The composition of the exoskeleton of two crustacea: The American lobster Homarus americanus and the edible crab Cancer pagurus
Introduction
Crustaceans constitute a widespread class of organisms of both marine and land-dwelling species. They possess an exoskeleton [1] that stabilizes the whole body of the animal and also serves for protection against predators. In the case of lobsters and crabs, the claws are part of the exoskeleton optimized to serve as a cutting tool. The exoskeleton of crustaceans [2] is a biomineralized [3], [4], [5], [6] structure which consists of an organic matrix (in this case α-chitin) together with an inorganic mineral, in this case mostly calcium carbonate [7]. This composite has a distinct microstructure which gives it an optimal performance with respect to mechanical strength (for protection) and flexibility (for movement). Raabe et al. have analysed this microstructure and found a strongly hierarchical twisted plywood structure [8], [9], [10] and pronounced crystallographic textures (preferred orientation distributions of calcite and chitin [11]. This paper reports the chemical composition of these exoskeletons, following our study of the exoskeleton (cuticle) of land-dwelling woodlice [12], [13].
Section snippets
Materials and methods
The American lobster Homarus americanus and the edible crab Cancer pagurus are in the class Malacostraca (higher crabs). The two animals are shown in Fig. 1. The claw, the finger and the carapace (Fig. 2) of the animals were investigated.
The composition of the shell parts was determined by thermogravimetry. As shown earlier with bone [14] and cuticles of woodlice [13] it is possible to separate the components water, organic matrix and calcium carbonate because the corresponding weight losses
Results and discussion
All thermogravimetric results are summarized in Table 1.
X-ray powder diffraction revealed the crystalline components of the exocuticle (Fig. 4). This was mainly calcite with broad reflections, indicating a nanocrystalline structure. Application of the Scherrer equation gave an estimate of the size of the crystalline domains [14], [15], [16]. The reflections (1 1 0) at 36.2° 2Θ and (2 0 2) at 43.4° 2Θ, were analysed with a form factor of K = 1. The typical full-width at half maximum was 0.4–0.5° 2Θ.
Conclusions
In both species, the mineral content increased from the carapace to the claw to the finger. This is explained by the different requirements for hardness. The finger is the movable part of the cutting device of the animal and must therefore be very hard. The claw is the fixed counterpart which must be more elastic, i.e. less mineralized. The carapace is the shell of the main body, i.e. it should be even more elastic to allow the movement and some bending of the animal. For comparable parts, the
Acknowledgement
We thank the Deutsche Forschungsgemeinschaft (DFG) for generous funding within the Priority Programme “Principles of Biomineralization” (to M.E.).
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