Asiatic Clams are common in brackish and fresh water in Asia, and they were introduced into North America in 1924 and have now spread throughout the continent. During the last two decades they have been reported in Europe, but the number of species here is uncertain. Populations of Corbicula from France and the Netherlands were analysed morphologically and genetically to quantify the degree of species and/or population differentiation. The morphological and genetic data, based on allozymes and mitochondrial sequences, were in full agreement. They indicate that there are two distinct species, identified as C. fluminalis and C. fluminea, in the two countries. Analyses of the mitochondrial COI gene revealed an unexpected divergent population of Corbicula in the Rhône. All these individuals were morphologically identified as C. fluminea, but had a COI sequence different from the two previous species. This population may, therefore, be a more ancient population, or a distinct species introduced via a different colonization route.

Evenness indices are numerous but the lack of knowledge of their properties is a limitation to their biological usefulness. 15 evenness indices, two of them being recently proposed, were studied in this work. We investigated the sensitivity of each index using (1) 189 macroinvertebrate communities sampled in the field, and (2) a set of communities modified in a controlled way.There is no single way to measure evenness. We demonstrated that a measure should be chosen considering (1) the kind of data analysed and, (2) the index properties wanted by users. Depending on the ecological data set the index should be more sensitive to variations in rare, median or abundant species. For most of macroinvertebrate community analyses, a convenient evenness index requirs to be symmetric, unsensitive to variation on rare taxa, with a large range of variation and can be compared with a diversity index. Depending on diversity measure used, five indices correspond to these criteria: E Pielou , E Hurlbert, E -ln(D) , E 1-D, E MI. Our results were summarized in a table which may help users to select a convenient evenness measure according to their specific data. Concerning index properties, three main features that an evenness index should meet are briefly discussed: dependence with richness, symmetry criteria and variation range. A revue of this controversial subject allowed a best understanding of values obtained with evenness measures depending on their own features.

Based on field observations performed in 1999 and 2000, the exotic amphipod Dikerogammarus villosus is now well established throughout the navigable sector of the River Moselle in France. D. villosus co-occurs with Gammarus pulex, G. tigrinus and D. haemobaphes, another recent exotic species found in this tributary of the River Rhine. The maximal size of individuals of D. villosus collected (15.7 mm) is lower in the River Moselle than in other basins, where this species could reach up to 30 mm (Neseman et al. 1995). Nevertheless it represents the largest gammarids found in the French sector of the River Moselle. Other particular traits are its wide phenotypic plasticity, varying from a melanic to a striped form, and its predatory behavior. These particular traits may favour the establishment of this species on a recipient ecosystem. The occurrence of D. villosus was noted in the Alsace Canal and in the River Rhône, showing that its immigration across French hydrosystems is still in progress. (Neseman et al. 1995), mais représente les plus grands Gammaridae du secteur français de la Moselle. Des traits particuliers, tels que sa grande plasticité phé-notypique, allant d'une forme mélanique à une forme zébrée, et son comportement prédateur, pourraient favoriser l'établissement de cette espèce dans dé nouveaux écosystèmes. La présence de D. villosus a été notée dans le canal d'Alsace et dans le Rhône, montrant que son immigration au sein des hydrosystèmes français est toujours en cours.

No abstract

No abstract