Reaction norms of size and age at maturity of Pomacea canaliculata (Gastropoda: Ampullariidae) under a gradient of food deprivation

摘要

Pomacea canaliculata, an apple snail native to South America, has become a serious pest of aquatic crops and a promoter of ecosystem changes in natural wetlands worldwide. Its success as an invader has been attributed to its great phenotypic plasticity in life-history traits. Our aims were to determine the reaction norms of size and age at maturity under a gradient of food deprivation. Full sibling experimental snails were reared in isolation from hatching and maintained until maturity under seven different levels of relative food deprivation based on size-specific ingestion rates. To detect the onset of sexual activity of experimental snails, fully mature virgin snails reared in the laboratory were used as consorts. The reaction norms for age and size at maturity of P. canaliculata showed marked sexual dimorphism. Shell length was the main component of variation in the male reaction norms for both copulation and egg-laying by female consorts, whereas age was the main component of variation for females. Irrespective of the intensity of food deprivation, males mature at the same age at the expense of size, since size is apparently irrelevant in the access to females and male fitness can be maximized through fast maturation. In contrast, a minimum size is required for females to reach maturity, perhaps as a result of their higher reproductive costs. The highly dimorphic reaction norms lead to an increasing lag between male and female maturity as deprivation increases; in temperate regions, males born early in the reproductive season would mature in the same season irrespective of food availability, while most females would have to overwinter before attaining sexual maturity in unproductive habitats or those dominated by unpalatable macrophytes. The great life-history plasticity reported in invaded areas could be a heritage from populations in the native range.