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The majority of marine gastropods breathe through a single gill, supplied with oxygen by a current of water through the mantle cavity. This current is U-shaped, so that it also flushes waste products away from the anus , which is located above the animal's head, and would otherwise cause a problem with fouling.
Determining whether some gastropods should be called sea snails is not always easy. Some species that live in brackish water (such as certain neritids) can be listed as either freshwater snails or marine snails, and some species that live at or just above the high tide level (for example, species in the genus Truncatella) are sometimes considered to be sea snails and sometimes listed as land ...
For these freshwater snails, the siphon is an anti-predator adaptation. It reduces their vulnerability to being attacked and eaten by birds because it enables the apple snails to breathe without having to come all the way up to the surface, where they are easily visible to predators. [6]
Siphonarioidea have a singular lung on the right side of their bodies that has two openings for inhaling and exhaling water, which cause their asymmetrical shell shape. When they are submerged in water, the Siphonarioidea breathe by inhaling water through the current created by the cilia on their gills and in their lung cavity.
Hygrophila is a clade of exclusively freshwater snails and limpets. They mostly breath oxygen from the air and regularly come to the surface to refill their pallial cavity with fresh air (this may not hold during the winter), but some live permanently submerged; secondary breathing surfaces for absorbing oxygen from water may be present.
Pulmonata or pulmonates is an informal group (previously an order, and before that, a subclass) of snails and slugs characterized by the ability to breathe air, by virtue of having a pallial lung instead of a gill, or gills. The group includes many land and freshwater families, and several marine families.
The snail's oesophageal gland houses symbiotic gammaproteobacteria from which the snail appears to obtain its nourishment. This species is considered to be one of the most peculiar deep-sea hydrothermal-vent gastropods, and it is the only known extant animal that incorporates iron sulfide into its skeleton (into both its sclerites and into its ...
Some groups of snails that live in freshwater respire using gills, whereas other groups need to reach the surface to breathe air. In addition, some are amphibious and have both gills and a lung (e.g. Ampullariidae). Most feed on algae, but many are detritivores and some are filter feeders.