Aim The distributional patterns of helminthological fauna of freshwater fishes were analysed to postulate a general hypothesis on the relationships of some Mexican hydrological systems.Location Eight hydrological systems of central and eastern Mexico were studied and compared with records from Nicaragua.Methods A Parsimony Analysis of Endemicity (PAE) was applied to the presence/ absence of ninety-two helminth parasite taxa (Monogeneans, Digeneans, Cestodes, Acanthocephalans and Nematodes) of freshwater fishes, from eight Mexican hydrological systems, using the Hennig86 program. ResultsThe results represent the first attempt for a biogeographical analysis through application of the PAE method to the distributional patterns of helminth parasites of freshwater fishes in Mexico. A single most parsimonious cladogram was obtained, which grouped all the Neotropical systems in accordance with previous proposals based on other plant and animal taxa. Main conclusionsThe most basal systems were Santiago and Lerma basins, which exhibited Nearctic affinities. The remaining areas of the cladogram showed Neotropical affinities. All the southeastern systems were grouped in a clade with the Nicaragua system, providing support for a ÔMesoamerican provinceÕ based on helminth parasites of cichlids. The cladogram also suggests that the treatment of the Lerma-Santiago basin as a single biogeographical unit is inaccurate and that they should be treated as separate systems.

A checklist based on previously published records and original data is presented for the helminth parasites reported in 35 fish species from nine families from the Rio Papaloapan basin, east Mexico. The checklist contains 85 taxa from 39 helminth families. Trematodes and nematodes were the most abundant taxonomic groups. The helminth fauna in the fish of the Papaloapan River basin predominantly consists of Neotropical species that are largely autogenic. The introduced species Centrocestus formosanus was the most widely distributed helminth, infecting 16 host species. Ten of the recorded helminth species have only been found in fish from the Papaloapan. This inventory contributes 157 new host records, and reports the presence of 30 helminth species in the Papaloapan for the first time . This inventory shows the richness of helminth parasite species in the fish of the Papaloapan River basin in comparison with the other hydrological basins in Mexico. It also demonstrates that this fauna is typically Neotropical and quite similar to that from the neighboring basins of the Grijalva-Usumacinta system and the Yucatan Peninsula. The data also suggest highly effective transmission between environments within the same basin and that the regional parasite fauna is strongly influenced by fish community composition.

From August 2008 to July 2010, 1,471 fish belonging to the subfamily Goodeinae (representing 28 species) were collected from 47 localities across central Mexico and analyzed for helminth parasites. In addition, a database with all available published accounts of the helminth parasite fauna of goodeines was assembled. Based on both sources of information, a checklist containing all the records was compiled as a necessary first step to address future questions in the areas of ecology, evolutionary biology and biogeography of this host-parasite association. The checklist is presented in two tables, a parasite-host list and a host-parasite list. The checklist contains 51 nominal species, from 34 genera and 26 families of helminth parasites. It includes 8 species of adult digeneans, 9 metacercarie, 6 monogeneans, 3 adult cestodes, 9 metacestodes, 1 adult acanthocephalan, 1 cystacanth, 6 adult nematodes and 8 larval nematodes. Based on the amount of information contained in the checklist, we pose that goodeines, a subfamily of viviparous freshwater fishes endemic to central Mexico, might be regarded as the first group of wildlife vertebrate for which a complete inventory of their helminth parasite fauna has been completed.

Distribution records of 152 adult helminth taxa parasites of freshwater fishes in Mexico were analysed to determine areas of high richness and endemism. Distribution maps were prepared for each taxon and overlaid onto a map of Mexico divided into 1 ¥ 1 degree grid-cells. Richness was determined by counting recorded helminth species in each grid-cell. A corrected weighted endemism index was calculated for each grid-cell, and the relationship between richness and endemicity was analysed with an Olmstead-Tukey corner test of association. Five areas of high richness and endemism were identified: (1) Los Tuxtlas and the Papaloapan river basin, on the Gulf of Mexico; (2) the Grijalva-Usumacinta basin near the Gulf of Mexico coastal plain; (3) the Yucatan Peninsula; (4) the Sierra de Manantlán Biosphere Reserve in western Mexico; and (5) the Pátzcuaro lake, in central Mexico. The distribution of richness and endemism of helminth parasites of freshwater fishes in Mexico is congruent with distributional patterns described for other freshwater taxa in Mexico. Patterns of richness and/or endemism in the studied areas can be explained by the ichthyological composition of their bodies of water. The present study establishes an objective way of analysing the relationship between richness and endemicity, and suggests that helminths can make valuable contributions to regionalization of geographical areas and for identification of rich and biologically complex areas with potential for conservation of aquatic systems.

The Asian fish tapeworm, Schyzocotyle acheilognathi (syn. Bothriocephalus acheilognathi) represents a threat to freshwater fish, mainly cyprinids, across the globe. This tapeworm possesses an extraordinary ability to adapt to different environmental conditions and, because of that, from its natural geographical origin in mainland Asia, it has colonized every continent except Antarctica. It is thought that this pathogenic tapeworm was first co-introduced into Mexico in 1965 from China, with the grass carp Ctenopharyngodon idella, although the first formal record of its presence was published in 1981. Over the past 35 years, the Asian fish tapeworm has invaded about 22% of the freshwater fish in Mexico. Because fish communities in Mexico are characterized by high species richness and levels of endemism, S. acheilognathi is considered as a co-introduced and co-invasive species. In this review, we update the geographic distribution and host spectrum of the Asian fish tapeworm in Mexico. Up until December 2016, the tapeworm had been recorded in 110 freshwater fish species (96 native and 14 introduced), included in 51 genera, 11 families and 4 orders; it was also widely distributed in all types of aquatic environments, and has been found in 214 localities. We present novel data from a survey aimed at establishing the distribution pattern of the tapeworm in native freshwater fishes of two rivers in north-central Mexico, and the genetic variation among individuals of this co-invasive species collected from different host species and localities. We discuss briefly the factors that have determined the remarkable invasive success of this parasite in freshwater systems in Mexico.