Challenging exploration of troubled waters: a decade of surveys of the giant freshwater pearl mussel Margaritifera auricularia in Europe

The critically endangered Giant Freshwater Pearl Mussel Margaritifera auricularia was presumed extinct before its rediscovery in Spain in 1985 and France in 2000. Since then, numerous surveys have been set up to search for living populations in France and Spain. This article presents an up-to-date account of species distribution based on available data, i.e. the literature, museum collections and recent field surveys, and provides unpublished molecular data for France. There are still three populations of the Giant Freshwater Pearl Mussel in the Ebro River in Spain, and eight populations in France (two in the Loire watershed, one in the Charente watershed, two in the Garonne watershed and three in the Adour watershed). The biggest population lives in the Charente River with an estimated 100,000 individuals. Recruitment is very scarce in all populations but living specimens estimated to be less than 10 years old have been found in the Ebro in Spain and in the Vienne, Charente, Dronne and Adour rivers in France. The recent rediscovery of populations in France was mainly a result of intensive dedicated surveys including scuba diving. Subsequent advances in knowledge show how large rivers and downstream ecosystems remain a terra incognita for the hydrobiologist.