The gastropods (), commonly known as snails and slugs, belong to a large taxonomic class
s within the phylum Mollusca
called Gastropoda .
[WoRMS (2020). Gastropoda. Accessed at: http://marinespecies.org/aphia.php?p=taxdetails&id=101 on 2020-09-29]
This class comprises snail
s and slug
s from saltwater, from freshwater, and from the land. There are many thousands of species of sea snail
s and slugs
, as well as freshwater snail
s, freshwater limpet
s, and land snail
s and slug
The class Gastropoda contains a vast total of named species, second only to the insect
s in overall number. The fossil history of this class goes back to the Late Cambrian
. , 721 families
of gastropods are known, of which 245 are extinct
and appear only in the fossil
record, while 476 are currently extant with
or without a fossil record.
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.
The class Gastropoda has an extraordinary diversification of habitat
s. Representatives live in gardens, woodland, deserts, and on mountains; in small ditches, great rivers and lakes; in estuaries
s, the rocky intertidal
, the sandy subtidal, in the abyssal
depths of the oceans including the hydrothermal vent
s, and numerous other ecological niches, including parasitic
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 can partly but not completely withdraw are termed semi-slugs.
The marine shelled species of gastropod include species such as abalone
s, and numerous other sea snails that produce seashell
s 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 limpet
s, the shell is coiled only in the larval
stage, and is a simple conical structure after that.
In the scientific literature, gastropods were described under "gasteropodes" by Georges Cuvier
[ Cuvier G. (1795). "Second mémoire sur l'organisation et les rapports des animaux à sang blanc, dans lequel on traite de la structure des Mollusques et de leur division en ordres, lu à la Société d'histoire naturelle de Paris, le 11 Prairial, an III". ''Magazin Encyclopédique, ou Journal des Sciences, des Lettres et des Arts'' 2: 433-449]
The word "gastropod" derives from the Ancient Greek
("stomach") and ("foot"), a reference to the fact that the animal's "foot" is positioned below its guts.
The earlier name ''univalve'' means "one valve" or shell, in contrast to ''bivalve
'' applied to other molluscs such as clams and meaning that those animals possess two valves
At all taxonomic
levels, gastropods are second only to the insect
s 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 of Mollusca with accepted names: about 85,000 (minimum 50,000, maximum 120,000).
[Chapman, A.D. (2009)]
Numbers of Living Species in Australia and the World, 2nd edition
. Australian Biological Resources Study, Canberra. Accessed 12 January 2010. (printed); (online).
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 terrestrial gastropods.
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. Many deep-sea species remain to be discovered as only 0.0001% of the deep-sea floor has been studied biologically.
. (2010). In Encyclopædia Britannica. Retrieved March 05, 2010, from Encyclopædia Britannica Online.
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 15,000 species.
In marine habitats, the continental slope
and the continental rise
are home to the highest diversity of marine gastropods, while the continental shelf and abyssal depths have a low diversity of marine gastropods.
Some of the more familiar and better-known gastropods are terrestrial gastropods
(the land snails and slugs). Some live in freshwater, but the majority of all named species of gastropods live in a marine environment.
Gastropods have a worldwide distribution, from the near Arctic
zones to the tropics. They have become adapted to almost every kind of existence on earth, having colonized nearly every available medium.
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 conchiolin
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 shore
s, and many other diverse areas.
Gastropods can be accidentally transferred from one habitat to another by other animals, e.g. by bird
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 degrees.
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), gonad
, 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 tentacle
s with eyes, and a ventral foot, which gives them their name (Greek
, and ''pous'', foot
). 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 protoconch
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.
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 cell
s, or to camouflage
them on the brightly colored hydroids
s and seaweeds on which many of the species are found.
Lateral outgrowths on the body of nudibranch
s are called cerata
. These contain an outpocketing of digestive gland called the diverticula
Sensory organs and nervous system
The sensory organs
of gastropods include olfactory organs
, eyes, statocyst
s and mechanoreceptor
Gastropods have no hearing.
[Chase R.: ''Sensory Organs and the Nervous System''. in Barker G. M. (ed.): ''The biology of terrestrial molluscs''. CABI Publishing, Oxon, UK, 2001, . 1-146, cited pages: 179-211.]
In terrestrial gastropods (land snails and slugs), the olfactory organs, located on the tips of the four tentacle
s, are the most important sensory organ.
The chemosensory organs of opisthobranch
marine gastropods are called rhinophore
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 nocturnal
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.
of a gastropod is usually adapted to the food that a species eats. The simplest gastropods are the limpet
s and abalone
s, herbivores that use their hard radula
to rasp at seaweed
s 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.
Almost all marine gastropods breathe with a gill
, but many freshwater species, and the majority of terrestrial species, have a pallial lung
. The respiratory protein in almost all gastropods is hemocyanin
, but one freshwater pulmonate
family, the Planorbidae
, have hemoglobin
as the 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, nudibranch
s. Some nudibranchs have smooth or wart
y backs with no visible gill mechanism, such that respiration may likely take place directly through the skin.
