65,000 to 80,000 species
Gastropoda or gastropods, more commonly known as snails and slugs,
are a large taxonomic class within the phylum Mollusca. The class
Gastropoda includes snails and slugs of all kinds and all sizes from
microscopic to Achatina achatina, the largest known land gastropod.
There are many thousands of species of sea snails and sea slugs, as
well as freshwater snails, freshwater limpets, land snails and land
Gastropoda contains a vast total of named species, second
only to the insects in overall number. The fossil history of this
class goes back to the Late Cambrian. As of 2017, there are 721
families of gastropods known, of which 245 are extinct and appear only
in the fossil record and 476 occur in the Recent with or without a
Gastropoda (previously known as univalves and sometimes spelled
"Gasteropoda") are a major part of the phylum Mollusca, and are the
most highly diversified class in the phylum, with 65,000 to
80,000 living snail and slug species. The anatomy, behavior,
feeding, and reproductive adaptations of gastropods vary significantly
from one clade or group to another. Therefore, it is difficult to
state many generalities for all gastropods.
Gastropoda has an extraordinary diversification of habitats.
Representatives live in gardens, woodland, deserts, and on mountains;
in small ditches, great rivers and lakes; in estuaries, mudflats, the
rocky intertidal, the sandy subtidal, in the abyssal depths of the
oceans including the hydrothermal vents, and numerous other ecological
niches, including parasitic ones.
Although the name "snail" can be, and often is, applied to all the
members of this class, commonly this word means only those species
with an external shell big enough that the soft parts can withdraw
completely into it. Those gastropods without a shell, and those with
only a very reduced or internal shell, are usually known as slugs;
those with a shell into which they cannot withdraw are termed limpets.
The marine shelled species of gastropod include species such as
abalone, conches, periwinkles, whelks, and numerous other sea snails
that produce seashells that are coiled in the adult stage—though in
some, the coiling may not be very visible, for example in cowries. In
a number of families of species, such as all the various limpets, the
shell is coiled only in the larval stage, and is a simple conical
structure after that.
4.2 Body wall
4.3 Sensory organs and nervous system
4.4 Digestive system
4.5 Respiratory system
4.6 Circulatory system
4.7 Excretory system
4.8 Reproductive system
5 Life cycle
6 Feeding behavior
8 Geological history and evolution
11 External links
In the scientific literature, gastropods were described under
Georges Cuvier in 1795. Cuvier chose "gastropod"
by derivation from the
Ancient Greek words γαστήρ (gastér)
"stomach", and ποδὸς (podòs) "foot".
The earlier name univalve means "one valve" or shell, in contrast to
bivalve applied to mollusks such as clams and meaning that those
animals possess two valves or shells.
At all taxonomic levels, gastropods are second only to the insects in
terms of their diversity.
Gastropods have the greatest numbers of named mollusc species.
However, estimates of the total number of gastropod species vary
widely, depending on cited sources. The number of gastropod species
can be ascertained from estimates of the number of described species
Mollusca with accepted names: about 85,000 (minimum 50,000, maximum
120,000). But an estimate of the total number of Mollusca,
including undescribed species, is about 240,000 species. The
estimate of 85,000 molluscs includes 24,000 described species of
Different estimates for aquatic gastropods (based on different
sources) give about 30,000 species of marine gastropods, and about
5,000 species of freshwater and brackish gastropods. The total
number of living species of freshwater snails is about 4,000.
There are 444 recently extinct species of gastropods (extinct since
the year 1500), 18 species that are now extinct in the wild (but still
existing in captivity) and 69 "possibly extinct" species.
The number of prehistoric (fossil) species of gastropods is at least
Main articles: sea snail, sea slug, Terrestrial animal
§ Gastropods, land snail, semi-slug, and slug
Some of the more familiar and better-known gastropods are terrestrial
gastropods (the land snails and slugs) and some live in freshwater,
but more than two thirds of all named species live in a marine
Gastropods have a worldwide distribution from the near Arctic and
Antarctic zones to the tropics. They have become adapted to almost
every kind of existence on earth, having colonized nearly every
In habitats where there is not enough calcium carbonate to build a
really solid shell, such as on some acidic soils on land, there are
still various species of slugs, and also some snails with a thin
translucent shell, mostly or entirely composed of the protein
Snails such as
Sphincterochila boissieri and
Xerocrassa seetzeni have
adapted to desert conditions. Other snails have adapted to an
existence in ditches, near deepwater hydrothermal vents, the pounding
surf of rocky shores, caves, and many other diverse areas.
