Gastropoda

Gastropods(/ˈɡæstrəpɒdz/), commonly known asslugs and snails, belong to a largetaxonomicclass ofinvertebrateswithin the phylumMolluscacalledGastropoda(/ɡæsˈtrɒpədə/).
This class comprisessnailsandslugsfrom saltwater, freshwater, and from the land. There are many thousands of species ofsea snailsandslugs, as well asfreshwater snails, freshwaterlimpets,land snailsandslugs.
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 theinsectsin overall number. The fossil history of this class goes back to theLate Cambrian. As of 2017, 721familiesof gastropods are known, of which 245 areextinctand appear only in thefossilrecord, while 476 are currentlyextantwithor without a fossil record.
Gastropoda (previously known asunivalvesand sometimes spelled "Gasteropoda") are a major part of the phylum Mollusca, and are the most highly diversifiedclassin thephylum, with 65,000 to 80,000living snail and slugspecies. Theanatomy, behavior, feeding, and reproductive adaptations of gastropods vary significantly from onecladeor group to another, so stating many generalities for all gastropods is difficult.
The class Gastropoda has an extraordinary diversification ofhabitats. Representatives live in gardens, woodland, deserts, and on mountains; in small ditches, great rivers, and lakes; inestuaries,mudflats, the rockyintertidal, the sandy subtidal, theabyssaldepths of the oceans, including thehydrothermal vents, and numerous other ecological niches, includingparasiticones.
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 externalshellbig 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 asabalone,conches,periwinkles,whelks, and numerous other sea snails that produceseashellsthat are coiled in the adult stage—though in some, the coiling may not be very visible, for example incowries. In a number offamiliesof species, such as all the variouslimpets, the shell is coiled only in thelarvalstage, and is a simple conical structure after that.
In the scientific literature, gastropods were described as "gasteropodes" byGeorges Cuvierin 1795.The wordgastropodcomes from Greekγαστήρ(gastḗr'stomach') andπούς(poús'foot'), a reference to the fact that the animal's "foot" is positioned below its guts.
The earlier name "univalve" means onevalve(or shell), in contrast tobivalves, such as clams, which have two valves or shells.
At alltaxonomiclevels, gastropods are second only to insects in terms of theirdiversity.
Gastropods have the greatest numbers of namedmolluskspecies. 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).But an estimate of the total number of Mollusca, including undescribed species, is about 240,000 species.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 andbrackishgastropods. Many deep-sea species remain to be discovered, as only 0.0001% of the deep-sea floor has been studied biologically.The total number of living species of freshwater snails is about 4,000.
Recentlyextinctspecies of gastropods (extinct since 1500) number 444, 18 species are nowextinct 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.
In marine habitats, thecontinental slopeand thecontinental riseare home to the highest diversity, while the continental shelf and abyssal depths have a low diversity of marine gastropods.
Gastropods are found in a wide range of aquatic and terrestrial habitats, from deep ocean trenches to deserts.
Some of the more familiar and better-known gastropods areterrestrial 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 nearArcticandAntarcticzones 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 enoughcalcium carbonateis 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 proteinconchiolin.
Snails such asSphincterochila boissieriandXerocrassa seetzenihave adapted to desert conditions. Other snails have adapted to an existence in ditches, near deepwaterhydrothermal vents, inoceanic trenches10,000 meters (6 miles) below the surface,the pounding surf ofrocky shores,caves, and many other diverse areas.
Gastropods can be accidentally transferred from one habitat to another by other animals, e.g. bybirds.
Snails are distinguished by an anatomical process known astorsion, where the visceral mass of the animal rotates 180° to one side during development, such that theanusis 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 theopisthobranch gastropodsare secondarily untorted to various degrees.
Torsion occurs in two stages. The first, mechanistic stage is muscular, and the second ismutagenetic. 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. Thegill-combs, theolfactory organs, thefoot slime-gland,nephridia, and theauricleof the heart are single or at least are more developed on one side of the body than the other. Furthermore, there is only onegenitalorifice, 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-definedheadwith two or four sensorytentacleswith 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 aprotoconch.
Most shelled gastropods have a one pieceshell(with exceptionalbivalved gastropods), typically coiled or spiraled, at least in the larval stage. This coiled shell usually opens on the right-hand side (as viewed with the shellapexpointing upward). Numerous species have anoperculum, 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 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.
Somesea slugsare very brightly colored. This serves either as awarning, when they arepoisonousor containstinging cells, or tocamouflagethem on the brightly coloredhydroids,sponges, and seaweeds on which many of the species are found.
