Radula
Encyclopedia
Chitin
Chitin n is a long-chain polymer of a N-acetylglucosamine, a derivative of glucose, and is found in many places throughout the natural world...
ous ribbon, which is typically used for scraping or cutting food before the food enters the esophagus
Esophagus
The esophagus is an organ in vertebrates which consists of a muscular tube through which food passes from the pharynx to the stomach. During swallowing, food passes from the mouth through the pharynx into the esophagus and travels via peristalsis to the stomach...
. The radula is unique to the molluscs, and is found in every class of mollusc except the bivalves
Bivalvia
Bivalvia is a taxonomic class of marine and freshwater molluscs. This class includes clams, oysters, mussels, scallops, and many other families of molluscs that have two hinged shells...
.
Within the gastropods, the radula is used in feeding by both herbivorous and carnivorous snail
Snail
Snail is a common name applied to most of the members of the molluscan class Gastropoda that have coiled shells in the adult stage. When the word is used in its most general sense, it includes sea snails, land snails and freshwater snails. The word snail without any qualifier is however more often...
s and slug
Slug
Slug is a common name that is normally applied to any gastropod mollusc that lacks a shell, has a very reduced shell, or has a small internal shell...
s. The arrangement of teeth (also known as denticles) on the radula ribbon varies considerably from one group to another.
In most of the more ancient lineages of gastropods, the radula is used to graze, by scraping diatom
Diatom
Diatoms are a major group of algae, and are one of the most common types of phytoplankton. Most diatoms are unicellular, although they can exist as colonies in the shape of filaments or ribbons , fans , zigzags , or stellate colonies . Diatoms are producers within the food chain...
s and other microscopic algae off rock surfaces and other substrates.
Predatory marine snails such as the Naticidae
Naticidae
Naticidae, common name the moon snails, is a family of minute to large-sized predatory sea snails, marine gastropod molluscs in the clade Littorinimorpha.Naticidae is the only family in the superfamily Naticoidea....
use the radula plus an acidic secretion to bore through the shell of other molluscs. Other predatory marine snails, such as the Conidae, use a specialized radula tooth as a poisoned harpoon. Predatory pulmonate land slug
Slug
Slug is a common name that is normally applied to any gastropod mollusc that lacks a shell, has a very reduced shell, or has a small internal shell...
s, such as the ghost slug
Ghost slug
The ghost slug, Selenochlamys ysbryda, is a species of predatory air-breathing land slug. It is a shell-less pulmonate gastropod mollusc in the family Trigonochlamydidae....
, use elongated razor-sharp teeth on the radula to seize and devour earthworms. Predatory cephalopods, such as squid
Squid
Squid are cephalopods of the order Teuthida, which comprises around 300 species. Like all other cephalopods, squid have a distinct head, bilateral symmetry, a mantle, and arms. Squid, like cuttlefish, have eight arms arranged in pairs and two, usually longer, tentacles...
, use the radula for cutting prey.
The introduction of the term "radula" is usually attributed to Alexander von Middendorff
Alexander von Middendorff
Alexander Theodor von Middendorff was a Baltic German zoologist and explorer.- Early life :Middendorff's mother Sophia Johanson was an Estonian peasant girl who had been sent to Saint Petersburg for education by her parents. There she met with the future director of the St...
in 1848.
Components
A typical radula comprises a number of bilaterally-symmetrical self-similar rows of teeth rooted in a radular membrane. Some species have teeth that bend with the membrane as it moves over the odontophore, whereas in other species, the teeth are firmly rooted in place, and the entire radular structure moves as one entity.Radular membrane
The radular membrane may be a single tongue, or may split into two (bipartite).Odontophore
The odontophore is the tongue of flesh underlying the radular membrane, and controls the organ's protrusion and return. It can be likened to a pulley wheel over which the radular 'string' is pulled.Flexibility
The radular teeth can generally bend in a sideways direction. In the patellogastropods, though, the teeth lost this ability and became fixed.Teeth
The radula comprises multiple, identical (or near enough) rows of teeth; often, each tooth in a row (along with its symmetric partner) will have a unique morphology.Each tooth can be divided into three sections: a base, a shaft, and a cusp. In radulae that just sweep, rather than rasp, the underlying substrate, the shaft and cusp are often continuous and cannot be differentiated.
