Material gradients in gastropod radulae and their biomechanical significance: a combined approach on the paludomid Lavigeria grandis

Abstract

The radula, a chitinous membrane spiked with teeth, is the molluscan autapomorphy for the gathering and processing of food. The teeth, as actual interfaces between the organism and the ingesta, act as load transmitting regions and have to withstand high stresses during foraging — without structural failure or high degrees of wear. Mechanisms contributing to this were studied previously in paludomid gastropods from Lake Tanganyika. For some species, gradients in hardness and Young’s modulus along the teeth were detected, enabling the bending and relying of teeth onto the next row, distributing the stresses more equally. The here presented study on one of them — Lavigeria grandis — aims at shedding light on the origin of these functional gradients. The mechanical properties were identified by nanoindentation technique and compared to the elemental composition, determined by elemental dispersive X-ray spectroscopy (EDX, EDS). This was done for the complete radular (mature and immature tooth rows), resulting in overall 236 EDX and 700 nanoindentation measurements. Even though teeth showed regional differences in elemental composition, we could not correlate the mechanical gradients with the elemental proportions. By applying confocal laser scanning microscopy (CLSM), we were finally able to relate the mechanical properties with the degree of tanning. CLSM is a common technique used on arthropod cuticle, but was never applied on radular teeth before. In general, we found that nanoindentation and CLSM techniques complement one another, as for example, CLSM is capable of revealing heterogeneities in material or micro-gradients, which leads to a better understanding of the functionalities of biological materials and structures.

Mechanical properties, degree of sclerotisation and elemental composition of the gastric mill in the red swamp crayfish Procambarus clarkii (Decapoda, Crustacea)

“…First, elemental compositions were analyzed by semi-quantitative EDX, afterwards nanoindentation was performed on the same sample localities. For EDX, the SEM Zeiss LEO 1525 (One Zeiss Drive, Thornwood, New York, USA) equipped with an Octane Silicon Drift Detector (SDD) (micro analyses system TEAM, EDAX Inc., New Jersey, USA) was employed (for a detailed protocol see Krings et al, 2022aKrings et al, , 2022bKrings et al, , 2022aKrings et al, , 2022dBrütt et al, 2022). The elemental composition of the embedded structures was studied, always using an acceleration voltage of 20 keV and the same settings (e.g., lens opening, working distance, etc.).…”

“…For nanoindentation (for detailed protocol see Krings et al, 2019Krings et al, , 2021e, 2022aKrings et al, , 2022dKrings, 2020;Gorb & Krings, 2021), a nanoindenter SA2 (MTS Nano Instrument, Oak Ridge, Tennessee, USA) equipped with a Berkovich indenter tip and a dynamic contact module (DCM) head was employed. A Poisson's ratio of 0.3 was used.…”

Coping with abrasion and structural failure – diverging composition of radular teeth in two Porifera-consuming nudibranch species (Mollusca, Gastropoda)

“…The origin of heterogeneities in the radular teeth Functional gradients and heterogeneities in biological materials can have their origin in the architecture of the structures (e.g., chitin bre density, porosity), the degree of tanning, or the inorganic content [for review, see 109]. For chiton and limpet radular teeth, functional gradients are in uenced by all parameters [for throughout reviews, see 81, 82, 83], whereas in the less-mineralized paludomid radular teeth the mechanical properties seem to have their origin in the architecture of chitin bres [74] and the degree of tanning [52]. The latter was determined by applying a protocol established on arthropods: for the chitinous insect cuticle, it is common practice to determine the degree of tanning via laser excitation by CLSM, according to the protocol established by Michels & Gorb [110] [see e.g., 111,112,113,114,115].…”

“…hardness, Young's modulus) and their gradients need to be tested, as they contribute to the understanding of overall functionality of the structure [e.g. 24,26,27,42,43,44,45,46,47,48,49,50,51,52,53]. Part of radular functioning is the reduction of structural failure or high degrees of wear during feeding.…”

“…(2) The radula consists of chitin bers with associated proteins [64], which form a brous composite organic matrix [65]. Besides of different local degrees of tanning, which result in different local mechanical properties [52], the matrix can vary in its bre orientation, density, or arrangement [45,46,66,67,68,69,70,71,72,73,74]. The tooth surface can consist of a rather homogenous layer, whereas the bres underneath are oriented perpendicularly to the tooth surface, probably absorbing shocks during interaction with the feeding substrate [60, 74,75,76,77,78,79] and transmitting stresses to the radular membrane [80].…”