Lateral outgrowths on the body ofnudibranchsare calledcerata. These contain an outpocketing ofdigestive glandscalled thediverticula.
Thesensory organsof gastropods includeolfactory organs, eyes,statocystsandmechanoreceptors.Gastropods have no hearing.
In terrestrial gastropods (land snails and slugs), the olfactory organs, located on the tips of the fourtentacles, are the most important sensory organ.The chemosensory organs ofopisthobranchmarine gastropods are calledrhinophores.
The majority of gastropods have simple visual organs, eye spots either at the tip orbaseof the tentacles. However, "eyes" in gastropods range from simpleocellithat only distinguish light and dark, to more complexpit eyes, and even tolens eyes.In land snails and slugs, vision is not the most important sense, because they are mainlynocturnalanimals.
The nervous system of gastropods includes theperipheral nervous systemand thecentral nervous system. The central nervous system consists ofgangliaconnected by nerve cells. It includes paired ganglia: the cerebral ganglia, pedal ganglia,osphradialganglia, pleural ganglia, parietal ganglia and the visceral ganglia. There are sometimes also buccal ganglia.
Theradulaof a gastropod is usually adapted to the food that a species eats. The simplest gastropods are thelimpetsandabalone, herbivores that use their hardradulato rasp atseaweedson rocks.
Many marine gastropods are burrowers, and have asiphonthat extends out from themantleedge. Sometimes the shell has asiphonal canalto accommodate this structure. A siphon enables the animal to draw water into theirmantle cavityand 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 agill, but many freshwater species, and the majority of terrestrial species, have a palliallung. The respiratory protein in almost all gastropods ishemocyanin, but one freshwaterpulmonatefamily, thePlanorbidae, havehemoglobinas 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,nudibranchs. Some nudibranchs have smooth orwartybacks with no visible gill mechanism, such that respiration may likely take place directly through the skin.
Gastropods haveopen circulatory systemand the transport fluid ishemolymph.Hemocyaninis present in thehemolymphas the respiratory pigment.
The primary organs of excretion in gastropods arenephridia, which produce eitherammoniaoruric acidas 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 thestomach.
Courtshipis a part ofmatingbehavior in some gastropods, including some of theHelicidae. Again, in some land snails, an unusual feature of thereproductive system of gastropodsis the presence and utilization oflove darts.
In many marine gastropods other than theopisthobranchs, there are separate sexes (dioecious/gonochoric); most land gastropods, however, arehermaphrodites.
Courtshipis a part of the behavior ofmatinggastropods with somepulmonatefamilies of land snails creating and utilizinglove darts, the throwing of which have been identified as a form ofsexual selection.
The main aspects of the life cycle of gastropods include:
The diet of gastropods differs according to the group considered. Marine gastropods include some that areherbivores,detritusfeeders,predatorycarnivores,scavengers,parasites, and also a few ciliary feeders, in which theradulais reduced or absent. Land-dwelling species can chew up leaves, bark, fruit, fungi, and decomposing animals while marine species can scrape algae off the rocks on the seafloor. Certain species such as theArchaeogastropodamaintain horizontal rows of slender marginal teeth. In some species that have evolved into endoparasites, such as theeulimidThyonicola doglieli, many of the standard gastropod features are strongly reduced or absent.
A fewsea slugsare 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:cone shells,Testacella,Daudebardia,turrids,ghost slugsand others.
Studies based on direct observations,fecalandgutanalyses, as well as food-choice experiments, have revealed that snails and slugs consume a wide variety of food resources.Their diet spans from living plants at various developmental stages such aspollen, seeds,seedlings, and wood, to decaying plant material like leaf litter. Additionally, they feed on fungi,lichens,algae, soil, and even other animals, both living and dead, including their feces. Given this diverse diet, terrestrial gastropods can be classified asherbivores,omnivores,carnivores, anddetritivores.However, the majority aremicrobivores, primarily consumingmicrobesassociated with decaying organic material.Despite theirecologicalimportance, there is a notable lack of research exploring the specific roles that terrestrial gastropods play within soil food webs.
Many terrestrial gastropod mollusks are known to consume fungi, a behavior observed in various species of snails and slugs across distinct families.Notable examples offungivoreslugs include members of the familyPhilomycidae, which feed onslime molds(myxomycetes), and theAriolimacidae, which primarily consume mushrooms (basidiomycetes).Snail families that contain fungivore species includeClausiliidae,Macrocyclidae,andPolygyridae.
