[A study of parasitic trematodes of Littorina saxatilis and their impact on this mollusc (author's transl)].

A monthly study of populations of Littorina saxatilis (mollusc, gasteropod) very strongly parasited has allowed to establish relations between the said mollusc and some parasite trematodes which live in it. A selection of animals was done in two stations of the Roscoff region (Finistere). A total of more than 1.000 individuals have been examined in each station. Eleven species of cercairs have been spotted. It has appeared that the Microphallus similis was the most represented: 79,86% of the total. This rate of parasitism does not change much throughout the year. The populations of molluscs strongly parasited by Microphallus similis show an important castration. A double association of cercairs remains rare but is yet possible. It appears that aged individuals are the most parasited. It seems that there would be a growth of parasites more important in females than in males.