The introduction of non-native freshwater fish is a primary cause of aquatic biodiversity loss at global scale. Such introductions have a severe impact on freshwater ecosystems in terms of competition, predation, habitat alteration, genetic pollution and transmission of diseases and parasites. A systematic review was conducted on the helminths parasites of freshwater fish in the context of species introduction and a total of 199 publications were retrieved between 1969 and November 2022. Several scenarios may arise when a new fish species arrive in a recipient area. Non-native fish hosts can co-introduce their parasites without transmitting them to native fish (e.g., the case of North American Centrarchidae and their Monogenea parasites). Another possible outcome is the transfer of these parasites to the native fish fauna (spillover, e.g., the cases of the Nematoda Anguillicola crassus Kuwahara, Niimi & Itagaki, 1974 and the Monogenea Gyrodactylus salaris Malmberg, 1957). Reciprocally, non-native fish hosts may acquire parasites in their new distribution range whether these parasites are native or were previously introduced (e.g., the cases of Ponto-Caspian Gobiidae and the Chinese sleeper Perccottus glenii Dybowski, 1877). Acquired parasites can then be spilled back to the native fauna. This phenomenon is of particular interest when non-native fish hosts influence the dynamics of zoonotic parasites.

There is an increasing interest in touristic observations of top predators in the wild. Sharks are probably the most sought-after animal in marine ecosystems by divers. Regulations have been put in place, and even if they are more or less respected, providing food is still used in some places in order to attract wild animals. Because of the difficulty in sampling shark guts, few studies have analyzed the microbiota of sharks, and none have evaluated the effect of feeding on this microbiota. In this work, we compare microbiota assemblages of black-tip sharks between sites with and without regular feeding. Our results revealed a significant feeding effect on both alpha and beta diversities of microbiota. Notably, the alpha diversity of fed sharks was lower than unfed sharks. We hypothesize that this result is related to a lower diversity of food intake by sharks in places where feeding is regularly provided. More studies need to be conducted in order to estimate the impact of feeding on shark physiology.

Introduced species have a major impact on freshwater ecosystems, particularly on islands. Numerous fish species have been introduced in Corsica (Mediterranean island, southern France) as part of planned programs or clandestinely. The introduction of non-native freshwater fish species can have a range of impacts on the recipient ecosystem, including through the co-introduction of its pathogens. A sample of introduced perch Perca fluviatilis Linnaeus, 1758 from the artificial reservoir of Padula was examined following a report of parasites by an angler. The analyses revealed the occurrence of Eustrongylides sp. (Nematoda) and Clinostomum complanatum (Digenea), two zoonotic parasites in P. fluviatilis. Both parasites are reported for the first time in France. Eustrongylides sp. and C. complanatum may have been introduced with their fish intermediate hosts or through their final bird hosts. The occurrence of the two parasites raises concerns from both a veterinary and human health perspective as they can use a wide range of amphibians as intermediate hosts and can be acquired in humans through the consumption of raw or undercooked fish.

The introduction of freshwater fish species is a leading cause of aquatic biodiversity erosion and can spread parasites to native populations. Hidden diversity evidenced by recent taxonomic revisions can add further complexity to the issue by rendering biological assessment data incomplete. The Eurasian minnows Phoxinus are one such example of cryptic diversity, with several described species being invasive. Current non-native fish populations in the small Mediterranean island of Corsica (France) are the result of successive waves of introductions, including several Phoxinus species. This study aims at determining which Phoxinus species were introduced to Corsica using the cytochrome oxidase subunit I barcoding marker, reconstructing their introduction routes and examining their parasite communities. The study found four species in Corsica: Phoxinus phoxinus and Phoxinus csikii mainly in the northernmost studied drainage basin and Phoxinus dragarum and Phoxinus septimaniae in the Tavignano drainage basin. P. phoxinus and P. csikii were most likely introduced through a live bait wholesaler while P. dragarum and P. septimaniae were probably introduced by recreational anglers bringing their bait from continental France. The molecular study of their Gyrodactylus (Platyhelminthes: Monogenea) parasites with the ITS marker allowed us to hypothesize inter-drainage basin secondary introduction routes for P. phoxinus and P. dragarum. In several sampling sites, Phoxinus minnows had black spot disease caused by encysted metacercariae of Digenea, likely Posthodiplostomum cuticola. These parasites were also found on the brown trout Salmo trutta in a locality where this patrimonial species co-occurs with Phoxinus minnows. Barcoding should be used in fish communities monitoring to help to accurately identify cryptic species.

An extensive survey of macroparasites in 320 European eel Anguilla anguilla (Linnaeus, 1758) was conducted in two brackish lagoons and eleven freshwater localities in the Mediterranean island of Corsica (France) between spring 2021 and winter 2021–2022. It resulted in the identification of nineteen parasites: two Monogea, four Digenea, one Copepoda, four Acanthocephala, three Cestoda, and five Nematoda, including the first geographical records, as Corsican freshwater sites were studied for the first time. The silvering stage was determined, and the eels were aged through otolithometry to compare parasite communities. Classic parasitology indices, a multivariate analysis, and an analysis of indicator values (IndVal) showed clear preferences towards the host’s habitat and salinity. Seasonal variations were shown for several parasites. A dataset from the same two coastal lagoons was used to study the changes in the parasite communities over the last decade, and this showed an increase in the prevalence and abundance of three invasive helminth species: the Monogenea Pseudodactylogyrus bini (Kikuchi, 1929), Pseudodactylogyrus anguillae (Yin and Sproston, 1948) Gusev, 1965 and the Nematoda Anguillicola crassus Kuwahara, Niimi, and Itagaki, 1974. These pathogenic parasites were found in all sampled localities, except for the two Monogenea in the polyhaline-to-euhaline Urbino lagoon. It is thus advised that future management measures take into account the environmental preferences of the most concerning parasites.