A few months back, we started a series on crystallography in invertebrates, in celebration of 2014 as the International Year of Crystallography. (In case you missed it: The Gastropod Radula; Diatoms.)
In this post, we focus on the cephalopod beak (the "rostrum"), which is the hard, tough feeding structure used by squids, octopuses, and nautiluses. The beak structure works like a pair of scissors, where the opposing sides are embedded in extremely strong muscle tissue, which forms the buccal mass.
The beak is made of chitin and highly cross-linked proteins. While chitin isn't a mineral, it does form in a crystalline structure (i.e., high degree of molecular ordering). The beak itself is composed of a tissue gradient, in terms of both composition and mechanical properties: the strongest and hardest part of the beak is its tip, where the beak contains the largest fraction of highly ordered chitin molecules.
From the tip to the muscle attachments, the beak becomes progressively "softer" and more compliant. This gradient helps the beak accommodate large stresses when biting into prey, and provides a slow transition into the soft tissue of the cephalopod's body. Remember, cephalopods are invertebrates, which means they don't have jaw bones - amazingly, their beaks are supported by squishy tissues and dense muscles.
Unconvinced that the cephalopod beak is an extraordinary example of a crystalline material in an invert? Check out these amazing 3D images of a nautilus beak (Nautilus pompilius), from the Tree of Life web project. Attention: you'll need a pair of red-cyan 3D glasses for proper viewing!
by Liz Boatman
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