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
SNAILS first evolved from marine gastropods over 350 millions
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 The operculum also acts to protect a withdrawn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
Operculum structures are of two
types. The first type is made up of a thin to relatively thick
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 PULMONTES have evolved lungs for respiration
and are less dependent on constant moisture.
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
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,
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
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
dentition that 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,
- 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