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Gastropods (), commonly known as slugs and snails, belong to a large taxonomic class of within the phylum Mollusca called Gastropoda ().
This class comprises and from saltwater, freshwater, and from the land. There are many thousands of species of and sea slug, as well as , freshwater , and .
The class Gastropoda is a diverse and highly successful class of mollusks within the phylum Mollusca. It contains a vast total of named species, second only to the in overall number. The fossil history of this class goes back to the Furongian. , 721 families of gastropods are known, of which 245 are extinct and appear only in the fossil record, while 476 are currently neontology living fossil 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, so stating many generalities for all gastropods is difficult.
The class Gastropoda has an extraordinary diversification of . Representatives live in gardens, woodland, deserts, and on mountains; in small ditches, great rivers, and lakes; in estuaries, , the rocky intertidal, the sandy subtidal, the Abyssal zone 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 Gastropod shell big enough that the soft parts can withdraw completely into it. Slugs are gastropods that have no shell or a very small, internal shell; semislugs are gastropods that have a shell that they can partially retreat into but not entirely.
The marine shelled species of gastropods include species such as abalone, , periwinkles, , and numerous other sea snails that produce 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 , the shell is coiled only in the larval stage, and is a simple conical structure after that.
The earlier name "univalve" means one valve (or shell), in contrast to , such as clams, which have two valves or shells.
Gastropods have the greatest numbers of named Mollusca 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.Appeltans W., Bouchet P., Boxshall G.A., Fauchald K., Gordon D.P., Hoeksema B.W., Poore G.C.B., van Soest R.W.M., Stöhr S., Walter T.C., Costello M.J. (eds) (2011). World Register of Marine Species. Accessed at marinespecies.org on 2011-03-07. The estimate of 85,000 mollusks 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 water gastropods. Many deep-sea species remain to be discovered, as only 0.0001% of the deep-sea floor has been studied biologically. "gastropod" . (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.
Recently extinct species of gastropods (extinct since 1500) number 444, 18 species are now extinct in the wild (but still exist in captivity), and 69 species are "possibly extinct".
The number of prehistoric (fossil) species of gastropods is at least 15,000 species. Nájera J. M. (1996). "Moluscos del suelo como plagas agrícolas y cuarentenarias". X Congreso Nacional Agronómico / II Congreso de Suelos 1996 51-56. PDF
In marine habitats, the continental slope and the continental rise are home to the highest diversity, 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 fresh water, but most named species of gastropods live in a marine environment.
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 available medium.
In habitats where not enough calcium carbonate is available to build a really solid shell, such as on some acidic soils on land, various species of slugs occur, and also some snails with thin, translucent shells, 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, in 10,000 meters (6 miles) below the surface, the pounding surf of , , and many other diverse areas.
Gastropods can be accidentally transferred from one habitat to another by other animals, e.g. by .RUSIECKI S. & RUSIECKA A. 2013. Hairy snail Trochulus hispidus (Linnaeus, 1758) in flight - a note on avian dispersal of snails. Folia Malacologica 21(2):111-112.
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 by asymmetrical growth, with the majority of growth occurring on the left side. This leads to the loss of right-side anatomy that in most bilaterians is a duplicate of the left side anatomy. The essential feature of this asymmetry is that the anus generally lies to one side of the median plane. The gill-combs, the osphradium, the foot slime-gland, Nephridium, 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. 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.
Gastropods typically have a well-defined head with two or four sensory with eyes, and a ventral 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.
Some gastropods have adult shells which are bottom heavy due to the presence of a thick, often broad, convex ventral callus deposit on the inner lip and adapical to the aperture which may be important for gravitational stability.
Lateral outgrowths on the body of are called cerata. These contain an outpocketing of called the diverticula.
In terrestrial gastropods (land snails and slugs), the olfactory organs, located on the tips of the four , are the most important sensory organ. The chemosensory organs of opisthobranch marine gastropods are called .
The majority of gastropods have simple visual organs, eye spots either at the tip or Leptopoma 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 Nocturnality animals.
The nervous system of gastropods includes the peripheral nervous system and the central nervous system. The central nervous system consists of ganglia connected by nerve cells. It includes paired ganglia: the cerebral ganglia, pedal ganglia, Osphradium ganglia, pleural ganglia, parietal ganglia and the visceral ganglia. There are sometimes also buccal ganglia.
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.
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, . Some nudibranchs have smooth or backs with no visible gill mechanism, such that respiration may likely take place directly through the skin.
In many marine gastropods other than the , there are separate sexes (dioecious/Gonochorism); most land gastropods, however, are .
The main aspects of the life cycle of gastropods include:
A few 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: , Testacella, Daudebardia, , and others.
Mushroom-producing fungi used as a food source by snails and slugs include species from several genera. Some examples are ( Lactarius spp.), the oyster mushroom ( Pleurotus ostreatus), and the Boletus. Additionally, slugs feed on fungi from other genera, such as Agaricus, Pleurocybella, and Russula. Snails have also been reported to feed on Boletus as well as Coprinellus, Aleurodiscus, Armillaria, Grifola, Marasmiellus, Mycena, Pholiota, and Ramaria. As for slime molds, commonly consumed species include Stemonitis axifera and Symphytocarpus flaccidus.