The teeth often tesselate with their neighbours, and this interlocking serves to make it more difficult to remove them from the radular ribbon.
Morphology
The morphology of the radula is related to diet. However, it is not fixed per species; some molluscs can adapt the form of their radular teeth according to which food sources are abundant.Pointed teeth are best suited to grazing on algal tissue, whereas blunt teeth are preferable if feeding habits entail scraping epiphytes from surfaces.
Use
The radula is used in two main ways: either as a 'rake', generally to comb up microscopic, filamentous algae from a surface; or as a rasp, to feed directly on a 'plant'.The rhipidoglossan (see below) and, to a lesser extent, the taenigloissan radula types are suited to less strenuous modes of feeding, brushing up smaller algae or feeding on soft forms; molluscs with such radulae are rarely able to feed on leathery or coralline algae. On the other hand, the docoglossan gastropod radula allows a very similar diet to the polyplacophora, feeding primarily on these resistant algae, although microalgae are also consumed by species with these radular types.
The sacoglossans (sea slugs) form an interesting anomaly in that their radula comprises a single row; they feed by sucking on cell contents, rather than rasping at tissue, and most species feed on a single genus or species of alga. Here, the shape of the radular teeth has a close match with the food substrate on which they are used. Triangular teeth are suited to diets of calcified algae, and are also present in radulae used to graze on Caulerpa; in both these cases the cell walls are predominantly composed of xylan. Sabot
Sabot
A sabot is a device used in a firearm or cannon to fire a projectile, such as a bullet, that is smaller than the bore diameter, or which must be held in a precise position. The term is also applied to a battery stub case, a device used similarly to make a small electrical battery usable in a...
-shaped teeth – rods with a groove along one side – are associated with diets of crossed-fibrillar cellulose-walled algae, such as the Siphonocladales and Cladophorales
Cladophorales
In taxonomy, the Cladophorales are an order of green algae, specifically the Ulvophyceae....
, whereas blade-shaped teeth are more generalist.
Early molluscs
The first bona fide radula dates to the Early Cambrian, although trace fossils from the earlier EdiacaranEdiacaran
The Ediacaran Period , named after the Ediacara Hills of South Australia, is the last geological period of the Neoproterozoic Era and of the Proterozoic Eon, immediately preceding the Cambrian Period, the first period of the Paleozoic Era and of the Phanerozoic Eon...
have been suggested to have been made by the radula of the organism Kimberella
Kimberella
Kimberella is a monospecific genus of bilaterian known only from rocks of the Ediacaran period. The slug-like organism fed by scratching the microbial surface on which it dwelt in a manner similar to the molluscs, although its affinity with this group is contentious.Specimens were first found in...
.
Another so-called radula has been reported from the early Cambrian in 1974, this one preserved with fragments of the mineral ilmenite
Ilmenite
Ilmenite is a weakly magnetic titanium-iron oxide mineral which is iron-black or steel-gray. It is a crystalline iron titanium oxide . It crystallizes in the trigonal system, and it has the same crystal structure as corundum and hematite....
suspended in a quartz
Quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...
matrix, and showing similarities to the radula of the modern cephalopod Sepia
Sepia (genus)
Sepia is a genus of cuttlefish in the family Sepiidae encompassing some of the best known and most common species. The cuttlebone is relatively ellipsoid in shape...
. However, this was since re-interpreted as Salterella
Salterella
Salterella is an enigmatic Cambrian–Ordovician genus with a calcareous shell that appears to be septate, but is rather filled with stratified laminar deposits. Its shell contains grains of sediment....
[/Volborthella
Volborthella
Volborthella is an animal of incertain classification, whose fossils pre-date . It has been considered for a period a cephalopod. However discoveries of more detailed fossils showed that Volborthella’s small, conical shell was not secreted but built from grains of the mineral silicon dioxide , and...
?].
