Occurrence of Echinorhynchus clavula (Dujardin, 1845) nec Hamann, 1892 (Acanthocephala) in the Fish of Llyn Tegid (Bala Lake), Merionethshire

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Abstract
The grayling Thymallus thymallus, pike Esox lucius. roach Rutilus rutilus, and eel Anguilla anguilla of Llyn Tegid were examined for the occurrence of Echinorhynchus clavula; the total infection of these species of fish was 46.0%, 11.5%, 16.1%, and 27.7%, respectively. The degree of infection may be correlated with the feeding of the fish on Asellus meridianus, the only known intermediate host of E. clavula in Llyn Tegid, although in the case of the pike of the size range examined, a secondary establishment of the worms is postulated. The percentage occurrence of male E. clavula was lower than that of female worms. The production of shelled acanthors by female worms may be correlated with the degree of concentration of E. clavula in the fish intestine; thus, in the eel, where maximum concentration was found, maximum production of shelled acanthors was also found. No seasonal periodicity of occurrence and maturation of E. clavula was observed, shelled acanthors were seen during all months except February and December, and the failure to find worms with shelled acanthors in these months is believed to be a sampling deficiency. The literature relevant to the distribution of E. clavula and to the seasonal periodicity of occurrence and development of Acanthocephala is reviewed. It is postulated that temperature may play a significant role in the seasonal periodicity of some Acanthocephala of fishes.