Feeding behaviors in slugs exhibit considerable variation. Some species display selectivity, consuming specific parts or developmental stages of fungi. For instance, certain slugs may target fungi only at particular stages of maturity, such as immature fruiting bodies or spore-producing structures. Conversely, other species show little to no selectivity, consuming entire mushrooms regardless of developmental stage. This variability stresses the diverse dietary adaptations among slug species and their ecological roles in fungal consumption. Moreover, by consuming fungi, snails and slugs can also indirectly help in their dispersal by carrying along some of their or the fungi themselves.
It is not until the Ordovician that true crown-group gastropods appear. By this time, gastropods had diversified into a variety of forms and inhabited a range of aquatic environments. Fossil gastropods from the early Paleozoic are often poorly preserved, making identification difficult. However, the Silurian genus Poleumita contains at least 15 identified species. Overall, gastropods were less common in the Paleozoic than bivalves.
Most Paleozoic gastropods belong to primitive groups, some of which still exist today. By the Carboniferous period, many gastropod shell shapes found in fossils resemble those of modern species, though most of these early forms are not directly related to living gastropods. It was during the Mesozoic era that the ancestors of many extant gastropods evolved. One of the earliest known terrestrial gastropods is Anthracopupa (or Maturipupa), found in the Carboniferous of Europe. However, land snails and their relatives were rare before the Cretaceous period.
In Mesozoic rocks, gastropods become more common in the fossil record, with well-preserved shells. Fossils are found in ancient beds from both freshwater and marine environments. Notable examples include the Purbeck Marble of the Jurassic and the Sussex Marble of the early Cretaceous, both from southern England. These limestones contain abundant remains of the pond snail Viviparus. Cenozoic rocks yield vast numbers of gastropod fossils, many of which are closely related to modern species. The diversity of gastropods increased significantly at the start of this era, alongside that of 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 are of debatable origin, some of them do resemble the trails made by living gastropods today.
Gastropod fossils may sometimes be confused with or other shelled . 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 also provide important evidence of faunal changes during the Pleistocene epoch, reflecting the impacts of advancing and retreating ice sheets.
Neomphaliones and Lower Heterobranchia are not included in the above cladogram.
In the older classification of the gastropods, there were four subclasses:Paul Jeffery. Suprageneric classification of class Gastropoda. The Natural History Museum, London, 2001.
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.
In 2004, Brian Simison and David R. Lindberg showed possible Paraphyly origins of the Gastropoda based on mitochondrial gene order and amino acid sequence analyses of complete genes.
In 2005, Philippe Bouchet and Jean-Pierre Rocroi made sweeping changes in the systematics, resulting in the Bouchet & Rocroi taxonomy, which is a step closer to the evolutionary history of the phylum.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. vliz.be Poppe G.T. & Tagaro S.P. 2006. The new classification of Gastropods according to Bouchet & Rocroi, 2005. Visaya, février 2006: 10 pp. journal-malaco.fr 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.
In 2017, Bouchet, Rocroi, and other collaborators published a significantly updated version of the 2005 taxonomy.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. In the Bouchet et al. taxonomy, the authors used unranked 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.
Fixed ranks like family, genus, and species however remain useful for practical classification and remain used in the World Register of Marine Species (WoRMS). Also many researchers continue to use traditional ranks because they are entrenched in the literature and familiar to specialists and non-specialists alike.
== Gallery ==
Genetics
Gastropods exhibit an important degree of variation in mitochondrial gene organization when compared to other animals. Main events of Mutation occurred at the origin of Patellogastropoda and Heterobranchia, whereas fewer changes occurred between the ancestors of Vetigastropoda (only 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 Transposon of tRNA genes.
Geological history and evolution
The first gastropods were exclusively marine, with the earliest known representatives appearing in the Furongian (e.g., Chippewaella, Strepsodiscus). However, their only definitive gastropod feature is a coiled shell, which raises the possibility that they may belong to the stem lineage of gastropods, or might not be gastropods at all.Budd, G. E., and S. Jensen. 2000: A critical reappraisal of the fossil record of the bilaterian phyla. Biological Reviews 75:253–295. Early Cambrian species such as Helcionella, Barskovia, and Scenella are no longer considered gastropods, and the small coiled Aldanella from the same period is probably not even a mollusk.
Phylogeny
A cladogram showing the phylogenic relationships of Gastropoda with example species:
Taxonomy
Current classification
The present backbone classification of gastropods relies on the results of phylogenomic analyses. Consensus has not been reached yet considering the relationships at the very base of the gastropod tree of life, but otherwise the major groups are known with confidence.
History
Since Charles 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 Gastropoda is constantly being revised and so the versions shown in various texts can differ in major ways.
Ecology and conservation
Many gastropod species face threats from habitat destruction, pollution, and climate change. Some species are endangered or have become extinct due to these factors. Conservation efforts often focus on protecting their habitats, especially in freshwater and terrestrial ecosystems.
Predators
Gastropods are prey to a wide range of organisms depending on the environment. In marine habitats, gastropods are preyed upon by fish, marine birds, , , and other mollusks such as . In terrestrial environments, gastropod predators include , (, harvestmen), , and , among others.
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