Ontogeny of the elemental composition and the biomechanics of radular teeth in the chiton Lepidochitona cinerea

Abstract

Background The radula, a chitinous membrane with embedded teeth, is one important molluscan autapomorphy. In some taxa (Polyplacophora and Patellogastropoda) one tooth type (the dominant lateral tooth) was studied intensively in the last decades with regard to its mechanical properties, chemical, and structural composition, and the relationship between these parameters. As the dominant lateral tooth is probably one of the best studied biological materials, it is surprising, that data on chemistry and mechanical properties of the other tooth types, present on a chiton radula, is lacking. Results We provide data on the elemental distribution and mechanical properties (hardness and elasticity, i.e. Young’s modulus) of all teeth from the Polyplacophora Lepidochitona cinerea (Linnaeus, 1767) [Chitonidae: Ischnochitonidae]. The ontogeny of elements, determined by energy-dispersive X-ray spectroscopy, and of the mechanical properties, detected by nanoindentation, was studied for every single tooth type. Additionally, we performed breaking stress experiments with teeth under dry and wet condition, highlighting the high influence of the water content on the mechanical behaviour of the radula. We thereby could determine the forces and stresses, teeth can resist, which was previously not studied in representatives of Polyplacophora. Overall, we were able to relate the mineral (iron, calcium) content, the mechanical parameters, breaking force, and stress for each tooth type. This led to a better understanding of the relationship between structure, material, and function in radular teeth. Further, we aimed at determining the role of calcium for the mechanical behaviour of the teeth: we decalcified radulae by ethylenediaminetetraacetic acid and afterwards performed elemental analyses, breaking stress experiments and determined the mechanical properties. Among other things, we detected that wet and decalcified radular teeth could resist highest forces, since teeth have a higher range of bending motion leading to a higher capability of teeth to gain mechanical support from the adjacent tooth row. Conclusion We present novel data on the chemistry, mechanical properties, and the mechanical behaviour of chiton teeth, which allows conclusions about tooth function. We could also relate the parameters mentioned, which contributes to our understanding on the origins of mechanical property gradients and the processes reducing structural failure in radular teeth.

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

“…It was previously determined, that Na, K, and S can be related to the protein bonding and degree of tanning [e.g., 92 – 94 ] —potentially this is also the case in crustaceans. The detected proportions of Fe and Zn potentially also increase the stiffness and hardness in the gastric mill, comparable to molluscan teeth—even though, the detected proportions are very small in comparison to the incorporations in Polyplacophora and limpets [e.g., 58 , 60 , 95 , 96 ]. But this again awaits further investigations.…”

“…The metallic ring ensured, that the sample surface was almost parallel to the sample holder. This specific epoxy was chosen, because, known from previous studies 39 – 43 , 57 , 58 , it does not infiltrate the structures. After polymerization, object slide and adhesive tape were removed and each sample was polished with sandpapers of distinct roughness until the structures targeted were on display.…”

“…the results on the proportions of lighter elements are not as reliable as the same of heavier elements. For the semi-quantitative EDX, we employed 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) [for a detailed protocol see 58 – 62 ]. For all measurements, we used an acceleration voltage of 20 keV and the same settings (e.g., lens opening, working distance, etc.).…”

“…For nanoindentation [for a detailed protocol see 39 , 42 , 43 , 57 , 58 , 61 ], 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.…”

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

“…After drying at room temperature, each stomach was surrounded by a metallic ring, which was lled with epoxy resin (RECKLI EPOXI WST, RECKLI GmbH, Herne, Germany), polymerizing for three days at room temperature (Young's modulus of the polymerized epoxy is 1.3 ± 0.3 GPa). The metallic ring ensured an almost parallel sample, and the speci c epoxy was chosen because, known from previous studies (Krings, 2020;Krings et al, 2019Krings et al, , 2021aKrings et al, , 2021bKrings et al, , 2021cKrings et al, , 2022aGorb & Krings, 2021), it does not in ltrate the structures. After polymerization, object slide and adhesive tape were removed and each sample was polished with sandpapers of distinct roughness until the structures targeted were on display.…”