You are what you eat: is the apple snail Pomacea canaliculata a macrophytophage or a detritivore in its native range (southern Pampas, Argentina)?

Abstract

Herbivorous invaders promote changes in community structure and ecosystem functioning. The apple snail, Pomacea canaliculata, is an invader with strong impacts on wetland vegetation and aquatic crops. While able to feed on diverse trophic resources using different feeding mechanisms it is usually regarded as a macrophytophage. However, studies showing direct evidence of what they actually eat in natural waterbodies are few and their results do not fully support such a feeding habit. We analyzed the digestive contents of P. canaliculata using a micrographic technique to describe the spatiotemporal variation of its natural diet within its native range. Stomach and intestine contents were similar but the intestines were never empty and their volume and diversity were generally higher. Detritus was the dominant food item (84.1% of total abundance) and was consumed by all the snails whereas macrophytes were eaten by only half of the snails and represented only an 8.6% of total abundance; filamentous algae, animal remains and grasses showed lower than 5% of total abundance. The spatiotemporal variation in these feeding patterns was minor, despite the among site variation in macrophyte coverage and richness. In the Encadenadas del Oeste basin, P. canaliculata behaves as a specialist on detritus, with some individuals occasionally consuming other resources and can thus be described as an opportunistic generalist omnivore. The impacts of apple snail invasions on both detritus and on other detritivores have seldom been studied although they may have important negative and positive effects, respectively, on dead vegetal matter decomposition.