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 ecosystemfunctioning. The apple snail, Pomacea canaliculata, is an invader with strong impactson wetland vegetation and aquatic crops. While able to feed on diverse trophicresources using different feeding mechanisms it is usually regarded as amacrophytophage. However, studies showing direct evidence of what they actually eatin natural waterbodies are few and their results do not fully support such a feedinghabit. We analyzed the digestive contents of P. canaliculata using a micrographictechnique to describe the spatiotemporal variation of its natural diet within its nativerange. Stomach and intestine contents were similar but the intestines were neverempty and their volume and diversity were generally higher. Detritus was the dominantfood item (84.1 % of total abundance) and was consumed by all the snails whereasmacrophytes were eaten by only half of the snails and represented only an 8.6 % oftotal abundance; filamentous algae, animal remains and grasses represented less than5 % of total abundance. The spatiotemporal variation in these feeding patterns wasminor, despite the among site variation in macrophyte coverage and richness. In theEncadenadas del Oeste basin, P. canaliculata behaves as a specialist on detritus, withsome individuals occasionally consuming other resources and can thus be describedas an opportunistic generalist omnivore. The impacts of apple snail invasions on bothdetritus and on other detritivores have seldom been studied although they may haveimportant negative and positive effects, respectively, on dead vegetal matterdecomposition.