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.

Disentangling the evolutionary and biogeographical history of the freshwater fish trematode genus Creptotrema (Digenea: Allocreadiidae) using an integrative taxonomy approach: the case of Creptotrema agonostomi in Middle American mountain mullets.

Species of the allocreadiid genus Creptotrema are parasites of freshwater fishes in the Americas. Species in the genus possess one pair of muscular oral lobes on the oral sucker. Currently, the genus contains eight species, six distributed in South America, one in Middle America and one in North America. Genetic data are only available for the North American species, Creptotrema funduli, a parasite of fundulids originally described from Oneida Lake, New York State. In this study, we obtained 28S ribosomal DNA sequences of trematodes morphologically similar to Creptotrema agonostomi from the mountain mullet, Dajaus monticola, across a wide geographical range in Middle America. Our molecular phylogenetic analyses showed that (1) the genus Creptotrema, as currently conceived, is not monophyletic; (2) the allocreadiids in mountain mullets should be re-allocated in the genus Pseudoparacreptotrema; and (3) the allocreadiid trematodes from D. monticola across Middle America represent four morphologically similar species, three of which can be distinguished genetically. These three new species are described herein using an integrative taxonomy approach. We contend that accurate estimates of species diversity and phylogenetic relationships among allocreadiids, and most likely other species of trematodes, necessarily require an integrative taxonomy approach that should consider at least DNA sequences and scanning electron microscopy.

A new genus and species of Brachycoeliidae (Digenea) from Chiropterotriton sp. (Caudata: Plethodontidae) in Mexico and its phylogenetic position within the Plagiorchiida based on partial sequences of the 28S ribosomal RNA gene.

Parabrachycoelium longicaecum n. gen., n. sp. (Digenea: Brachycoeliidae) is described from the intestine of a plethodontid salamander Chiropterotriton sp. Hosts were collected in bromeliads at the cloud forest of Tlaquilpa, Veracruz, Mexico. Members of the Brachycoeliidae Looss, 1899 (sensu Yamaguti, 1971) are characterized by having a spined tegument; ceca usually short, not passing level of gonads, but longer in some species; gonads posterior to, or in region of, acetabulum, with ovary anterior to testes; a well developed cirrus pouch containing a bipartite seminal vesicle; and uterus occupying entire hind-body posterior to testes. However, this combination of morphological traits prevents the inclusion of the new taxon in any of the genera in that family; a new genus was, therefore, erected to accommodate the new species. The new taxon is readily distinguished from members belonging to Brachycoelium Dujardin, 1845, Mesocoelium Odhner, 1910, and Tremiorchis Mehra and Negi, 1925, by having long ceca extending into the posterior third of the body, slightly surpassing the testes, and vitellaria extending along the body. The new species morphologically resembles Caudouterina rhyacotritoni Martin, 1966, a digenean parasitizing a plethodontid salamander; however, the latter species lacks spines in the tegument and is actually placed within the Allocreadiidae. To demonstrate further the phylogenetic position of the new taxon, we sequenced the D1-D3 regions of 28S rRNA gene and conducted a phylogenetic analysis of available sequences for the order to which brachycoeliids belong (Plagiorchiida). Sequence divergence of the partial 28S rRNA gene confirms its distinction from the aforementioned brachycoeliids, and the phylogenetic position within the Plagiorchiida places the new species as closely related to a clade formed by Brachycoelium + Mesocoelium. Divergence levels and phylogenetic position within the Plagiorchiida verifies the validity of the new genus and its inclusion in Brachycoeliidae.

A new species of Sprostoniella Bychowsky and Nagibina, 1967 (Monogenea: Capsalidae) from Chaetodipterus zonatus (Osteichthyes: Ephippidae) in Chamela Bay, Mexico.

A new species of Sprostoniella is described from the gills of Chaetodipterus zonatus in Chamela Bay, on the west coast of Mexico. The new species differs from the 2 previously described congeners by having smaller testes each of which is not nearly as long as the ovary. It further differs from the type species, Sprostoniella multitestis, by having a poorly developed pair of accessory sclerites and the cecal diverticula are not confluent posteriorly. The new species also differs from Sprostoniella micrancyra, the other species of the genus, by having a shorter peduncle, the glands of Goto are smaller than the testes and ovary, and by having a slightly different haptoral structure, with 1 pair of hamuli rather than 2 pairs, and 1 central haptoral loculus instead of 2 loculi.