Gastropods can be accidentally transferred from one habitat to another
by other animals, e.g. by birds. The smallest bird species reported to
carry a gastropod was a great tit (Parus major), as a hairy snail
Trochulus hispidus was found in the plumage of a wintering great tit
Poland in 2010.
The anatomy of a common air-breathing land snail. Note that much of
this anatomy does not apply to gastropods in other clades or groups.
The anatomy of an aquatic snail with a gill, a male prosobranch
gastropod. Note that much of this anatomy does not apply to gastropods
in other clades.
Light yellow - body
Brown - shell and operculum
Green - digestive system
Light purple - gills
Yellow - osphradium
Red - heart
Dark violet -
2. cerebral ganglion
4. upper commissure
7. pleural ganglion
8. atrium of heart
9. visceral ganglion
15. tentacle (chemosensory, 2 or 4)
17. penis (everted, normally internal)
18. esophageal nerve ring
19. pedal ganglion
20. lower commissura
21. vas deferens
22. pallial cavity / mantle cavity / respiratory cavity
23. parietal ganglion
Snails are distinguished by an anatomical process known as torsion,
where the visceral mass of the animal rotates 180° to one side during
development, such that the anus is situated more or less above the
head. This process is unrelated to the coiling of the shell, which is
a separate phenomenon. Torsion is present in all gastropods, but the
opisthobranch gastropods are secondarily de-torted to various
Torsion occurs in two stages. The first, mechanistic stage, is
muscular, and the second is mutagenetic. The effects of torsion are
primarily physiological - the organism develops an asymmetrical
growth, with the majority occurring on the left side. This leads to
the loss of right-paired appendages (e.g., ctenidia (comb-like
respiratory apparatus), gonads, nephridia, etc.). Furthermore, the
anus becomes redirected to the same space as the head. This is
speculated to have some evolutionary function, as prior to torsion,
when retracting into the shell, first the posterior end would get
pulled in, and then the anterior. Now, the front can be retracted more
easily, perhaps suggesting a defensive purpose.
However, this "rotation hypothesis" is being challenged by the
"asymmetry hypothesis" in which the gastropod mantle cavity originated
from one side only of a bilateral set of mantle cavities.
Gastropods typically have a well-defined head with two or four sensory
tentacles with eyes, and a ventral foot, which gives them their name
(Greek gaster, stomach, and poda, feet). The foremost division of the
foot is called the propodium. Its function is to push away sediment as
the snail crawls. The larval shell of a gastropod is called a
The principal characteristic of the
Gastropoda is the asymmetry of
their principal organs. The essential feature of this asymmetry is
that the anus generally lies to one side of the median plane.; The
ctenidium (gill-combs), the osphradium (olfactory organs), the
hypobranchial gland (or pallial mucous gland), and the auricle of the
heart are single or at least are more developed on one side of the
body than the other ; Furthermore, there is only one genital
orifice, which lies on the same side of the body as the anus.
Main article: Gastropod shell
The shell of Zonitoides nitidus, a small land snail, has dextral
coiling, which is typical (but not universal) in gastropod shells.
Upper image: dorsal view of the shell, showing the apex
Central image: lateral view showing the spire and aperture of the
Lower image: basal view showing the umbilicus
Most shelled gastropods have a one piece shell, typically coiled or
spiraled, at least in the larval stage. This coiled shell usually
opens on the right-hand side (as viewed with the shell apex pointing
upward). Numerous species have an operculum, which in many species
acts as a trapdoor to close the shell. This is usually made of a
horn-like material, but in some molluscs it is calcareous. In the land
slugs, the shell is reduced or absent, and the body is streamlined.
Some sea slugs are very brightly colored. This serves either as a
warning, when they are poisonous or contain stinging cells, or to
camouflage them on the brightly colored hydroids, sponges and seaweeds
on which many of the species are found.
Lateral outgrowths on the body of nudibranchs are called cerata. These
contain an outpocketing of digestive gland called the diverticula.
Sensory organs and nervous system
The upper pair of tentacles on the head of
Helix pomatia have eye
spots, but the main sensory organs of the snail are sensory receptors
for olfaction, situated in the epithelium of the tentacles.
Sensory organs of gastropods
Sensory organs of gastropods and Nervous system of
Sensory organs of gastropods
Sensory organs of gastropods include olfactory organs, eyes,
statocysts and mechanoreceptors. Gastropods have no hearing.
