Radula

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Radula from Aplysia juliana

As a radula ( lat. "Scrapers", "rasp") or rasp or friction strip the characteristic is mouth parts of molluscs called. In its basic training it is used to grate, chop and collect food in the throat. Carnivore species can also use the radula to grab and hold prey.

The radula occurs in all molluscs with the exception of the filter feeder mussels as well as some worm molluscs and snails , which suckle. For this reason, it represents an essential apomorphism (common feature) of this group. At the same time, the various modifications of the radula are considered to be the main reason for the development of different food sources and thus for the wide distribution and the high number of species of molluscs, especially snails.

construction

Basic structure

Radula: structure and functionality

The radula is a tongue-like lamella in the throat area, which is usually designed as a toothed chitin membrane ( radula membrane). It sits near the mouth opening in the pharynx (corresponding to the throat ) in a radula pocket. In most cases the membrane is equipped with a few to many teeth made of chitin (up to 800,000), conchin and mineral salts , arranged in regular transverse or longitudinal rows .

The radula sits on a supporting apparatus (odontophor) and is moved by muscle groups together with it. In many molluscs, especially snails, the radula sits on a pad with which it can be used to graze substrates and which, when pushed forward, graze the surface. The modes of movement are different depending on the species-specific use, whereby the use of the substrate-scraping pastureland such as the beetle snails or the limpets is considered to be the most original form. A “jaw” is often formed as an abutment on the rear wall of the throat, for example in the case of Roman snails . The radula force of the spotted garden snail was measured while it was eating. 107 millinewtons were registered, which corresponds to a pressure of up to 4700 bar per tooth tip.

variation

According to the very different way of life and diet of the molluscs, the radula is partly heavily modified, especially in the number, size and shape of the teeth in the different animal groups. In the mussels (Bivalvia), which feed on a filtering diet, as well as in some furrows and snails , which feed on a sucking diet, the radula has been reduced and is therefore completely absent.

The radula of the tortoiseshell (caudofoveata) is in most cases very simple and has only a single row of transverse teeth, with which the animals take their food (especially detritus , foraminifera and diatoms ) from the sediment of the seabed. In the second group of worm mollusks , the parsnips (Solenogastres), there are two rows of teeth on the radula; however, it is completely reduced in around a third of the known species. Beetle snails (Placophora), on the other hand, have a very large radula, which is about a third of the length of the body. It consists of 17 longitudinal rows and often more than 40 transverse rows of teeth, which are hardened by embedded magnetite , especially in the tip area. The animals scrape algae growth from the substrate with these teeth and transport it into the throat and the adjoining foregut. A similar structure is also the throat area of the Einschaler (Monoplacophora) with a radula, which is also comparable to that of the beetle screw as with the tongue of the beam prosobranch (docoglosser type).

It shows the greatest variability in the very diverse snails; There is a strong differentiation especially within the hind gill snails . A distinction is made here, for example. Beam tongues, ribbon tongues, brush tongues, fan tongues, spring tongues, lard tongues and poison tongues, which are typical for kinship groups and are also reflected in the naming of the different taxa . The most important types of radula of snails are:

  • Beam tongue (docoglosser type): With the docoglosser type, there are some intermediate plates in a transverse row on both sides of a central tooth, followed by side plates. The intermediate and side plates are often hardened by the inclusion of opal and goethite . The docoglose radula type is primarily characterized by grazing herbivores, and the species are often specialized in diatoms or foraminifera.
  • Fan tongue (rhipidoglosser type): The rhipidoglosser type consists of a strong central tooth, to which one to ten intermediate plates and several side plates adjoin. The type is common in grazers on algae and other growth.
  • Ribbon tongue (taenioglosser type): The taenioglosse type is the most common type of radula and occurs in almost all species that were formerly grouped as middle snails. It consists of a central tooth in each transverse row, which is flanked by a central plate and two side plates.
  • Lard tongue (rhachi or shorthand type): The shorthand type consists of a central tooth and a central plate and a side plate on both sides. It occurs mainly in the carnivorous new snails .
  • Arrow or poison tongue (toxoglosser type): The toxoglosse type is the radula shape that has been modified most from the original type. The radula membrane is reduced, and the radula consists of a few teeth with an arrow-like shape that can inject poison into a potential victim like a cannula. After use, the tooth is replaced with a reserve tooth. This type is developed in the highly poisonous cone snails (genus Conus ), which inject their victims with a strongly paralyzing poison with the help of the radula.

In contrast to the front-gill snails, lung snails and hind- gill snails have more uniform radulae, which are usually covered with tens of thousands of similar teeth, which can only be differentiated under an electron microscope.

In cephalopods (cephalopods), the toothed radula is very well developed; in addition, many species above the radula have a narrowing of the intestinal tract by lateral lobes, which in the squid are covered with a toothed chitin layer and are intended to prevent the prey from sliding back. Octopuses have additional teeth under the radula and in the back of the throat.

education

In the radula pouch (also known as the radula sac), the radula is formed by glandular secretions , with specialized cells secreting the membrane components ( proteins and chitin-like glycoproteins ), while groups of odontoblasts form the teeth. Each tooth consists of a basal plate, a middle and a tip part, which is particularly strengthened by mineral salts.

Worn teeth and chitin parts are constantly being re-formed in the radula pocket, so that the radula constantly grows back from behind. In some pulmonary snails, around three transverse rows are created each day, for example in Lymnaea stagnalis an average of 2.8 rows per day.

supporting documents

Individual evidence

  1. ↑ a b c d e f g h i Herder Lexicon of Biology. 2003.
  2. ↑ a b c d e K-J. Götting: Mollusca, molluscs. 1996, p. 279.
  3. ↑ a b c K-J. Götting: Mollusca, molluscs. 1996, p. 298.
  4. ↑ Nadja Podbregar: Snail tongue is a "power miracle". In: Wissenschaft.de . July 26, 2019, accessed August 3, 2019 .
  5. ↑ KJ. Götting: Mollusca, molluscs. 1996, p. 285.
  6. ↑ KJ. Götting: Mollusca, molluscs. 1996, p. 286.
  7. ↑ a b c K-J. Götting: Mollusca, molluscs. 1996, p. 288.
  8. ↑ KJ. Götting: Mollusca, molluscs. 1996, p. 291.
  9. ↑ KJ. Götting: Mollusca, molluscs. 1996, p. 314.

literature

  • Radula. In: Herder-Lexikon der Biologie. Spektrum Akademischer Verlag, Heidelberg 2003, ISBN 3-8274-0354-5 .
  • Klaus-Jürgen Götting: Mollusca, molluscs. In: W. Westheide, R. Rieger: Special Zoology. Part 1: Protozoa and invertebrates. Elsevier Spektrum Akademischer Verlag, Heidelberg 1996, ISBN 3-437-20515-3 , pp. 276-330.

Web links

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