The beaks of jumbo squid are flexible near the body and stiff near the tip, as defined by varying degrees of water content in a composite of chitin nanofibrils infused with cross-linked proteins.

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Squid, like other cephalopods, have a hard, sharp beak for catching and devouring prey. While most other animals that have to bite and tear their food achieve the required level of hardness and strength through the incorporation of minerals, metal ions, or halogen atoms (fluorine, chlorine, bromine, or iodine) in their structure, the squid's beak lacks any of these features. Instead, it's a composite of reinforcing chitin nanofibrils infused with proteins containing catechol functional groups that form many strong cross linkages that act like cement. Given that squids live underwater, the process starts out in an aqueous environment. However, during the cross linking/hardening process, water is continuously expelled. The degree of water remaining in different areas of the composite material is thought to contribute to the gradient of flexibility along the length of the beak – flexible where it is attached to the body and increasingly stiff towards the far end. In other words, water content imparts flexibility, so a high water content is found at the flexible base of the beak where it attaches to the body. The water content gradually decreases further from the body – the tip of the beak is strong, stiff, and dehydrated. Layering keeps cracks from propagating.

Video of Jumbo squid eating prey:
ARKive video - Humboldt squid fed by diver Edit Summary

References

"Cephalopods such as squids, cuttlefish, and octopods are equipped with a hard beak that is as sharp as a knife and crucial for disabling prey and feeding. Beak chemistry has attracted much recent attention in materials science: in contrast to mammalian hard tissues, the beak is devoid of minerals, consisting instead of a composite of proteins and chitin fibres with varying degrees of hydration along the beak structure. In animals, this is a unique material design for hard tissues that function in biting...As the beak lacks any of the known strengthening entities previously associated with wear-resistant tissues such as biomineralization, metal ion cross-linking, or protein halogenation, it begs a question, namely, what sort of molecular processing can impart such impressive physical properties? A deeper understanding of the mechanisms by which beaks are sclerotized is also likely to reveal novel chemical paradigms for the fabrication of robust and biocompatible composites for a variety of restorative applications. Furthermore, synthesis of such polymer-based composite materials could inspire environmentally friendly routes as Dosidicus beak is formed under ambient seawater conditions and is wholly non-toxic." (Miserez et al. 2010:38115)

"Jumbo squid beaks (and the beaks of other cephalopods such as octopus and cuttlefish) represent an intriguing paradigm of composite materials processing for several reasons. Beaks are produced in a wet environment, grow continuously during the lifespan of the animal, and are fabricated with macroscopic biochemical gradients resulting in a wide range of mechanical properties, namely a flexible proximal region attached to the buccal mass and a hard, stiff rostrum at the distal end. Previous investigations established the presence of His-dopa adducts in squid beak and proposed them to be stabilizing cross-links of the composite proteinaceous matrix...Bioprocessing of the chitin/water/protein composite shares parallels with the impregnation processes by which many thermoset polymer matrix composites are fabricated. In this scenario, chitin nanofibrils are assumed to form the initial template, similar to glass fibers or carbon fiber mats in composite processing. Protein “fillers” and catechols are then secreted through the chitinous preform. Once oxidation of catechols moieties has been triggered (either by autoxidation or enzymatic processes), cross-linking and hardening (sclerotization) ensue (see Fig. 5d). In the chitin/protein biocomposite, however, there is a critical additional variable, water. As sclerotization proceeds, water is indeed progressively expelled from the material. This precise role of water removal on the structural properties of the beak is especially critical and not yet fully understood, in part because the effect of water on the individual components of the composite is poorly understood." (Miserez et al. 2010:38120-22)

"Covalent cross-linking is undoubtedly important in the stiffening and hardening of beak. It is, however, only one of many strategies, both molecular and microstructural, in Nature for tuning the overall mechanical properties of macromolecules. Water access to chitin nanofibers encased by a highly cross-linked supramolecular network saturated with hydrophobic and H-bonding catecholic rings is highly restricted. From a thermodynamic perspective, cross-linking may serve to increase the glass transition temperature of the biocomposite. As in other structural tissues such as silk, understanding the complexities of protein solvation may be key to designing chitin/protein composites with physical properties that are tailored on a macroscopic scale...Indeed, squid beak is an excellent model system for the bioinspired fabrication of environmentally sustainable load-bearing materials." (Miserez et al. 2010:38123)

Journal article
Cross-linking Chemistry of Squid BeakJournal of Biological ChemistrySeptember 25, 2010
A. Miserez, D. Rubin, J. H. Waite

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Living System/s

Organism
Humboldt SquidDosidicus gigasSpecies

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