Based on the bipartite nature of the radular dentition pattern in solenogasters, larval gastropods and larval polyplacophora, it has been postulated that the ancestral mollusc bore a bipartite radula (although the radular membrane may not have been bipartite).
In chitons
Each row of the polyplacophoran radula has two mineralized teeth used to abrade the substrate, and two longer teeth that sweep up any debris. The other 13 teeth on each row do not appear to be involved in feeding.The teeth of Chaetopleura apiculata
Chaetopleura apiculata
Chaetopleura apiculata is a species of chiton in the family Chaetopleuridae. It is a marine mollusc.- Description :Teeth of the radula of this species has been studied by atom-probe tomography for the chemical structure and it have been published in 2011. It has been shown, that teeth contain...
comprise fibres surrounded by magnetite
Magnetite
Magnetite is a ferrimagnetic mineral with chemical formula Fe3O4, one of several iron oxides and a member of the spinel group. The chemical IUPAC name is iron oxide and the common chemical name is ferrous-ferric oxide. The formula for magnetite may also be written as FeO·Fe2O3, which is one part...
, sodium
Sodium
Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...
and magnesium
Magnesium
Magnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...
.
In gastropods
Anatomy and method of functioning
The mouth of the gastropods is located below the anterior part of the mollusc. It opens into a pocket-like buccal cavity, containing the radula sac, an evaginated pocket in the posterior wall of this cavity.The radula apparatus consists of two parts :
- the cartilaginous base (the odontophore), with the odontophore protractor muscle, the radula protractor muscle and the radula retractor muscle.
- the radula itself, with its longitudinal rows of chitinChitinChitin n is a long-chain polymer of a N-acetylglucosamine, a derivative of glucose, and is found in many places throughout the natural world...
ous and recurved teethToothTeeth are small, calcified, whitish structures found in the jaws of many vertebrates that are used to break down food. Some animals, particularly carnivores, also use teeth for hunting or for defensive purposes. The roots of teeth are embedded in the Mandible bone or the Maxillary bone and are...
, the cuticula.
The odontophore is movable and protrusible, and the radula itself is movable over the odontophore. Through this action the radula teeth are being erected. The tip of the odontophore then scrapes the surface, while the teeth cut and scoop up the food and convey the particles through the esophagus
Esophagus
The esophagus is an organ in vertebrates which consists of a muscular tube through which food passes from the pharynx to the stomach. During swallowing, food passes from the mouth through the pharynx into the esophagus and travels via peristalsis to the stomach...
to the digestive tract.
In a flexoglossate radula (the primitive condition), the teeth flex outwards to the sides as they round the tip of the odontophore, before flexing back inwards. In the derived stereoglossate condition, the teeth do not flex.
These actions continually wear down the frontal teeth. New teeth are continuously formed at the posterior end of the buccal cavity in the radula sac. They are slowly brought forward to the tip by a slow forward movement of the ribbon, to be replaced in their turn when they are worn out.
Teeth production is rapid (some species produce up to five rows per day). The radular teeth are produced by odontoblasts, cells in the radula sac.
The number of teeth present depends on the species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
of mollusc and may number more than 100,000. Large numbers of teeth in a row (actually v-shaped on the ribbon in many species) is presumed to be a more primitive condition, but this may not always be true.
The greatest number of teeth per row is found in Pleurotomaria
Pleurotomariidae
Pleurotomariidae, common name the '"slit snails", is a family of large marine gastropods in the clade Vetigastropoda. This family is very ancient; there were numerous species in the geological past. It is currently represented by a group of species that live only in deep water...
(deep water gastropods in an ancient lineage) which has over 200 teeth per row (Hyman, 1967).
The shape and arrangement of the radular teeth is an adaptation to the feeding regimen of the species.
The teeth of the radula are lubricated by the mucus of the salivary gland
Salivary gland
The salivary glands in mammals are exocrine glands, glands with ducts, that produce saliva. They also secrete amylase, an enzyme that breaks down starch into maltose...
