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The radula (; : radulae or radulas) is an anatomical structure found in most Mollusca, serving as their primary feeding tool. Often compared to a tongue., this minutely toothed, ribbon typically functions by scraping or cutting food before it enters the esophagus. Mollusks in every class possess a radula, except for Bivalvia, which instead employ waving cilia to draw in minute organisms for feeding.
Within the gastropods, the radula is used in feeding by both herbivore and carnivore and . The arrangement of teeth (denticles) on the radular 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 and other microscopic off rock surfaces and other substrates.
Predatory marine snails such as the Naticidae use the radula plus an acidic secretion to bore through the shell of other mollusks. Other predatory marine snails, such as the Conidae, use a specialized radular tooth as a poisoned harpoon. Predatory pulmonate land , such as the ghost slug, use elongated razor-sharp teeth on the radula to seize and devour earthworms. Predatory cephalopods, such as squid, use the radula for cutting prey.
The introduction of the term "radula" (Latin, "little scraper") is usually attributed to Alexander von Middendorff in 1847. von Middendorff A. T. (1847). Beiträge zu einer malacozoologia rossica: Chitonen. St. Petersburg, 151 pp. + plates. p .54.
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.
Each row of radular teeth consists of
This arrangement is expressed in a radular tooth formula, with the following abbreviations :
3 + D + 2 + R + 2 + D + 3
This formula means: Across the radula there are 3 marginal teeth, 1 dominant lateral tooth, 2 lateral teeth, and one central tooth.
Another formula for describing radulae omits the use of letters and simply gives a sequence of numbers in the order marginal-lateral-rachidian-lateral-marginal, thus:
1-1-1-1-1
This particular formula, which is common to the , means one marginal tooth, one lateral tooth, one rachidian tooth, one lateral tooth, and one marginal tooth across the ribbon.
Pointed teeth are best suited to grazing on algal tissue, whereas blunt teeth are preferable if feeding habits entail scraping epiphytes from surfaces.
The () 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-shaped teeth – rods with a groove along one side – are associated with diets of crossed-fibrillar cellulose-walled algae, such as the Siphonocladaceae and Cladophorales, whereas blade-shaped teeth are more generalist.
A so-called radula from the early Cambrian was discovered 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. However, this was since re-interpreted as Salterella. /''
Based on the bipartite nature of the radular dentition pattern in solenogasters, larval gastropods and larval polyplacophora, it has been postulated that the ancestral mollusk bore a bipartite radula (although the radular membrane may not have been bipartite).
The teeth of Chaetopleura apiculata comprise fibres surrounded by magnetite, sodium and magnesium.
The radula apparatus consists of two parts :
The odontophore is movable and protractible, and the radula itself is movable over the odontophore. Through this action the radular 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.
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 radular 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 radular sac.
The number of teeth present depends on the species of mollusk and may number more than 100000. 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 Pleurotomariidae (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, just above the radula. Food particles are trapped into this sticky mucus, smoothing the progress of transfering food into the esophagus.
Certain gastropods use their radular teeth to hunt other gastropods and bivalve mollusks, scraping away the soft parts for ingestion. Cone shells have a single radular tooth, that can be thrust like a harpoon into its prey, releasing a neurotoxin.
These radular 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.
Some marine gastropods lack a radula. For example, all species of sea slugs in the family Tethydidae have no radula,Rudman W. B. (14 October 2002) "http://www.seaslugforum.net/factsheet.cfm?base=tethfimb". Sea Slug Forum, accessed 29 December 2010. and a clade of dorids (the Porostomata) as well as all species of the genus Clathromangelia (family Clathurellidae) likewise lack the organ. The radula has been lost a number of times in the Opisthobrancha.
The cephalopod radula rarely fossilizes: it has been found in around one in five ammonite genera, and is rarer still in non-ammonoid forms. Indeed, it is known from only three non-ammonoid taxa in the Palaeozoic era: Michelinoceras, Paleocadmus, and an unnamed species from the Soom Shale.
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 radular types are suited to less strenuous modes of feeding, brushing up smaller algae or feeding on soft forms; mollusks 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.
Early mollusks
The first bona fide radula dates to the Early Cambrian, although trace fossils from the earlier Ediacaran have been suggested to have been made by the radula of the organism Kimberella.
In chitons
Each row of the 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.
In gastropods
[[File:Radula diagram3.png|thumb|right|300px|Diagrammatic saggital 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 radular 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 mollusk. It opens into a pocket-like buccal cavity, containing the radular sac, an evaginated pocket in the posterior wall of this cavity.
Seven basic types
'' showing outer marginal teeth (on the left), inner marginal teeth and immediately next to them lateral teeth, central teeth. Scale bar is 20 μm.]]
Gastropods with no radula
The streptaxid Careoradula perelegans is the only known terrestrial gastropod which has no radula.
In cephalopods
Most possess a radula as well as a horny cephalopod beak, although the radula is reduced in and absent in Spirula.
In solenogasters
The solenogaster radula is akin to that of other mollusks, 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 Caudofoveata Falcidens 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.
See also
Further reading
External links
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