Radula

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Transverse view of the buccal cavity with the radula

Radula types chart. Red=central tooth (rachis), Green=lateral teeth, Yellow=marginal teeth.

The radula is the toothed chitinous ribbon in the mouth of most mollusks, used for cutting and chewing food before it enters the esophagus. It is present in all mollusks except bivalves, and only in mollusks. For example, Gastropods use it to graze and scrape diatoms and other microscopic algae off rock surfaces and other substrata. Squid use it for cutting prey. The remainder of this article is about radula in gastropods.

The mouth of the gastropods is located below the anterior part of the mollusk. 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 mollusk 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 largest number of teeth per row is found in Pleurotomaria (gastropod) with over 200 teeth per row (Hyman, 1967). The shape and arrangement of 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 mollusks, 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.

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 : one or more marginal teeth.

This arrangement of the teeth is expressed in a teeth 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 ∞

docoglossan radula

This can be expressed in a typical formula such as:

3 + D + 2 + R + 2 + D + 3

meaning : on each side 3 marginal teeth, 1 dominant lateral tooth, 2 lateral teeth and one central tooth.

This results in 7 basic types :

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 first mollusks (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 e.g. 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 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, in accordance with the first three types. Carnivorous types need fewer teeth, especially laterals and marginals. The ptenoglossan radula is situated between the two extremes and is typical for gastropods, adapted to a life as parasites on polyps.