, just above the radula. Food particles are trapped into this sticky mucus
Mucus
In vertebrates, mucus is a slippery secretion produced by, and covering, mucous membranes. Mucous fluid is typically produced from mucous cells found in mucous glands. Mucous cells secrete products that are rich in glycoproteins and water. Mucous fluid may also originate from mixed glands, which...
, smoothing the progress of food into the oesophagus.
Certain gastropods use their radula teeth to hunt other gastropods and bivalve molluscs, scraping away the soft parts for ingestion. Cone shells have a single radula tooth, that can be thrust like a harpoon into its prey, releasing a neurotoxin
Neurotoxin
A neurotoxin is a toxin that acts specifically on nerve cells , usually by interacting with membrane proteins such as ion channels. Some sources are more general, and define the effect of neurotoxins as occurring at nerve tissue...
.
Radula formulae
Each row of radula teeth consists of
- One central or median tooth (or rachidian tooth, rachis tooth)
- On each side: one or more lateral teeth
- And then beyond that: one or more marginal teeth.
This arrangement is expressed in a radular tooth formula, with the following abbreviations :
- R : designates the central tooth or the rachis tooth (in case of lack of central tooth : the zero sign 0)
- the lateral teeth on each side are expressed by a specific number or D, in case the outer lateral tooth is dominant.
- the marginal teeth are designated by a specific number or, in case they are in a very large numbers, the infinity symbol ∞
3 + D + 2 + R + 2 + D + 3
This formula means: on each side of the radula there are 3 marginal teeth, 1 dominant lateral tooth, 2 lateral teeth, and one central tooth.
The seven basic types
- The docoglossan or stereoglossan radula: in each row there is one usually small central tooth, flanked by 1-3 laterals (with the outer one dominant) and a few (3 at the most) hooked marginals. The central tooth may even be absent. The teeth are fixed in a stiff position on the radular ribbon. This is the most primitive radula type, and we could assume it represents the plesiomorphic condition i.e., the primitive character state, that is taken from an ancestor without change, such as would be possessed by the earliest molluscs (EogastropodaEogastropodaEogastropoda was a previously used taxonomic category of snails or gastropods, a subclass which was erected by Ponder and Lindberg in 1997. It was one of two great divisions of the class Gastropoda, the snails. The other subclass of gastropods was the Orthogastropoda.Eogastropoda were the more...
, also Polyplacophora; limpet families PatellidaePatellidaePatellidae is a taxonomic family of sea snails or true limpets, marine gastropod molluscs in the clade Patellogastropoda.Patellidae is the only family in the superfamily Patelloidea...
, LottiidaeLottiidaeLottiidae is a family of sea snails, specifically true limpets, marine gastropod mollusks in the superfamily Lottioidea and the clade Patellogastropoda .- 2005 taxonomy :...
, LepetidaeLepetidaeLepetidae is a family of sea snails or small, deep-water true limpets, marine gastropod molluscs in the clade Patellogastropoda the true limpets.- Taxonomy :...
). The radula operates like a chain of 'shovels', and the rigid structure operates like a rasp, scraping at hardened macroalgae. Accordingly, docoglossan radulae are often hardened by biomineralization. Spaces between the teeth make the radula ill-suited to collecting microalgae.- Formula: 3 + D + 2 + R + 2 + D + 3
- Or: 3 + D + 2 + 0 + 2 + D + 3
- Rhipidoglossan radula : a large central and symmetrical tooth, flanked on each side by several (usually five) lateral teeth and numerous closely packed flabellate marginals, called uncini (typical examples: VetigastropodaVetigastropodaVetigastropoda is a major taxonomic group of sea snails, marine gastropod mollusks that form a very ancient lineage. Taxonomically Vetigastropoda are sometimes treated as an order although they are a clade in Bouchet and Rocroi, 2005....
, Neritomorpha). This already marks an improvement over the simple docoglossan state. These radulae generally operate like 'brooms', brushing up loose microalgae.- Formula: ∞ + 5 + R + 5 + ∞
- In case of a dominant lateral tooth: ∞ + D + 4 + R + 4 + D + ∞
- Hystrichoglossan radula : each row with lamellate and hooked lateral teeth and hundreds of uniform marginal teeth that are tufted at their ends (typical example : PleurotomariidaePleurotomariidaePleurotomariidae, common name the '"slit snails", is a family of large marine gastropods in the clade Vetigastropoda. This family is very ancient; there were numerous species in the geological past. It is currently represented by a group of species that live only in deep water...
