Bothriocestus claviceps (Cestoda: Bothriocephalidae) in pike in southwestern France (Teleostei: Esocidae): molecular evidence.

Bothriocestus claviceps (Goeze, 1782) is a cestode parasite infecting several fish species. One of its principal definitive hosts is the European eel Anguilla anguilla. The parasite is widely distributed across Europe and has been reported from numerous countries, such as Poland, Czechia, and France. In southern France, the European eel co-occurs with the Aquitanian pike, Esox aquitanicus, an endemic species. This pike is threatened by human activities and hybridization with the introduced Northern pike Esox lucius and is therefore classified as "vulnerable" on the French Red List of the International Union for Conservation of Nature (IUCN). Despite its conservation status, little information is available regarding its biology, particularly its parasitofauna, which remains undocumented. Over a three-year period, Aquitanian pike were collected and examined for parasites. Cestodes recovered from the intestinal tract were analyzed using a molecular approach. Molecular identification of the species was based on partial sequences of 28S ribosomal DNA and cytochrome oxidase subunit I (COI). Morphometric data were also obtained to provide additional data on the identified cestode species. Our results provide the first evidence of the presence of B. claviceps in the intestine of E. aquitanicus, as well as in one hybrid (E. aquitanicus × E. lucius), and suggest that these fish can act as definitive or post-cyclic hosts for this cestode species. This study contributes new insights into the parasitofauna of the Aquitanian pike and expands current knowledge of B. claviceps.