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The radula is an anatomical structure found in mollusks and used for feeding. It is a minutely toothed, 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 ribbon. It is typically used for scraping or cutting food before the food enters the esophagus
Esophagus

The esophagus or oesophagus , sometimes known as the gullet, is an Organ in vertebrates which consists of a Muscle tube through which food passes from the pharynx to the stomach....
. The radula is unique to the molluscs, and is found in all clades of mollusks except for the bivalves
Bivalvia

Bivalves are molluscs belonging to the class Bivalvia. They have two-part animal shells, and typically both valves are symmetry along the hinge line....
.

Within the gastropods, the radula is used in feeding by both herbivorous and carnivorous snail
Snail

The word snail is a common name for almost all members of the molluscan class Gastropoda that have coiled animal shells in the adult stage. When the word snail is used in a general sense, it includes sea snails, land snails and freshwater snails....
s and slug
Slug

Slug is a common non-scientific word, which is often applied to any gastropod Mollusca whatsoever that has a very reduced shell, a small internal shell, or no shell at all....
s.






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Radula Types
The radula is an anatomical structure found in mollusks and used for feeding. It is a minutely toothed, 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 ribbon. It is typically used for scraping or cutting food before the food enters the esophagus
Esophagus

The esophagus or oesophagus , sometimes known as the gullet, is an Organ in vertebrates which consists of a Muscle tube through which food passes from the pharynx to the stomach....
. The radula is unique to the molluscs, and is found in all clades of mollusks except for the bivalves
Bivalvia

Bivalves are molluscs belonging to the class Bivalvia. They have two-part animal shells, and typically both valves are symmetry along the hinge line....
.

Within the gastropods, the radula is used in feeding by both herbivorous and carnivorous snail
Snail

The word snail is a common name for almost all members of the molluscan class Gastropoda that have coiled animal shells in the adult stage. When the word snail is used in a general sense, it includes sea snails, land snails and freshwater snails....
s and slug
Slug

Slug is a common non-scientific word, which is often applied to any gastropod Mollusca whatsoever that has a very reduced shell, a small internal shell, or no shell at all....
s. The arrangement of teeth on the radula ribbon varies considerably however from one group to another.

In most of the more ancient lineages of gastropods, the radula is used to graze and scrape diatom
Diatom

Diatoms are a major group of eukaryote algae, and are one of the most common types of phytoplankton. Most diatoms are unicellular, although they can exist as Colony in the shape of filaments or ribbons , fans , zigzags , or stellate colonies ....
s and other microscopic algae off rock surfaces and other substrata.

Predatory marine snails such as the Naticidae
Naticidae

Naticidae, the moon snails or naticoids, is a family of minute to large-sized predatory sea snails, marine gastropod molluscs in the clade Littorinimorpha....
 use the radula plus an acidic secretion in order to bore through the shell of other molluscs. 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 non-scientific word, which is often applied to any gastropod Mollusca whatsoever that has a very reduced shell, a small internal shell, or no shell at all....
s such as the ghost slug
Ghost slug

The ghost slug, Selenochlamys ysbryda, is a species of predatory air-breathing land slug, a shell-less pulmonate gastropod mollusk in the family Trigonochlamydidae....
 use elongated razor-sharp teeth on the radula in order to seize and devour earthworms.

Predatory cephalopods such as squid
Squid

Squid are marine cephalopods of the order Teuthida, which comprises around 300 species. Like all other cephalopods, squid have a distinct head, Symmetry #Bilateral_symmetry, a mantle , and cephalopod arms....
 use the radula for cutting prey.

The gastropod radula

Radula Diagram3

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.

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 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 and recurved teeth
    Tooth

    Teeth are small whitish structures found in the jaws of many vertebrates that are used to tear, scrape, and chew food. Some animals, particularly carnivores, also use teeth for hunting or defense....
    , 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 or oesophagus , sometimes known as the gullet, is an Organ in vertebrates which consists of a Muscle tube through which food passes from the pharynx to the stomach....
 to the digestive tract.

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 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....
 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 shells, is a family of large sea snails, marine gastropod mollusks in the clade Vetigastropoda .The interior of the shells of these sea snails is pearly ....
 (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 regime 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 that produce saliva. They also secrete amylase, an enzyme that breaks down starch into glucose. In other organisms such as Insecta, salivary glands are often used to produce biologically important proteins like silk or glues, and fly salivary glands contain polytene chromosomes that have been usefu...
, 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. It is a viscous colloid containing antiseptic enzymes and immunoglobulins that serves to protect Epithelium in the respiratory,...
, 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....
.