).- The radula formula of, for example, PleurotomariaPleurotomariaPleurotomaria is a genus of sea snails, marine gastropod mollusks in the family Pleurotomariidae.-Species:Species within the genus Pleurotomaria include:Species brought into synonymy :...
(Entemnotrochus) rumphii is : ∞. 14. 27. 1. 27. 14. ∞
- The radula formula of, for example, Pleurotomaria
- Taenioglossan radula: seven teeth in each row: one middle tooth, flanked on each side by one lateral and two marginal teeth (characteristic of the majority of the CaenogastropodaCaenogastropodaCaenogastropoda is a taxonomic clade of a large diverse group of mostly marine gastropods.Caenogastropoda contains a majority of the families of shelled marine molluscs including such families as the periwinkles, cowries, wentletraps, moon snails, and cone snails.About 60% of all living gastropods...
). These operate like 'rakes', scraping algae and gathering the resultant detritus.- Formula : 2 + 1 + R + 1 + 2
- Ptenoglossan radula: rows with no central tooth but a series of several uniform, pointed marginal teeth (typical example : Epitoniodea).
- Formula : n + 0 + n
- Stenoglossan or rachiglossan radula: each row has one central tooth and one lateral tooth on each side (or no lateral teeth in some cases) (most NeogastropodaNeogastropodaNeogastropoda is an unranked taxonomic clade of sea snails, marine gastropod mollusks. For many years prior to 2005 Neogastropoda was an order.- Description :...
).- Formula : 1 + R + 1
- Or : 0 + R + 0
- Toxoglossan radula: The middle teeth are very small or lack completely. Each row has only two teeth of which only one is in use at a time. These grooved teeth are very long and pointed, with venom channels (neurotoxins) and barbs, and are not firmly fixed to the basal plate. The teeth can therefore be individually transferred to the proboscis and ejected like a harpoon into the prey (typical example : ConoideaConoideaConoidea is a superfamily of predatory sea snails, marine gastropod mollusks within the suborder Hypsogastropoda. This superfamily is a very large group of marine mollusks, estimated at about 340 recent valid genera and subgenera, and considered by one authority to contain 4,000 named living species...
).- formula : 1 + 0 + 1
These radula types show the evolution in the gastropods from herbivorous to carnivorous feeding patterns. Scraping algae requires many teeth, as is found in the first three types.
Carnivorous gastropods generally need fewer teeth, especially laterals and marginals. The ptenoglossan radula is situated between the two extremes and is typical for those gastropods which are adapted to a life as parasites on polyp
Polyp
A polyp in zoology is one of two forms found in the phylum Cnidaria, the other being the medusa. Polyps are approximately cylindrical in shape and elongated at the axis of the body...
s.
Gastropods with no radula
The streptaxid Careoradula perelegansCareoradula perelegans
Careoradula perelegans is a species of air-breathing land snail, terrestrial pulmonate gastropod mollusk in the family Streptaxidae.Careoradula perelegans is the only species in the genus Careoradula....
is the only known terrestrial gastropod which has no radula.
There are also various marine gastropods that lack a radula. For example, all species of sea slugs in the family Tethydidae
Tethydidae
Tethydidae is a family of sea slugs, marine gastropod mollusks in the superfamily Tritonioidea.- Taxonomy :This family is within the clade Cladobranchia and has no subfamilies ....
have no radula, and a clade of dorids (the Porostomata) as well as all species of the genus Clathromangelia
Clathromangelia
Clathromangelia is a genus of sea snails, marine gastropod mollusks in the family Clathurellidae, .The species in this genus lack a radula...
(family Clathurellidae) likewise lack the organ. The radula has been lost a number of times in the Opisthobrancha.
