The radula (; : radulae or radulas) is an anatomical structure found in most mollusks, serving as their primary feeding tool. Often compared to a tongue, this minutely toothed, chitinous ribbon typically functions by scraping or cutting food before it enters the esophagus. Mollusks in every class possess a radula, except for bivalves, which instead employ waving cilia to draw in minute organisms for feeding.
The radula typically functions as a rasping organ to scrape food particles. However, its form and use have diversified significantly; it is modified for drilling holes in prey shells (e.g., in Muricidae), transformed into venomous harpoons (e.g., in Conidae using conotoxins), or reduced/lost in fluid feeders (such as in the Pyramidellidae where the highly specialized, needle-like radula is called a stylet).
Within the gastropods, the radula is used in feeding by both herbivorous and carnivorous snails and slugs. 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 diatoms and other microscopic algae 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 slugs, 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 the Russian zoologist Alexander von Middendorff in 1847.
Components
A typical radula comprises a number of bilaterally-symmetrical self-similar rows of teeth rooted in a radular membrane in the floor of their mouth cavity. 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 elastic, delicate radular membrane may be a single tongue, or may split into two (bipartite).
Hyaline shield
See Hyaline shield for more details.
Odontophore
The odontophore is the eversible, fleshy tongue underlying the radular membrane. It 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.
The teeth often tesselate with their neighbours, and this interlocking serves to make it more difficult for them to be removed from the radular ribbon.
Morphology
The morphology of the radula is related to diet. However, it is not fixed per species; some mollusks 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.
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.
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. [/Volborthella?]
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).
In gastropods
[[File:Radula diagram.svg|thumb|right|300px|Diagrammatic saggital view of the buccal cavity of a gastropod, showing the radula and how it is used. <br>
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]]
thumb|right|Upper right: Mouth of a [[Planorbarius corneus freshwater snail with the radula visible.]]
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.
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 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.
Some marine gastropods lack a radula. For example, all species of sea slugs in the family Tethydidae have no radula, 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 Opisthobranchia.
In cephalopods
thumb|Radular teeth of the squid [[Illex illecebrosus]]
Most cephalopods possess a radula as well as a horny chitinous beak, although the radula is reduced in octopuses and absent in Spirula.
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.
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. 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.