Interactions between

Field investigations were carried out in mid-April for two years in road ditches located in the department of Indre (central France) to determine whether there was competition between Anisus spirorbis and Galba truncatula in habitats where the two species live together. Compared to control populations, the number of A. spirorbis living in a community was 70 per cent lower in 2023 and 59 per cent lower in 2024, while the density of G. truncatula showed no significant variation. Shell diameter (A. spirorbis) or shell height (G. truncatula) did not differ significantly between community and control snails. Laboratory studies were also conducted from March to June for two years by placing juvenile, pre-adult or adult planorbids in the presence of juvenile, pre-adult or adult G. truncatula in Petri dishes for 30 days. The life stage of G. truncatula had a significant influence on the survival of A. spirorbis. In pairwise-raised snails, this survival was significantly lower for juvenile planorbids than for pre-adults. In contrast, survival of adult planorbids was slightly lower than that noted in corresponding controls, while their reproductive activity was significantly lower. This interspecific competition between A. spirorbis and G. truncatula would not be due to a limitation in food resources because food was abundant in their natural habitat or breeding dishes. It might be due to the action of mucus and/or toxicity of faecal pellets excreted by adult G. truncatula.