Effect of Food Availability on Morphometric and Somatic Indices of the Apple SnailPomacea canaliculata(Caenogastropoda, Ampullariidae)

Pomecee canaliculata is a freshwater snail native to South America that together with some congeners has invaded natural wetlands and paddy fields in several continents, especially in Southern Asia. The high variability in shape, color and thickness of Pomacea shells and the sexual dimorphism in many traits blurs the species limits and hampers taxonomic identification. Ecological characterization of habitat productivity based on shells was previously proposed for P. canaliculata but was never methodically explored. Using full siblings of P. canaliculata, we studied the effects of different chronic levels of food availability (from 100% to 20% of daily ingestion rate) on shell shape, somatic indices and sexual dimorphism at maturity. The eight specific morphometric and somatic indices investigated showed different combinations of the effects of food availability and sex: changes related to food availability but independent of sex (relative aperture width), sexual dimorphism independent of food availability (shell globosity and relative aperture expansion), and changes related to food availability and sex, without a noticeable interaction (organic density); a significant interaction that increases the intersexual differences when food availability increases was detected in some indices (relative operculum weight, overall shell density and relative shell investment). The organic density can be used as a condition index to indicate the actual trophic availability in the field, although it should be estimated separately for males and females. The relative aperture width and the overall shell density can be used as paleo-environmental indicators of productivity, as they can be measured on empty shells. The effect of water alkalinity should be taken into account should the latter be used.