In terrestrial gastropods (land snails and slugs), the olfactory
organs, located on the tips of the four tentacles, are the most
important sensory organ. The chemosensory organs of opisthobranch
marine gastropods are called rhinophores.
The majority of gastropods have simple visual organs, eye spots either
at the tip or base of the tentacles. However, "eyes" in gastropods
range from simple ocelli that only distinguish light and dark, to more
complex pit eyes, and even to lens eyes. In land snails and slugs,
vision is not the most important sense, because they are mainly
The nervous system of gastropods includes the peripheral nervous
system and the central nervous system. The central nervous system
consist of ganglia connected by nerve cells. It includes paired
ganglia: the cerebral ganglia, pedal ganglia, osphradial ganglia,
pleural ganglia, parietal ganglia and the visceral ganglia. There are
sometimes also buccal ganglia.
Digestive system of gastropods
Digestive system of gastropods and Radula
The radula of a gastropod is usually adapted to the food that a
species eats. The simplest gastropods are the limpets and abalones,
herbivores that use their hard radula to rasp at seaweeds on rocks.
Many marine gastropods are burrowers, and have a siphon that extends
out from the mantle edge. Sometimes the shell has a siphonal canal to
accommodate this structure. A siphon enables the animal to draw water
into their mantle cavity and over the gill. They use the siphon
primarily to "taste" the water to detect prey from a distance.
Gastropods with siphons tend to be either predators or scavengers.
Respiratory system of gastropods
Respiratory system of gastropods and Respiratory system
Anatomy of respiratory system in invertebrates
Almost all marine gastropods breathe with a gill, but many freshwater
species, and the majority of terrestrial species, have a pallial lung.
Gastropods with a lung belong to one group with common descent, the
Pulmonata, however, gastropods with gills are paraphyletic. The
respiratory protein in almost all gastropods is hemocyanin, but a
Planorbidae have hemoglobin as respiratory protein.
In one large group of sea slugs, the gills are arranged as a rosette
of feathery plumes on their backs, which gives rise to their other
name, nudibranchs. Some nudibranchs have smooth or warty backs and
have no visible gill mechanism, such that respiration may likely take
place directly through the skin.
Main article: Circulatory system of gastropods
Gastropods have open circulatory system and the transport fluid is
Hemocyanin is present in the hemolymph as the respiratory
Main article: Excretory system of gastropods
The primary organs of excretion in gastropods are nephridia, which
produce either ammonia or uric acid as a waste product. The nephridium
also plays an important role in maintaining water balance in
freshwater and terrestrial species. Additional organs of excretion, at
least in some species, include pericardial glands in the body cavity,
and digestive glands opening into the stomach.
Main article: Reproductive system of gastropods
Courtship is a part of mating behavior in some gastropods including
some of the Helicidae. Again, in some land snails, an unusual feature
of the reproductive system of gastropods is the presence and
utilization of love darts.
In many marine gastropods other than the opisthobranchs, there are
separate sexes; most land gastropods, however, are hermaphrodites.
A 9-hour-old trochophore of Haliotis asinina
sf - shell field
mating behaviour of Elysia timida
Egg strings of an Aplysia species.
Main article: Reproductive system of gastropods
Mating of gastropods
Courtship is a part of the behavior of mating gastropods with some
pulmonate families of land snails creating and utilizing love darts,
the throwing of which have been identified as a form of sexual
The main aspects of the life cycle of gastropods include:
Egg laying and the eggs of gastropods
The Embryonic development of gastropods
The larvae or larval stadium: some gastropods may be trochophore
Estivation and hibernation (each of these are present in some
The growth of gastropods
Courtship and mating in gastropods: fertilization is internal or
external according to the species. External fertilization is common in
Snail eating a dandelion flower
The diet of gastropods differs according to the group considered.
Marine gastropods include some that are herbivores, detritus feeders,
predatory carnivores, scavengers, parasites, and also a few ciliary
feeders, in which the radula is reduced or absent. Land-dwelling
species can chew up leaves, bark, fruit and decomposing animals while
marine species can scrape algae off the rocks on the sea floor. In
some species that have evolved into endoparasites, such as the eulimid
Thyonicola doglieli, many of the standard gastropod features are
strongly reduced or absent.