Gastropods have open circulatory system
and the transport fluid is hemolymph
is present in the hemolymph
as the respiratory pigment.
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
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 dart
In many marine gastropods other than the opisthobranch
s, there are separate sexes(dioecious
); most land gastropods, however, are hermaphrodite
is a part of the behavior of mating
gastropods with some pulmonate
families of land snails creating and utilizing love dart
s, the throwing of which have been identified as a form of sexual selection
The main aspects of the life cycle of gastropods include:
* Egg laying and the eggs of gastropods
* The Embryonic development
* The larvae or larval stadium: some gastropods may be trochophore
(each of these are present in some gastropods only)
* The growth of gastropods
* Courtship and mating in gastropods: fertilization
is internal or external according to the species. External fertilization is common in marine gastropods.
The diet of gastropods differs according to the group considered. Marine gastropods include some that are herbivore
feeders, predatory carnivore
s, 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. Certain species such as thArchaeogastropda
maintain horizontal rows of slender marginal teeth. In some species that have evolved into endoparasites, such as the eulimid
'', many of the standard gastropod features are strongly reduced or absent.
A few sea slug
s 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 shell
'', Ghost slug
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 Patellogastropoda
, whereas fewer changes occurred between the ancestors of Vetigastropoda
s D, C and N) and Caenogastropoda
(a large single inversion
, and translocation
s of the tRNAs D and N).
Within Heterobranchia, gene order seems relatively conserved, and gene rearrangements are mostly related with transposition
of tRNA genes.
Geological history and evolution
The first gastropods were exclusively marine, with the earliest representatives of the group appearing in the Late Cambrian
''), though their only gastropod character is a coiled shell, so they could lie in the stem lineage, if they are gastropods at all. Earliest Cambrian
organisms like ''Helcionella
'' and ''Scenella
'' are no longer considered gastropods, 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 Silurian
'' contains fifteen identified species. Fossil gastropods were less common during the Palaeozoic
era than bivalve
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 during the Mesozoic
era that the ancestors of many of the living gastropods evolved.
One of the earliest known terrestrial (land-dwelling) gastropods is ''Maturipupa
'', which is found in the Coal Measure
s of the Carboniferous
period in Europe, but relatives of the modern land snails are rare before the Cretaceous
period, when the familiar ''Helix
'' first appeared.
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 early Cretaceous period, which both occur in southern England, are limestone
s containing the tightly packed remains of the pond snail ''Viviparus
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 bivalve
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 fossil
s are of debatable origin, some of them do resemble the trails made by living gastropods today.
Gastropod fossils may sometimes be confused with ammonite
s or other shelled cephalopod
s. 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 cephalopod.
Gastropods are one of the groups that record the changes in fauna caused by the advance and retreat of the Ice Sheets
during the Pleistocene
showing the phylogenic relationships of Gastropoda with example species:
and Lower Heterobranchia
are not included in the above cladogram.
, 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 Gastropoda is 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 subclasses:
(gills to the right and behind the heart).
(gills in front of the heart).
(with a lung instead of gills)
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.
, 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)
[Bouchet P. & Rocroi J.-P. (Ed.); Frýda J., Hausdorf B., Ponder W., Valdes A. & Warén A. 2005. ''Classification and nomenclator of gastropod families''. Malacologia: International Journal of Malacology, 47(1-2). ConchBooks: Hackenheim, Germany. . 397 pp]
/ref> 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 the ''Malacologia'' journal (available online from 4 January 2018), a significantly updated version of the 2005 "Bouchet & Rocroi" taxonomy was published in the paper "Revised Classification, Nomenclator and Typification of Gastropod and Monoplacophoran Families". [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 Families
'. Malacologia, 61(1-2): 1-526.
This article incorporates CC-BY-2.0 text from the reference.
* 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.
* 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.
* Parkinson, B., Hemmen, J. & Groh, K. (1987): ''Tropical Landshells of the World.'' 279 S., Verlag Christa Hemmen. Wiesbaden.
* Ponder, W. F. & Lindberg, D. R. (1997): ''Towards a phylogeny of gastropod molluscs: an analysis using morphological characters''. ''Zoological Journal of the Linnean Society'', 119 83–265.
* Robin, A. (2008): ''Encyclopedia of Marine Gastropods.'' 480 S., Verlag ConchBooks. Hackenheim.
Gastropod reproductive behavior
* - An article about social learning also in gastropods.
Gastropod photo gallery
mostly fossils, a few modern shells
A video of a crawling Garden Snail
(''Cornu aspersum''), YouTube
Grove, S.J. (2018). A Guide to the Seashells and other Marine Molluscs of Tasmania: Molluscs of Tasmania with images
Category:Articles containing video clips
Category:Extant Cambrian first appearances
Category:Taxa named by Georges Cuvier