Differences in the Young modulus and hardness reflect different functions of teeth within the taenioglossan radula of...

One important autapomorphy of molluscs is the radula, which is the anatomical structure used for feeding in most species of Mollusca. As this phylum represents the second species-richest animal group inhabiting very diverse environments, it is not surprising that the morphology of the radula and its teeth is also very diverse between taxa. However, the taenioglossan radulae are remarkable because its tooth types (central, lateral, and marginal teeth) are highly distinct in their morphology within the same radula. There are several hypotheses attempting to explain this tooth diversity by their functional specialisations. Here, for the first time, the functional morphology of taenioglossan radular teeth from one single species was analysed and their material properties (hardness and elasticity modulus) were characterised by nanoindentation. Spekia zonata Bourguignat, 1879 belongs to the Paludomidae inhabiting and feeding on solid substrates in Lake Tanganyika. All tooth types show gradual and significant differences in their stiffness and hardness: from the basis, as the softest and most flexible area, to the stylus and the cusps as the stiffest and hardest areas. The flexibility of the stylus allows the teeth to act as one single feeding organ: the central and lateral teeth can stabilize each other during feeding by the rear teeth providing support to the next row. Tooth types also differ significantly in their stiffness and hardness: the central teeth consist of the hardest and stiffest material, followed by the lateral teeth and finally marginal teeth. This can be explained by different functional loads of teeth: central and lateral ones are used for scratching over the substrate while the marginals serve mainly as brooms collecting food particles.