by Richard Goldberg
Among the approximately 140,000 known molluscan species, just over one-third inhabit the continents and islands of the world. Fossil records indicate that the terrestrial or LAND SNAILS first evolved from marine gastropods over 350 million years ago.
Unlike the marine (ocean) environment which is inhabited by all seven classes of mollusks and the freshwater (rivers, streams and lakes) environs where both gastropods and bivalves are found, gastropods or snails are the only class of mollusk to have adapted to the terrestrial environment. The land snails are air breathers with the ability to reproduce independent of an aquatic or marine environment (oviparously). Two subclasses of land mollusks exist.
The earliest land snail species to evolve were marine operculate or PROSOBRANCH gastropods which adapted to the terrestrial environment. PROSOBRANCHS absorb oxygen through specialized tissue (mantle tissue) in the body of the snail. An operculum, or trap door, attached to the top of the snail's foot allows it to effectively seal itself inside when it withdraws into its shell to retain moisture during dry periods. The operculum also acts to protect a withdrawn snail against predatory insects. The majority of land snail PROSOBRANCHS are limited to tropical latitudes due to their dependence on moisture. About one-fifth of all land snail species are PROSOBRANCHS.
Operculum structures are of two types. The first type is made up of a thin to relatively thick corneous material which is flexible, single layered, and circular to subcircular in shape. The second type is multi-layered, with a corneous base and a calcareous overlay which is sometimes sculptured with spiral structures and grooves. The operculum sculpture is a useful characteristic to help differentiate and classify similar groups (genera) of land operculates.
PULMONATES, the second subclass of land snail, evolved independently of the PROSOBRANCHS, yet through speciation now outnumber the operculate species due to their adaptability in both arid and humid or moist environments. Some species of PULMONATES have adapted to living in arid desert environments. The PULMONATES have evolved lungs for respiration and are less dependent on constant moisture.
PULMONATE land snails, much like their distant cousins the PROSOBRANCHS, can withdraw and seal themselves inside the shell to retain moisture during dry seasons. The difference is that the PULMONATES secrete an epiphragm, a temporary mucus door over the aperture (opening) which hardens to seal the snail inside. When wet weather returns, the snail will end its torpid state and push through the epiphragm. The ability for any snail to seal itself inside its shell, either with an epiphragm (PULMONATES), or with an operculum (PROSOBRANCHS), provides a means by which a snail can survive through periods of drought, sometimes as long as a few months or a few years.
PROSOBRANCHS and PULMONATES can be further differentiated by the external anatomy of the snail. PROSOBRANCHS have only one pair of tentacles on the head with eyes at the bases. The PULMONATES have two sets of tentacles, the longer set with eyes at the distal end of each and a shorter pair which probably act as sensory probes.
The shells produced by land snails are sometimes brightly colored or patterned, especially species inhabiting tropical environments. Others, such as the ground-dwelling species, tend to be rather drab, but often with an exquisite sculpture of ribs, cords, and/or spines. Yet others have oddly shaped shells and apertures. Color, sculpture, and shape are functional and adaptive characteristics. Bright colors can ward off predators or camouflage the snail in its surroundings. Structures such as ribs and spines, often a characteristic of arid environment snails, are believed to help radiate heat away from the snail.
A number of land snail PULMONATES have evolved intricate dentition that effectively block off the opening to the shell's aperture. The dentition provides protection for the snail against insect predators. Some of the snails have evolved such elaborate dentitionthat it is hard to imagine that a snail can withdraw itself back into the shell.
Land snails are calciphiles, thriving in limestone environments. Relatively few land snails are found to inhabit areas of igneous rock and shale. Land snails are quite habitat specific. Ground dwelling species often prefer leaf litter where moisture is retained, or in scree which affords a great deal of protection from predators and desiccation. Arboreal or tree dwelling snails tend to prefer living on specific botanical species. An extreme example of this specificity is exhibited by the Hawaiian Achatinella tree snails which only live on the endemic Hawaiian tree, Ohia Lehua (Metrosideros polymorpha). The Achatinella is so habitat specific that if removed from the Ohia tree that the snail was born on and transplanted to another, its chances of survival are slim to none.
The beauty and intriguing ecology of land snails are just further reasons to take a second look at this overlooked group of mollusks.
- Abbott, R. Tucker, 1989, COMPENDIUM OF LANDSHELLS, American Malacologists, Burlington, MA.
- Goldberg, R. L., Severns, M., 1997, Isolation and Evolution of the Amphidromus in Nusa Tenggara, American Conchologist, Vol. 25, No. 2 (June): 3-7.
- Parkinson, B., Hemmen, J., Groh, K., 1987, TROPICAL LAND SHELLS OF THE WORLD, Verlag Christa Hemmen, Wiesbaden, Germany
- American Conchologist, Quarterly Bulletin of the Conchologists of America, Inc. March 1988, Vol. 16, No. 1, Special land shell issue #1
- American Conchologist, Quarterly Bulletin of the Conchologists of America, Inc. March 1993, Vol. 21, No. 1, Special land shell issue #2