There are over 40,000 species of snails worldwide. Some are adapted land dwellers, some are aquatic, while others are amphibious.
The snails that occupy this intermediate habitat must be able to cope with the variation in conditions, and some are amphibious. The many kinds of snails characteristic of wetlands comprise representatives of two distinct evolutionary pathways. Those that have adapted to wetland life from a purely aquatic ancestry; and those that originate from snails with fully terrestrial lifestyles.
Marshes and riverside fields provide unstable habitats, changing between land and water quickly. For much of the winter, they may be completely submerged, whereas, in a hot summer, they can dry out with a layer of hard, caked mud.
Life In The Water
One group of snails called the Basommatophora, because their eyes are set at the base of their tentacles, are primarily aquatic. Still, at one stage in their evolution, they lost their gills and developed the capacity to breathe air from the atmosphere.
This allowed them to survive regular exposure on the seashore or to breathe at the surface when oxygen levels in ponds and rivers became too low. They can also breathe through their skin, while some groups, having reverted to a fully aquatic existence, have developed a new form of gills.
Snails of one basommatophoran family, the Lymnaeidae, can be recognized by their flat, triangular tentacles. Most can be found in freshwater but can often survive buried in the mud of a pond if it should dry up. Some inhabit marshy places, and Lymnaea palustris is fully amphibious, crawling freely through damp grass or living a aquatic existence in ponds or streams.
Another member of the Basommatophora is the tiny snail Carychium minimum, a member of the family Ellobiidae. This family is mostly composed of larger snails inhabiting tropical mangrove swamps. Still, Carychium minimum lives in a wide range of damp habitats, particularly abundant in large rivers’ floodplains.
Stylommatophora
Most land snails belong to a group called the Stylommatophora, whose eyes are set at the tips of their tentacles. When the tentacles are withdrawn, a muscle attached to the back of the eyes contracts to turn the tentacles. Unlike the Basommatophora, the Stylommatophora are never fully aquatic, but they can inhabit wet zones intermediate between land and fresh water.
One stylommatophoran family, the Succiniidae, or amber snails, nearly all inhabit the zone extending from the waterside plants to damp meadows. Oxyloma pfeifferi favors the immediate neighborhood of river banks, whereas Succinea putris can be found all over wet meadows.
Some authorities claim that the Succiniidae are a primitive group demonstrating the transitional stage that land snails passed through when they evolved from freshwater snails.
Studies have shown that they evolved from true land snails but have secondarily acquired features that appear primitive because of their lifestyle in a wet environment.
Rare Aquatic Snails
Some snails that have undoubtedly evolved from fully terrestrial forms now inhabit only very wet conditions. The family Clausiliidae includes members that inhabit a range of conditions.
In southeastern Europe, many species spend the summer cemented to the hot surfaces of rocks and walls. Most species live in woodland, but one, Balea biplicata, lives in rivers in a few widely scattered colonies. While once abundant, this species is not quite rare in Britain.
Balea biplicata lives under the loose flood debris that lies scattered between the large trees that cover this vanishing type of habitat. A large piece of debris can harbor some tens of this species under it, including numerous young who, because the adults retain the eggs in their bodies until they hatch, are fully developed.
Another rare snail that occurs is a member of the family Helicidae, Perforatella rubiginosa. This is now becoming more common in Britain, although it has been known from parts of Continental Europe since the middle of the 19th century.
Perforatella rubiginosa has been overlooked, probably because of its close but superficial resemblance to several other members of the Helicidae that live in damp conditions, such as Trichia plebeia and Ashfordia granulate.
All these snails have globular shells that are covered in numerous hair-like growths, and it is easy to see how they have been confused, although internally, they are quite distinct.
Perforaretella rubiginosa can live in wet conditions but will drown in water. In winter, it survives by burrowing into the substrate, and hibernating under a barrier of dried mucus.