Parasites in European eelAnguilla Anguilla(L.) from Danish Freshwater, Brackishand Marine Localities

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Abstract
The parasite fauna of 370 specimens of European eel, Anguilla anguilla(L.), (35–55 cm long) caught in three lakes in Denmark, three brackish localities and in the western Kattegat, was examined. A total of 46 species of parasites were found, the highest number (27) was found in a lake, and the lowest (10) in the western Kattegat. Myxidium giardi and Myxobolus sp. were found at all seven localities. Pseudodactylogyrus anguillae, Diplostomum sp. metacercariae, Bothriocephalus clauiceps, Proteocephalus macrocephalus, Raphidascaris acus, Anguillicola crassus and Ergasilus gibbus were found in both freshwater and brackish localities. Podocotyle atomon, Deropristis inflate, Hemiurus communis, Brachyphallus crenatus, Derogenes varicus, Lecithaster gibbosus, Lecithochinum rufotnride and Cryptocotyle lingua metacercariae occurred in brackish or marine localities. Gyrodactylus spp., tetraphyllidean larvae and Hysterothylacium aduncum occurred in brackish localities. The freshwater dracunculoid nematodes Philometra ouata, Daniconema anguillae and Anguillicola crassus were found in the swim-bladder; the latter was common in the smallest lake only. Third-stage larvae of Contracaecum rudolphii, Paracuaria adunca and Cosmocephalus obuelatus occurred encapsulated in the intestinal wall of eels from brackish localities. Acanthocephalus lucii, A. anguillae and Pomphorhynchus laevis were found, the latter in a brackish locality only. Copepodids of Lemaeocera branchialis were found on the gills of eels from the western Kattegat. Most parasite species live in the digestive system. They have no or little pathogenic effect on the host due to the low intensity of infection. The gills are often seriouslydamaged by the parasites, and M. giardi, P. anguillae and E. gibbus especially are pathogenic to the eels. It is suggested that under stressed conditions, e.g. oxygen deficiency, the presence of these parasites may be a hazard to the eels.