With both a gill and a lung, apple snails are able to breathe both in and out of water. Perhaps because they carry their houses on their backs, apple snails are not exactly picky about their habitats. Most species live in stagnant, swampy waters that are rich in rotting plants and decaying organisms but poor in oxygen. In addition to the low oxygen levels, apple snails in these tropical habitats often have to contend with seasonal droughts. To cope with these conditions, they have evolved a lung that enables them to draw oxygen directly from the air.
During this evolutionary process, apple snails never lost their gill – an advantage that enables them to make fewer trips to the surface, where they become vulnerable to predators. In grimy waters, the gill alone cannot always provide enough oxygen, but apple snails have another adaptation to solve that problem: a snorkel-like device called a siphon. Attached to the left side of an individual’s neck, this tube shaped organ can be extended up to two and a half times the length of the snail’s body to bring air back down to its lung. So, even when land-locked predators are prowling, apple snails can still breathe easy.