Snails exhibit a wide range of respiratory strategies tailored to their diverse habitats, meaning the question of whether they breathe underwater has a nuanced answer. Some snail species are fully aquatic and can extract oxygen directly from water, while others, even those living in water, must surface to breathe air. There are also terrestrial snails that are exclusively air-breathers.
Aquatic Snails: Gills and Water Breathing
Many aquatic snails possess specialized respiratory organs called gills, or ctenidia, which allow them to absorb dissolved oxygen directly from the water. These comb-like or feather-like structures are typically located within the snail’s mantle cavity. Water flows over the ctenidia, enabling the exchange of gases where oxygen diffuses into the snail’s bloodstream and carbon dioxide is released.
Gilled snails, including many marine and some freshwater varieties like ramshorn snails, are truly aquatic. Their reliance on dissolved oxygen means they are often sensitive to water quality and oxygen levels. Their efficient gills allow them to remain submerged indefinitely.
Aquatic Snails: Lungs and Surface Breathing
Some aquatic snails, known as pulmonates, have evolved a lung-like structure instead of gills, even though they live in water. This “lung” is a modified mantle cavity with a rich network of blood vessels, functioning similarly to a mammalian lung. These snails primarily breathe atmospheric air and must periodically come to the water surface to do so.
A small opening called a pneumostome allows air to enter and exit this lung-like cavity. Snails like common pond snails and most apple snails exhibit this breathing method, often extending a siphon to reach the surface while remaining submerged. This adaptation enables them to survive in oxygen-poor waters, as they are not dependent on dissolved oxygen. Some species, like apple snails, possess both a gill and a lung, offering versatility in their respiratory approach.
Land Snails: Life Without Water
Terrestrial snails, which are also pulmonates, exclusively breathe atmospheric air and do not possess gills. They have a lung-like structure, or pallial lung, within their mantle cavity, similar to their aquatic lunged relatives. Air enters and exits this lung through a breathing pore, also called a pneumostome.
These snails are highly dependent on humidity for their survival and activity. While they breathe air, their permeable skin means they can lose water rapidly in dry conditions. Land snails often seek moist environments and are most active during humid periods, as desiccation can quickly immobilize or even be fatal to them.