Freshwater mollusk “Aplexa marmorata” (10 mm), in an aquarium. This species is one of the most frequent in the pools of Guadeloupe and reproduces preferentially by self-fertilization. A systematic study of a community of 30 freshwater molluscs, present in 230 mares of Guadeloupe, made it possible to compare the diversity of the species of an environment and the genetic diversity within these species. These two diversities are strongly correlated and influenced by the same factors. Ponds with strong connectivity (some pools overflow during the rainy season and are then connected to each other) are the environments where these diversities are the strongest.