A few sea slugs are herbivores and some are carnivores. The
carnivorous habit is due to specialisation. Many gastropods have
distinct dietary preferences and regularly occur in close association
with their food species.
Some predatory carnivorous gastropods include, for example: Cone
shells, Testacella, Daudebardia,
Ghost slug and others.
Gastropods exhibit an important degree of variation in mitochondrial
gene organization when compared to other animals. Main events of
gene rearrangement occurred at the origin of
Heterobranchia, whereas fewer changes occurred between the ancestors
Vetigastropoda (only tRNAs D, C and N) and
Caenogastropoda (a large
single inversion, and translocations of the tRNAs D and N). Within
Heterobranchia, gene order seems relatively conserved, and gene
rearrangements are mostly related with transposition of tRNA
Geological history and evolution
Fossil gastropod and attached mytilid bivalves on a
bedding plane of the
Matmor Formation in southern Israel.
Cornu aspersum (formerly Helix aspersa): a European pulmonate land
snail that has been accidentally introduced in many countries
throughout the world.
See also: fr:
Gastropoda (classification phylogénétique) and List of
marine gastropod genera in the fossil record
The first gastropods were exclusively marine, with the earliest
representatives of the group appearing in the Late Cambrian
(Chippewaella, Strepsodiscus), though their only gastropod
character is a coiled shell, so they could lie in the stem lineage, if
they are gastropods at all. Early
Cambrian organisms like
Scenella are no longer considered gastropods,[citation
needed] and the tiny coiled
Aldanella of earliest
Cambrian time is
probably not even a mollusk.
As such, it's not until the
Ordovician that the first crown-group
members arise. By the
Ordovician period the gastropods were a
varied group present in a range of aquatic habitats. Commonly, fossil
gastropods from the rocks of the early
Palaeozoic era are too poorly
preserved for accurate identification. Still, the
Poleumita contains fifteen identified species.
Fossil gastropods were
less common during the
Palaeozoic era than bivalves.
Most of the gastropods of the
Palaeozoic era belong to primitive
groups, a few of which still survive. By the
Carboniferous period many
of the shapes seen in living gastropods can be matched in the fossil
record, but despite these similarities in appearance the majority of
these older forms are not directly related to living forms. It was
Mesozoic era that the ancestors of many of the living
One of the earliest known terrestrial (land-dwelling) gastropods is
Maturipupa, which is found in the Coal Measures of the Carboniferous
period in Europe, but relatives of the modern land snails are rare
Cretaceous period, when the familiar Helix first appeared.
Cepaea nemoralis: another European pulmonate land snail, which has
been introduced to many other countries
In rocks of the
Mesozoic era, gastropods are slightly more common as
fossils; their shells are often well preserved. Their fossils occur in
ancient beds deposited in both freshwater and marine environments. The
"Purbeck Marble" of the
Jurassic period and the "Sussex Marble" of the
Cretaceous period, which both occur in southern England, are
limestones containing the tightly packed remains of the pond snail
Rocks of the
Cenozoic era yield very large numbers of gastropod
fossils, many of these fossils being closely related to modern living
forms. The diversity of the gastropods increased markedly at the
beginning of this era, along with that of the bivalves.
Certain trail-like markings preserved in ancient sedimentary rocks are
thought to have been made by gastropods crawling over the soft mud and
sand. Although these trace fossils are of debatable origin, some of
them do resemble the trails made by living gastropods today.
Gastropod fossils may sometimes be confused with ammonites or other
shelled cephalopods. An example of this is Bellerophon from the
limestones of the
Carboniferous period in Europe, the shell of which
is planispirally coiled and can be mistaken for the shell of a
Gastropods are one of the groups that record the changes in fauna
caused by the advance and retreat of the Ice Sheets during the
A cladogram showing the phylogenic relationships of
Neomphalina and Lower
Heterobranchia are not included
in the above cladogram.
Main articles: Taxonomy of the
Gastropoda (Ponder & Lindberg,
1997); Taxonomy of the
Gastropoda (Bouchet & Rocroi, 2005); and
Changes in the taxonomy of gastropods since 2005
A group of fossil shells of
Turritella cingulifera from the Pliocene
Five views of a shell of a
Microphoto (35x) of
Gastropoda sp. from Holocene sediments of Amuq
Plain SSE Turkey
Since Darwin, biological taxonomy has attempted to reflect the
phylogeny of organisms, i.e., the tree of life. The classifications
used in taxonomy attempt to represent the precise interrelatedness of
the various taxa. However, the taxonomy of the
constantly being revised and so the versions shown in various texts
can differ in major ways.
