Factors affecting the distribution of Pomacea canaliculata (Gastropoda : Ampullariidae) along its southernmost natural limit

The Argentinean apple snail Pomacea canaliculata is a recent invader in pad dy fields in most Southeast Asian countries, where it has dispersed explosi vely since about 1980. Determinants of the natural range of P. canaliculata in its original area are poorly understood. In this study, factors affecti ng its distribution in the southern limit of its native area in Buenos Aire s Province were investigated. Salty, alkaline, poorly vegetated aquatic env ironments with high desiccation risk in the western part of the study area probably act as a natural barrier for its dispersal toward the south and we st. Sites inhabited by P. canaliculata are generally shallow, quiet, turbid , with low Na+/(K+ + Mg++) ratios compared to the uninhabited sites. Contra ry to previous statements, P canaliculata is frequently found in streams, w here it reaches the highest densities. Most of the inhabited environments a re located on the northern slope of the Tandilia and Ventania mountains, th is physical barrier, and not temperature, being probably the main factor im peding the species' spread southwards in Buenos Aires Province. Pomacea can aliculata also inhabits isolated sites on the southern slopes of these moun tains, where it appears to have been introduced. Its spread is slow within and among water bodies in this area. Repeated introductions, tropical clima te, and integrated and flood-prone drainage systems probably account for th e faster expansion of this species in southeastern Asia.