Gastropod Radula formula

The number, shape, and specialized arrangement of teeth in each transverse row is consistent on a radula, and the different patterns can be used as a diagnostic characteristic to identify the species in many cases.

Each row of radula teeth consists of
  • One central tooth (or 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 8
This can be expressed in a typical formula such as:

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 7 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. 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 (Eogastropoda
    Eogastropoda

    Eogastropoda is a taxonomic category. It is one of the two great divisions of the class Gastropoda, the snails. The other subclass of gastropods is the Orthogastropoda....
    , also Polyplacophora).
    • 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 (typical examples: Vetigastropoda
    Vetigastropoda

    Vetigastropoda is a major taxonomy grouping of sea snails, marine gastropod molluscs. This is a very ancient lineage .In the current taxonomy of the Gastropoda , Vetigastropoda is one of the main clades....
    , Neritomorpha). This already marks an improvement over the simple docoglossan state.
    • Formula: 8 + 5 + R + 5 + 8
    • In case of a dominant lateral tooth: 8 + D + 4 + R + 4 + D + 8


  • Hystrichoglossan radula : each row with lamellate and hooked lateral teeth and hundreds of uniform marginal teeth that are tufted at their ends (typical example : Pleurotomariidae
    Pleurotomariidae

    Pleurotomariidae, common name the slit shells, is a family of large sea snails, marine gastropod mollusks in the clade Vetigastropoda .The interior of the shells of these sea snails is pearly ....
    ).
    • The radula formula of, for example, Pleurotomaria (Entemnotrochus) rumphii is : 8. 14. 27. 1. 27. 14. 8


  • 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 Caenogastropoda
    Caenogastropoda

    Caenogastropoda is a very large taxonomic group, a superorder of saltwater snails, . The Caenogastropoda are within the gastropod infraclass Apogastropoda....
    ).
    • 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 Neogastropoda
    Neogastropoda

    The Neogastropoda was, for many years, an order of sea snails, marine gastropod mollusks. In the current version of gastropod taxonomy however, Neogastropoda is an infraorder of sea snails....
    ).
    • 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 : Conoidea
    Conoidea

    Conoidea is a taxonomic superfamily of sophisticated predatory sea snails.This superfamily includes the Turridae, the Terebridae and the Conidae or cone snails....
    ).
    • 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

In zoology, a polyp is one of two forms of individuals found in many species of cnidarians. The two are the polyp or hydroid and the medusa . Polyps are approximately cylindrical, elongated on the axis of the body....
s.

The term "radula" was first used by Alexander von Middendorff
Alexander von Middendorff

Alexander Theodor von Middendorff was a Russian Empire zoologist and explorer of Baltic German origin.Middendorff was born in Saint Petersburg, where he received his early education....
 in 1848.

Radulæ in other molluscs

Cephalopods 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.

Evolution of the radula


The first bona fide radula dates to the Early Cambrian, although trace fossils from the earlier Ediacaran
Ediacaran

The Ediacaran Period 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 genus of fossils known only from rocks of the Ediacaran period, and only one species, Kimberella quadrata, has been recognized....
.

Another so-called radula was been reported from the early Cambrian in 1974, this one preserved with fragments of the mineral ilmenite
Ilmenite

Ilmenite is a weakly Magnetism 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 most abundant mineral in the Earth's continental crust . It is made up of a Crystal structure of silica tetrahedra. Quartz has a hardness of 7 on the Mohs scale and a density of 2.65 g/cm?....
 matrix, and showing similarities to the radula of the modern cephalopod Sepia
Sepia

Sepia may refer to:...
. However, this was since re-interpreted as Salterella [/Volborthella
Volborthella

fossil_range = |regnum=Animalia|phylum=incertae sedis...
?].

See also

  • Hypostome
    Hypostome

    A hypostome , is a Osseous_tissue harpoon-like structure near the mouth area of certain Parasitism arthropods including ticks and mites , that allows them to anchor themselves firmly in place on a host mammal while sucking blood....
  • Cone snail#Harpoon and venoms
    Cone snail

    The cone snails or cone shells, sometimes simply known as "cones", , are a taxonomic family of approximately 500 medium-sized to large, sophisticated predatory sea snails, marine gastropod mollusks....