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

We followed the invasion dynamics of the Oriental thiarid snail Thiara gran ifera on the Martinique island, French Antilles. This freshwater species wa s first discovered in 1991 in the Charpentier River, and its spread has sin ce been analysed based on a yearly survey of the malacological fauna at mor e than 100 sites covering the whole island and representing 50 river system s and three pools. Four river systems were sampled at many sites. Thirteen river systems were colonized by 1997. Colonization within river systems occ urred at a speed greater than 1 km per year, probably resulting from both a ctive and passive dispersal. Our results can, on the whole, be explained by a simple diffusion process, However, stratified diffusion has to be invoke d in at least one river. Moreover, colonization was faster downstream than upstream, suggesting that current velocity plays a significant role in disp ersal. Dispersal also occurred between river systems at a mean distance of almost 10 km, though with a large variance, in accordance with the scattere d colony model of stratified diffusion. The relative frequencies of T. gran ifera and Melanoides tuberculata, another recent invader of Martinique, wer e followed at three sites on the Lezarde River. The first species quickly o utnumbered 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. gr anifera on the local freshwater fauna.