Abstract
Exposed regions of the arthropod exoskeleton have specialized structure and mineral composition. Their study can provide insights into the evolutionary optimization of the cuticle as a material. We determined the structural and compositional features of claws in the crustacean Ligia pallasii using X-ray micro-computed tomography, scanning electron microscopy (SEM), and analytical scanning transmission electron microscopy (STEM). In addition, we used nanoindentation to determine how these features fine-tune the mechanical properties of the claw cuticle. We found that the inner layer of the claw cuticle—the endocuticle—contains amorphous calcium phosphate, while the outer layer—the exocuticle—is not mineralized and contains elevated amounts of bromine. While the chitin–protein fibers in crustacean exoskeletons generally shift their orientation, they are aligned axially in the claws of L. pallasii. As a consequence, the claw cuticle has larger elastic modulus and hardness in the axial direction. We show that amorphous calcium phosphate mineralization and the brominated cuticle are widespread in isopod crustaceans inhabiting terrestrial habitats. We discuss how the features of the claw cuticle may aid in minimizing the likelihood of fracture. Ultimately, our study points out the features that increase the durability of thin skeletal elements.
Highlights
Exoskeletal elements that are specialized for pray-handling, mastication, and locomotion differ in structure and composition from the general exoskeleton and are optimized by the process of natural selection in response to the functions they perform [1,2,3]
The cuticle of mandibles and claws of arthropods may be mineralized with calcite, apatite, amorphous calcium phosphate, or silica [2,4,5], or may not be mineralized at all but instead incorporate halogens, manganese (Mn), or zinc (Zn), resulting in fine-tuned mechanical properties that likely contribute to preventing mechanical failure [3]
It has previously been demonstrated that amorphous calcium phosphate (ACP) is present in the claws of the terrestrial isopod P. scaber, which have a similar mineral composition and distribution of mineralized and non-mineralized cuticular layers [5]
Summary
The arthropod exoskeleton—their cuticle—consists of chitin fibers intimately linked with proteins [6,7]. In crustaceans, this organic matrix is generally mineralized, incorporating calcium carbonate in the form of calcite and amorphous calcium carbonate, which may be present as a mixture with calcium phosphate [8,9,10]. The two inner and generally mineralized layers are the exocuticle and the endocuticle These two layers are generally strongly mineralized and consist of chitin–protein fibers, arranged in layers parallel to the body surface [8]. The parallel fibers in sequential layers shift their orientation helicoidally, resulting in a plywood-like periodical pattern known as the Bouligand structure [14]
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