Quite the appetite: juvenile island apple snails (Pomacea insularum) survive consuming only exotic invasive plants

Most aquatic snails derive their energy by grazing periphyton. However, certain species, including the invasive island apple snail, Pomacea insularum, readily consume aquatic macrophytes. These snails often overlap in their distribution with other exotic, invasive plants. We sought to discover if juvenile P. insularum could survive and grow when fed only three reportedly less palatable food sources: Eurasian watermilfoil (Myriophyllum spicatum), wild taro (Colocasia esculenta) and water hyacinth (Eichhornia crassipes). Snails received nonrooted macrophytes simultaneously in a multiple-resource experiment. Using enclosures with compartments that separated plants but allowed snails full access, we housed individual early (10.7+ 0.9 mm operculum width; 0.56 –1.11 g blotted wet weight; mean+ 1 SD) or late (23+ 2 mm; 4.60– 14.82 g) juvenile P. insularum. We monitored snail survival and growth for 6 weeks. As controls, we placed standardized nonrooted macrophytes in enclosures without snails for 1 week. Replacing plants weekly, we calculated average daily consumption rates. Both size classes of snails grew substantially in terms of operculum width, shell height and blotted wet weight, with early juveniles exhibiting relatively larger gains in size over the 6-week period. No mortality occurred. Both size classes consumed significantly more watermilfoil than taro and more taro than water hyacinth. Collectively, these results translate into the potential for one single 10-g P. insularum (i.e. representative of the larger snails used in our experiment) to consume a combined 0.35 g of dried plant matter (or c. 3.5 g) of live macrophytes per day. Our study suggests that available resources offered by a community of exotic invasive plants may sustain snail survival.