth in the middle of the prostom. Rhabdochona xiphophori is characterized mainly by a unique structure of the prostom (presence of 6 anterior teeth; dorsal and ventral teeth unusually broad, with 2 lateral horns) and the shape of the deirids (bifurcated, with markedly long base).

Rhabdochona mexicana sp. n. (Nematoda: Rhabdochonidae) from the intestine of characid fishes in Mexico.

A new nematode, Rhabdochona mexicana sp. n., is described based on specimens recovered from the intestine of two species of fishes, Astyanax mexicanus (De Filippi) (type host) and Astyanax fasciatus (Cuvier) (Characidae: Characiformes) in central Mexico. This species is characterised by the following characters: 10 anteriorly directed teeth in the prostom, a larger (left) spicule which is slender in form with a small bifurcation at its distal tip covered by a cuticular membrane, a smaller (right) spicule without a barb at its distal tip, eggs bearing an irregular flock-like coating, and a conical tail without a cuticular spike (in both sexes).

Spinitectus mexicanus n. sp. (Nematoda : Cystidicolidae) from the intestine of the freshwater fish Heterandria bimaculata in Mexico.

A new nematode, Spinitectus mexicanus n. sp., is described on the basis of the specimens recovered from the intestine of Heterandria bimaculata (Heckel) (Poeciliidae, Cyprinodontiformes) from 3 rivers of the Papaloapan River basin (type locality La Basura River), Los Tuxtlas, Veracruz State, Mexico. It differs from its congeners mainly in having the spination of the cuticle separated into 4 longitudinal sectors, each with posteriorly diminishing numbers of larger spines at the anterior part of body. It is the first species of Spinitectus described from a poeciliid fish and the second reported from freshwater fishes in Mexico.

Observations on cucullanid nematodes from freshwater fishes in Mexico, including Dichelyne mexicanus sp. n.

A new cucullanid nematode, Dichelyne mexicanus sp. n., is described from the intestine of three species of fishes, Agonostomus monticola (Bancroft) (Mugilidae, Perciformes) (type host), Ictalurus balsanus (Jordan et Snyder) (Ictaluridae, Siluriformes) and Cichlasoma beani (Jordan) (Cichlidae, Perciformes), from three rivers (La Maquina River, Veracruz; Chontalcoatlán River, Guerrero and Santiago River, Nayarit) in central Mexico. This species is characterised by the absence of a ventral sucker in the male (subgenus Dichelyne) and it differs from its congeners mainly in possessing very unequal and dissimilar spicules (left 0.465-0.768 mm and right 293-548 mm long), an asymmetrical gubernaculum, and two intestinal caeca. Another cucullanid nematode, Cucullanus caballeroi Petter, 1977, is reported from Dormitator maculatus (Bloch) (Eleotridae, Perciformes) from the La Palma and La Maquina Rivers and Balzapote stream, Veracruz, being briefly described and illustrated; this represents a new host record. Findings of D. mexicanus and C. caballeroi represent a new record of cucullanid nematodes from fishes in Mexican fresh waters.

Creptotrema agonostomi n. sp. (Trematoda: Allocreadiidae) from the intestine of freshwater fish of México.

Creptotrema agonostomi n. sp. is described from the mugilid fish Agonostomus monticola from Río Cuitzmala, Jalisco, east México, from Río Las Palmas and Río Máquinas, Veracruz, west México, and from the ictalurid, Ictalurus balsanus from Río Chontalcoatlán, Guerrero, east México. It is distinguished from other species of Creptotrema by its small size, large acetabulum with vertical incision, cirrus sac not reaching the posterior border of acetabulum, and very small eggs, measuring 0.041-0.057 x 0.020-0.033 mm.