The radula (US: ; pl.: 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.
Teeth The radula comprises multiple, identical (or near-enough) rows of teeth, fine, flat, or spiney out-growths; often, each tooth in a row (along with its symmetric partner) will have a unique morphology. Each tooth can be divided into three sections: a base, a shaft, and a cusp. In radulae that 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 for them to be removed from the radular ribbon.
Radula formulae
The number, shape, and specialized arrangement of molluscan 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 radular teeth consists of
One central or median tooth (or rachidian tooth, 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 very large numbers, the infinity symbol ∞
This can be expressed in a typical formula such as: 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 scaphopods, means one marginal tooth, one lateral tooth, one rachidian tooth, one lateral tooth, and one marginal tooth across the 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.
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. The sacoglossans (sea slugs) 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.
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