Animal skeletons are high-performing composite materials that may help inspire materials and designs in a broad spectrum of industrial and biomedical applications. The study of various skeletal elements can provide important insights into evolutionary solutions to mechanical demands of animal locomotion, feeding and reproduction, as well as reveal the mechanisms controlling skeletal formation and biomineralization.

The extracellular matrix forming the crustacean exoskeleton comprises chitin-protein fibers embedded in a calcified inorganic matrix that consists of calcite and amorphous calcium carbonate. In crustaceans, the cuticle can be subdivided into a thin external layer – the epicuticle – and two internal layers, which are heavily calcified – the exocuticle and the endocuticle [1]. The crustacean cuticle generally consists of stacked sheets of parallel chitin-protein fibers, which helicoidally shift their orientation in each sequential sheet, resulting in a structure referred to as the Bouligand pattern [2]. This organization of the fibers strengthens the cuticle in different directions.

In our study, we analyzed the structure and composition of the walking leg claw of the woodlouse Porcellio scaber. Woodlice are terrestrial crustaceans that support their bodies with 7 pairs of legs, each ending in a claw. The claws are thin skeletal elements predominantly subjected to unidirectional loads. To study the nano-structure of the matrix, we imaged fractured claws with field-emission scanning electron microscopy using a Jeol 7500F microscope. We then analyzed the elemental composition and the distribution of mineral components in the claws with energy-dispersive X-ray spectroscopy (EDX) and electron energy-loss spectroscopy (EELS) combined with high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) at high spatial and high energy resolution using Zeiss SESAM and Jeol ARM200F microscopes at different accelerating voltages.

Our results demonstrate that the exocuticle of the claw is not calcified and is heavily brominated instead. The endocuticle, on the other hand, is mineralized predominantly with stable amorphous calcium phosphate, which is a highly unusual feature of an animal exoskeleton. Furthermore, we established that the claw endocuticle is highly structurally anisotropic, consisting of axially oriented chitin-protein fibers and amorphous calcium phosphate particles, all oriented in the direction of loading.

The presence of amorphous calcium phosphate in the mineralized endocuticle and a non-calcified, brominated external exocuticle may help increase fracture resistance of the claw cuticle. The brominated exocuticle, which is likely more elastic than the mineralized endocuticle, is distributed in areas subjected to maximum stress during axial loading of the claw. These structural and compositional features of the claw cuticle likely result in greater resistance of the claw to fracture and wear when exposed to axial loading.

Acknowledgments

The research leading to these results has received funding from the European Union Seventh Framework Program [FP/2007-2013] under grant agreement No.312483 (ESTEEM2). The work was supported by the Slovenian Research Agency in the scope of the research program P1-0184 (Integrative zoology and speleobiology).

References

[1] R Roer and R Dillaman, American Zoologist 24 (1984), pp 893-909.

[2] Y Bouligand, Tissue & Cell 4 (1972), pp. 189-217.

Figures:

Fig. 1. A: The woodlouse claw. B: SEM image of the fractured claw. C: Higher magnification of the fractured claw, showing the exocuticle and the endocuticle with unidirectionally oriented fibers. D: HAADF-STEM image of a similar region as shown in C, showing the greater electron density of the claw endocuticle as compared to the exocuticle.

Fig. 2. A: HAADF-STEM image of the claw endocuticle, showing unidirectionally oriented particles of calcium phosphate. B: Electron diffraction of the region in A, demonstrating the amorphous nature of the calcium phosphate. C: Energy-dispersive X-ray spectrum of the region in A, characteristic of calcium phosphate.

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EMC Abstracts - https://emc-proceedings.com/abstract/interplay-of-organic-matrix-and-amorphous-calcium-phosphate-strengthens-the-isopod-claw/