Introduction and spread of Thiara granifera (Lamarck, 1822) in Martinique, French West Indies

We followed the invasion dynamics of the Oriental thiarid snail Thiara granifera on the Martinique island, French Antilles. This freshwater species was first discovered in 1991 in the Charpentier River, and its spread has since been analysed based on a yearly survey of the malacological fauna at more than 100 sites covering the whole island and representing 50 river systems and three pools. Four river systems were sampled at many sites. Thirteen river systems were colonized by 1997. Colonization within river systems occurred at a speed greater than 1km per year, probably resulting from both active and passive dispersal. Our results can, on the whole, be explained by a simple diffusion process. However, stratified diffusion has to be invoked in at least one river. Moreover, colonization was faster downstream than upstream, suggesting that current velocity plays a significant role in dispersal. Dispersal also occurred between river systems at a mean distance of almost 10km, though with a large variance, in accordance with the scattered colony model of stratified diffusion. The relative frequencies of T. granifera and Melanoides tuberculata, another recent invader of Martinique, were followed at three sites on the Lézarde River. The first species quickly outnumbered the second, though never wiped it out. The data therefore do not support any exclusion phenomena between these two parthenogenetic invaders. Our analysis does not indicate any obvious influence of the rise of T. granifera on the local freshwater fauna.