In cephalopods
Most cephalopodCephalopod
A cephalopod is any member of the molluscan class Cephalopoda . These exclusively marine animals are characterized by bilateral body symmetry, a prominent head, and a set of arms or tentacles modified from the primitive molluscan foot...
s possess a radula as well as a horny chitin
Chitin
Chitin n is a long-chain polymer of a N-acetylglucosamine, a derivative of glucose, and is found in many places throughout the natural world...
ous beak, although it is reduced in octopus
Octopus
The octopus is a cephalopod mollusc of the order Octopoda. Octopuses have two eyes and four pairs of arms, and like other cephalopods they are bilaterally symmetric. An octopus has a hard beak, with its mouth at the center point of the arms...
es and absent in Spirula.
The cephalopod radula rarely fossilizes: it has been found in around one in five ammonite
Ammonite
Ammonite, as a zoological or paleontological term, refers to any member of the Ammonoidea an extinct subclass within the Molluscan class Cephalopoda which are more closely related to living coleoids Ammonite, as a zoological or paleontological term, refers to any member of the Ammonoidea an extinct...
genera, and is rarer still in non-ammonoid forms. Indeed, it is known from only three non-ammonoid taxa in the Palaeozoic era: Michelinoceras
Michelinoceras
Michelinoceras is the oldest known genus of the Michelinocerida, more commonly known as the Orthocerida, characterized by long, slender, nearly cylindrical orthocones with a circular cross section, long camerae, very long body chambers, and a central or near central tubular siphuncle free of...
, Paleocadmus
Paleocadmus
Paleocadmus is a genus of radula known only from the Mazon Creek biota. It is only known from isolated fossils around a centimetre in length, and a few mm wide , but its morphology aligns it with the nautiloids, or perhaps the bactritoids or belemnoids....
, and an unnamed species from the Soom Shale
Soom Shale
The Soom Shale is a member of the Late Ordovician Cedarberg Formation in South Africa, renowned for its remarkable preservation of soft-tissue in fossil material....
.
In solenogasters
The solenogaster radula is akin to that of other molluscs, with regularly spaced rows of teeth produced at one end and shed at the other. The teeth within each row are similar in shape, and get larger in size towards the outer extreme. A number of teeth occur on each row; this number is usually constant but prone to small variations from row to row; indeed, it increases over time, with teeth being added to the middle of rows by addition or by the division of existing teeth.A number of radular formulae are exhibited by this class: 1:0:1 is most common, followed by 0:1:0 and n:0:n.
In caudofoveates
The radula of the caudofoveate FalcidensFalcidens
Falcidens is one of three genera within the Falcidentidae; its radula consists of a single row of teeth which are mineralized in crystaline hydroxyapatite , and its teeth are not periodically shed and replaced like in other molluscs. For details of the radula, see Radula#In_caudofoveates....
is unlike the conchiferan radula. It has a reduced form, comprising just a single row of teeth. On each side of the apparatus, two teeth appear at the front; behind these, the third teeth fuse to form a mineralized axial plate. Bars occur posterior to this, behind which a sheath encircles the apparatus. The rear of the apparatus consists of a large plate, the 'radular cone'. The unusual form of the radula is accompanied by an unusual purpose: rather than rasping substrates, Falcidens uses its teeth as pincers to grasp prey items.
Further reading
- Molluscan buccal structures and radula
- A Comparison of the feeding behaviour and the functional morphology of radula structure in Nudibranchs
- Katsuno S. & Sasaki T. (2008). "Comparative HistologyHistologyHistology is the study of the microscopic anatomy of cells and tissues of plants and animals. It is performed by examining cells and tissues commonly by sectioning and staining; followed by examination under a light microscope or electron microscope...
of Radula-Supporting Structures in Gastropoda". MalacologiaMalacologiaMalacologia is a peer-reviewed scientific journal in the field of malacology, the study of mollusks. The journal publishes articles in the fields of molluscan systematics, ecology, population ecology, genetics, molecular genetics, evolution, and phylogenetics.The journal specializes in publishing...
50(1-2): 13-56. doi:10.4002/0076-2997-50.1.13