Studies on Crepidostomum metoecus (Braun) and C. farionis (Müller), parasitic in Salmo trutta L. and S. salar L. in Britain

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Abstract
1. Crepidostomum farionis (Müller, 1784) and C. metoecus (Braun, 1900) were found to occur together in the same host fish, Salmo trutta L., S. salar L. and Anguilla anguilla (L.) in mid-Wales. It is believed that the latter species constituted an accidental host.2. C. metoecus is recorded in Britain for the first time.3. There was some evidence of a habitat isolation in individual fish as C. metoecus occurred predominantly within the pyloric caeca, while C. farionis usually occupied a more posterior station in the intestine.4. A detailed description is given of G. metoecus and the salient features of this species are compared with those of C. farionis.5. The two species of Crepidostomum commonly occurred in large numbers in their fish hosts, individual fish harbouring up to 157 worms. No lesions attributable to Crepidostomum were, however, detected and there was no evidence of pathogenicity.6. It would appear that the eel is physiologically immune to both species of Crepidostomum and that the salmon parr is partially resistant.7. The degree of infestation of trout and parr with both the Crepidostomum species is at a maximum during the winter months and at a minimum in the summer months. This seasonal variation can be attributed to a periodicity in the swarming of the cercariae.8. There is no evidence of age resistance or acquired immunity of the trout to infestation by Crepidostomum species.