Shell interpopulation variation and its origin in Pomacea canaliculata (Gastropoda : Ampullariidae) from Southern Pampas, Argentina

摘要

Despite its widely recognized conchological variation, studies on the shell variability of Pomacea canaliculata are limited to its sexual and ontogenetic components. Here, we analyse the interpopulation variation in conchological and somatic traits, and sex-related growth patterns of P. canaliculata to discover if it is ecophenotypically or genetically determined. Pomacea canaliculata showed variation in shell shape, shell and body weight, and body ash content among populations from three environmentally different sites. Shell shape was also different when snails from the three sources were reared under homogeneous laboratory conditions, indicating a genetic basis for the differences. Shell shapes of laboratory snails differed from their field counterparts, suggesting an environmental influence on shape. The genetic differentiation of shell shape among the studied populations does not seem to be the outcome of adaptation to local conditions or of genetic drift, but is probably a side effect of adaptive differentiation in some life-history traits. On the other hand, weight and ash content differences disappeared under homogeneous conditions, suggesting that their variation is mainly ecophenotypic. Variability in shell thickness, body weight and ash content seems to be more related to trophic availability than to water chemistry. In the laboratory, females showed slightly higher growth rates than males, but these inter-sex differences varied among snails from the three sources. However, shell length was not different between sexes in the field populations, probably due to a greater effect of food shortage on female growth rates. The widespread pattern that shells of freshwater snails from contrasting environments are different has been attributed mostly to cumulative environmental effects or to adaptation to local conditions. However, we suggest that different shell shapes could arise as a collateral outcome of genetically different reproductive behaviours and that it would be misleading to study the shell as a trait exposed separately to selective pressures or environmental influences.