The family Clinostomidae Lühe, 1901 contains 29 species allocated to seven genera, of which Clinostomum Leidy, 1856 is the most diverse, with c. 14 valid species. The diversity of Clinostomum has been assessed, combining morphological and molecular data. The genetic library for species in this genus has increased steadily, although there is little or no information for the other genera included in the family. Molecular phylogenetic relationships among the genera of clinostomids have not been assessed, and their classification is still based on morphological traits. The monotypic Ithyoclinostomum was described from a fish-eating bird in Brazil, and its metacercariae have been found in several locations in South America, parasitizing erythrinid freshwater fishes. We collected unusually large metacercariae from the body cavity of cichlids in several locations across Middle America. These metacercariae exhibited some resemblance to Ithyoclinostomum, although several differences prevent their inclusion in Ithyoclinostomum dimorphum, casting doubt on their taxonomic identification. The main objective of this paper was to characterize the metacercariae collected in cichlids using both morphology and molecular data from three molecular markers, and to assess the molecular phylogenetic relationships among the genera of Clinostomidae to establish the position of the newly generated sequences. We took a conservative position and tentatively placed the metacercariae as belonging to Ithyoclinostomum.

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.

The phylogenetic position of Clinostomum heluans Braun, 1899 within the genus Clinostomum Leidy, 1856 is reported in this study based on sequences of the barcoding region of the mitochondrial cytochrome c oxidase subunit 1 gene ( COX1). Additionally, molecular data are used to link the adult and the metacercariae of the species. The metacercariae of C. heluans were found encysted infecting the cichlid fish Australoheros sp. in Minas Gerais, Brazil, whereas the adults were obtained from the mouth cavity of the Great White Egret, Ardea alba, in Campeche, Mexico. The COX1 sequences obtained for the Mexican clinostomes and the Brazilian metacercaria were almost identical (0.2% molecular divergence), indicating conspecificity. Similar molecular divergence (0.2-0.4%) was found between sequences of C. heluans reported here and Clinostomum sp. 6 previously obtained from a metacercaria recovered from the cichlid Cichlasoma boliviense in Santa Cruz, Bolivia. Both maximum likelihood and Bayesian inference analyses unequivocally showed the conspecificity between C. heluans and Clinostomum sp. 6, which form a monophyletic clade with high nodal support and very low genetic divergence. Moreover, tree topology reveals that C. heluans occupies a basal position with respect to New World species of Clinostomum, although a denser taxon sampling of species within the genus is further required. The metacercaria of C. heluans seems to be specific to cichlid fish because both samples from South America were recovered from species of this fish family, although not closely related.

individual fish belonging to 23 species were collected in 23 localities along the Nazas River drainage, with sample sizes per species varying from 2 to 145 specimens ( Figure 1; Table 1). Fish were collected under the Cartilla Nacional de Colector Científico de Flora y Fauna Silvestre (FAUT-0057 issued to G.P.P.L.). Fish were collected with seine nets and electrofishing, and were kept alive and studied for helminths no more that 4 hours after capture. All internal organs were analyzed separately in Petri dishes with 0.65 % saline under the stereoscope. Gills from each host were also obtained and placed in tap water to search for monogeneans. Monogeneans were fixed in glycerin ammonium-picrate (GAP) in order to study their sclerotized structures. After the morphological evaluation, specimens fixed with GAP were remounted in Canada balsam following Ergens (1969); some specimens were fixed in hot formaline 4 %, stained with Gomori's trichrome and mounted in Canada balsam to study their internal organs. Endoparasites were removed to a Petri dish with saline 0.65 %, prior to fixation. Digeneans and cestodes were fixed with hot (steaming) 4 % formalin. Acanthocephalans were maintained at 4 °C for 12 hours in distilled water, and then fixed in 70 % ethanol. Nematodes were fixed with hot (steaming) 4 % formalin or 70 % ethanol. All helminths were processed following standard procedures. Identification was made using specialized literature, and representative specimens of all helminth species were deposited at the Colección Nacional de Helmintos, Instituto de Biología, Universidad Nacional Autónoma de México (UNAM), Mexico City (CNHE). Results and DiscussionIn total, 41 helminth species were found, representing 26 genera and 17 families. Of the 41 species we record herein, 19 were monogeneas, 10 digeneans, seven Abstract: This paper represents the first study of the helminth parasites of freshwater fishes from the Nazas River basin in northern Mexico. Between July 2005 and December 2008, 906 individual fish were collected and examined for helminth parasites in 23 localities along the river basin. Twenty-three species of fish were examined as a part of this inventory work. In total, 41 helminth species were identified: 19 monogeneans, 10 digeneans, seven cestodes, one acanthocephalan, and four nematodes. The biogeographical implications of our findings are briefly discussed.