In the older classification of the gastropods, there were four
Opisthobranchia (gills to the right and behind the heart).
Gymnomorpha (no shell)
Prosobranchia (gills in front of the heart).
Pulmonata (with a lung instead of gills)
The taxonomy of the
Gastropoda is still under revision, and more and
more of the old taxonomy is being abandoned, as the results of DNA
studies slowly become clearer. Nevertheless, a few of the older terms
such as "opisthobranch" and "prosobranch" are still sometimes used in
a descriptive way.
New insights based on
DNA sequencing of gastropods have produced some
revolutionary new taxonomic insights. In the case of the Gastropoda,
the taxonomy is now gradually being rewritten to embody strictly
monophyletic groups (only one lineage of gastropods in each group).
Integrating new findings into a working taxonomy remain challenging.
Consistent ranks within the taxonomy at the level of subclass,
superorder, order, and suborder have already been abandoned as
unworkable. Ongoing revisions of the higher taxonomic levels are
expected in the near future.
Convergent evolution, which appears to exist at especially high
frequency in gastropods, may account for the observed differences
between the older phylogenies, which were based on morphological data,
and more recent gene-sequencing studies.
Bouchet & Rocroi (2005) made sweeping changes in the
systematics, resulting in a taxonomy that is a step closer to the
evolutionary history of the phylum. The Bouchet & Rocroi
classification system is based partly on the older systems of
classification, and partly on new cladistic research.
In the past, the taxonomy of gastropods was largely based on phenetic
morphological characters of the taxa. The recent advances are more
based on molecular characters from DNA and RNA research. This has
made the taxonomical ranks and their hierarchy controversial. The
debate about these issues is not likely to end soon.
In the Bouchet, Rocroi et al. taxonomy, the authors have used unranked
clades for taxa above the rank of superfamily (replacing the ranks
suborder, order, superorder and subclass), while using the traditional
Linnaean approach for all taxa below the rank of superfamily. Whenever
monophyly has not been tested, or is known to be paraphyletic or
polyphyletic, the term "group" or "informal group" has been used. The
classification of families into subfamilies is often not well
resolved, and should be regarded as the best possible hypothesis.
In 2004, Brian Simison and
David R. Lindberg showed possible
diphyletic origins of the
Gastropoda based on mitochondrial gene order
and amino acid sequence analyses of complete genes.
In the 2017 issue of "Malacologia" journal (available online from 4
January 2018) new much updated version of 2005 "Bouchet & Rocroi"
taxonomy was published: "Revised Classification, Nomenclator and
Typification of Gastropod and Monoplacophoran Families".
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^ Philippe Bouchet, Jean-Pierre Rocroi, Bernhard Hausdorf, Andrzej
Kaim, Yasunori Kano, Alexander Nützel, Pavel Parkhaev, Michael
Schrödl and Ellen E. Strong. 2017. Revised Classification,
Nomenclator and Typification of Gastropod and Monoplacophoran
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Abbott, R. T. (1989): Compendium of Landshells. A color guide to more
than 2,000 of the World's Terrestrial Shells. 240 S., American
Malacologists. Melbourne, Fl, Burlington, Ma. ISBN 0-915826-23-2
Abbott, R. T. & Dance, S. P. (1998): Compendium of Seashells. A
full-color guide to more than 4,200 of the world's marine shells. 413
S., Odyssey Publishing. El Cajon, Calif. ISBN 0-9661720-0-0
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Ponder, W. F. & Lindberg, D. R. (1997): Towards a phylogeny of
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Robin, A. (2008): Encyclopedia of Marine Gastropods. 480 S., Verlag
ConchBooks. Hackenheim. ISBN 978-3-939767-09-1
Look up gastropod or univalve in Wiktionary, the free dictionary.
Wikispecies has information related to Gastropoda
Wikimedia Commons has media related to Gastropoda.
Gastropod reproductive behavior
2004 Linnean taxonomy of gastropods
Webster, S.; Fiorito, G. (2001). "Socially guided behaviour in
Animal Cognition. 4 (2): 69.
doi:10.1007/s100710100108. - An article about social learning
also in gastropods.
Gastropod photo gallery, mostly fossils, a few modern shells
A video of a crawling Garden
Snail (Helix aspersa), YouTube
Classes of Molluscs
Fauna Europaea: 11369