The invasive giant apple snail (Pomacea maculata) will eat frog and toad eggs

Background/Question/Methods: Giant apple snails (Pomacea maculata) are an invasive, aquatic, freshwater snail native to South America. They are often described as a generalist herbivore of aquatic vegetation. A sister species, Pomacea canaliculata, has been reported to be a predator of frog eggs in its introduced range (a wetland in Hong Kong, People’s Republic of China). Our question: Are giant apple snails (Pomacea maculata) a potential predator of anuran eggs? We presented the eggs of American toads (Anaxyrus americanus- AT), northern leopard frogs (Lithobates pipiens- NLF), and pickerel frogs (Lithobates palustris- PF) to lab raised giant apple snails (henceforth ‘snails’). Anuran eggs from a single egg mass were, divided into four groups of approximately equal number, and then randomly assigned to one of four treatments: (Snail Test 1) with snails; (Snail Test 2) with snails and an alternative snail food, lettuce (Lactuca sativa); (Control 1) with only lettuce; (Control 2) with neither lettuce nor snails. The eggs were incubated until either all the eggs had hatched into tadpoles or the remaining eggs showed signs of decomposition in at least one of the replicates. The incubation period varied between 2 and 6 days. At the conclusion of the experiment we counted the number of eggs and tadpoles in each container. The number of replicates for each experiment varied depending on egg mass availability. The number of replicates were: 8, 4, and 4 for PF, NLF and AT respectively.

Results/Conclusions: Statistics were performed using one-sample T-tests with a 0.05 alpha level. For the snail exposed eggs (Snail Test 1 and 2) the average number of eggs lost varied as follows: PF- 8.13% and 10.25%; NLF-18.76 and 21.79%; and AT- 9.13 and 10.85%. All three anuran species had a significant loss of eggs (p values < 0.03) with snails as compared to controls and there were no significant differences (p value = 0.6781) between snail treatments with and without lettuce. For all three anuran species, there were no differences in egg loss between Controls 1 and 2. We conclude that P. maculata eat anuran eggs. Many anuran species lay their eggs in habitats that are being invaded by these snails and our results are a potential cause for concern. However, it remains to be demonstrated that snails will prey on eggs in the wild.