Mushroom-producing fungi used as a food source by snails and slugs include species from several genera. Some examples aremilk-caps(Lactariusspp.), theoyster mushroom(Pleurotus ostreatus), and thepenny bun. Additionally, slugs feed on fungi from other genera, such asAgaricus,Pleurocybella, andRussula.Snails have also been reported to feed onpenny bunsas well asCoprinellus,Aleurodiscus,Armillaria,Grifola,Marasmiellus,Mycena,Pholiota, andRamaria.As for slime molds, commonly consumed species includeStemonitis axiferaandSymphytocarpus 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 theirdispersalby carrying along some of theirsporesor the fungi themselves.
Gastropods exhibit an important degree of variation inmitochondrial geneorganization when compared to other animals.Main events ofgene rearrangementoccurred at the origin ofPatellogastropodaandHeterobranchia, whereas fewer changes occurred between the ancestors ofVetigastropoda(onlytRNAsD, C and N) andCaenogastropoda(a large singleinversion, andtranslocationsof the tRNAs D and N).Within Heterobranchia, gene order seems relatively conserved, and gene rearrangements are mostly related withtranspositionof tRNA genes.
The first gastropods were exclusively marine, with the earliest known representatives appearing in theLate Cambrian(e.g.,Chippewaella,Strepsodiscus).However, their only definitive gastropod feature is acoiled shell, which raises the possibility that they may belong to the stem lineage of gastropods, or might not be gastropods at all.Early Cambrian species such asHelcionella,Barskovia, andScenellaare no longer considered gastropods, and the small coiledAldanellafrom the same period is probably not even a mollusk.
It is not until theOrdovicianthat truecrown-groupgastropods appear.By this time, gastropods had diversified into a variety of forms and inhabited a range ofaquatic environments. Fossil gastropods from the earlyPaleozoicare often poorly preserved, making identification difficult. However, theSiluriangenusPoleumitacontains at least 15 identified species. Overall, gastropods were less common in thePaleozoicthanbivalves.
Most Paleozoic gastropods belong to primitive groups, some of which still exist today. By theCarboniferousperiod, 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 theMesozoicera that the ancestors of many extant gastropods evolved.One of the earliest known terrestrial gastropods isAnthracopupa(orMaturipupa), found in theCarboniferousCoal Measuresof Europe.However, land snails and their relatives were rare before theCretaceousperiod.
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 thePurbeck Marbleof theJurassicand theSussex Marbleof the early Cretaceous, both fromsouthern England. These limestones contain abundant remains of the pond snailViviparus.Cenozoicrocks 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 thesetrace fossilsare of debatable origin, some of them do resemble the trails made by living gastropods today.
Gastropod fossils may sometimes be confused withammonitesor other shelledcephalopods. An example of this isBellerophonfrom the limestones of theCarboniferousperiod 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 thePleistoceneepoch, reflecting the impacts of advancing and retreatingice sheets.
Acladogramshowing the phylogenic relationships of Gastropoda with example species:
Cocculiniformia,NeomphalinaandLower Heterobranchiaare not included in the above cladogram.
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.
SinceDarwin, biological taxonomy has attempted to reflect thephylogenyof organisms, i.e., thetree 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:
Thetaxonomyof the Gastropoda is still under revision, and more and more of the old taxonomy is being abandoned, as the results ofDNAstudies 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 strictlymonophyleticgroups (only one lineage of gastropods in each group). Integrating new findings into a workingtaxonomyremain 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 andDavid R. Lindbergshowed possiblediphyleticorigins of the Gastropoda based onmitochondrialgene order andamino acid sequenceanalyses of complete genes.
In 2005,Philippe BouchetandJean-Pierre Rocroimade sweeping changes in thesystematics, resulting in theBouchet & Rocroi taxonomy, which is a step closer to the evolutionary history of thephylum.The Bouchet & Rocroi classification system is based partly on the older systems of classification, and partly on newcladisticresearch. In the past, the taxonomy of gastropods was largely based onpheneticmorphological characters of the taxa. The recent advances are more based on molecular characters fromDNAandRNAresearch. 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.In theBouchet et al. taxonomy, the authors used unrankedcladesfor taxa above the rank of superfamily (replacing the ranks suborder, order, superorder and subclass), while using the traditionalLinnaean approachfor all taxa below the rank of superfamily. Whenevermonophylyhas 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 theWorld 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.
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.
Gastropods arepreyto a wide range of organisms depending on the environment. In marine habitats, gastropods are preyed upon byfish,marine birds,marine mammals,crustaceans, and other mollusks such ascephalopods. In terrestrial environments, gastropod predators includeinsects,arachnids(spiders,harvestmen),birds, andmammals, among others.