radula

A chart of the arrangement of the teeth in five basic varieties of gastropod radula. Red=central tooth (rachis), Green=lateral teeth, Yellow=marginal teeth.

The radula (plural radulae or radulas^[1]) is an anatomical structure found in mollusks and used for feeding. It is a minutely toothed, chitinous ribbon. It is typically used for scraping or cutting food before the food enters the esophagus. The radula is unique to the molluscs, and is found in all clades of mollusks except for the bivalves.

Within the gastropods, the radula is used in feeding by both herbivorous and carnivorous snails and slugs. 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 diatoms and other microscopic algae off rock surfaces and other substrata.

Predatory marine snails such as the Naticidae 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 slugs such as the ghost slug use elongated razor-sharp teeth on the radula in order to seize and devour earthworms.

Predatory cephalopods such as squid use the radula for cutting prey.

The introduction of the term "radula" is usually attributed to Alexander von Middendorff in 1848.

In gastropods

Diagrammatic transverse view of the buccal cavity of a gastropod, showing the radula and how it is used.
The rest of the body of the snail is shown in green. The food is shown in blue. Muscles that control the radula are shown in brown. The surface of the radula ribbon, with numerous teeth, is shown as a zig-zag line

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 chitinous and recurved teeth, 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 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 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 (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, just above the radula. Food particles are trapped into this sticky mucus, 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.

Radula formulae

Radula of the ghost slug, Selenochlamys ysbryda.

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 ∞

Microscopic detail of a docoglossan radula showing the denticles or teeth

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 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. 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, 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, Neritomorpha). This already marks an improvement over the simple docoglossan state.
  • Formula: ∞ + 5 + R + 5 + ∞
  • In case of a dominant lateral tooth: ∞ + D + 4 + R + 4 + D + ∞

Radula (magn. 400x) of the gray garden slug (Deroceras laeve) showing the chitinous ribbons with numerous inward-pointing denticles

• 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).
  • The radula formula of, for example, Pleurotomaria (Entemnotrochus) rumphii is : ∞. 14. 27. 1. 27. 14. ∞

• 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).
  • 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).
  • 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).
  • 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 polyps.

In other molluscs

Cephalopods possess a radula as well as a horny chitinous beak.^[2]

This section requires expansion.

Evolution

The first bona fide radula dates to the Early Cambrian,^[3] although trace fossils from the earlier Ediacaran have been suggested to have been made by the radula of the organism Kimberella.

Another so-called radula has been reported from the early Cambrian in 1974, this one preserved with fragments of the mineral ilmenite suspended in a quartz matrix, and showing similarities to the radula of the modern cephalopod Sepia.^[4] However, this was since re-interpreted as Salterella^[5] [/Volborthella?]^{[verification needed]}.

This section requires expansion.

See also

Look up radula in Wiktionary, the free dictionary.

• Hypostome

References

This article includes a list of references, related reading or external links, but its sources remain unclear because it lacks inline citations. Please improve this article by introducing more precise citations where appropriate. (March 2009)

• Molluscan buccal structures and radula
• A Comparison of the feeding behaviour and the functional morphology of radula structure in Nudibranchs
• Hickman Carole, S (1980). "Gastropod Radulae and the Assessment of Form in Evolutionary Paleontology". Paleobiology 6 (3): 276–294. 
• Amélie H. Scheltema, Klaus Kerth, Alan M. Kuzirian (April 2006). "Original molluscan radula: Comparisons among Aplacophora, Polyplacophora, Gastropoda, and the Cambrian fossil Wiwaxia corrugata". Journal of Morphology 257 (2): 219–245. doi:10.1002/jmor.10121. PMID 12833382. http://www3.interscience.wiley.com/cgi-bin/abstract/104528759/ABSTRACT?CRETRY=1&SRETRY=0. 

Further reading

• Katsuno S. & Sasaki T. (January 2008) "Comparative Histology of Radula-Supporting Structures in Gastropoda". Malacologia, 50(1-2): 13-56. doi:10.4002/0076-2997-50.1.13

External links

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