Although “mollusk” comes from the Latin word for “soft,” squid beaks are so hard they can crack a fish’s spine with one bite. Yet somehow, a squid’s soft-tissue mouth can clench the beak without cutting itself—something akin to holding the naked blade of an X-Acto knife with a block of Jell-O, says biochemist Herbert Waite of the University of California, Santa Barbara.
The secret, Waite and his collaborators write in the March 28 Science, is that squid beaks are not uniformly hard. Instead, they transition smoothly from a relatively soft base—soft enough not to damage the muscle they rest on—to a tip that’s about a hundred times harder.
Counterintuitively, the researchers also found that chitin, a complex carbohydrate that makes insect exoskeletons hard, is not more abundant at the tip of the squid’s beak than at the base. But it’s not just chitin that matters. Another molecule, a protein, is actually more abundant at the tip. The protein “micromanages” how water molecules wrap around chitin, as Waite puts it. More of the protein squeezes the water out. “You pack the bigger molecules more tightly,” and those molecules can link more robustly, leading to a harder tissue, Waite says.
Similar transitions exist elsewhere in nature where soft and hard tissues meet, says Waite, such as between our teeth’s enamel and dentin layers (enamel is a lot harder) or on the inside of an insect’s exoskeleton.
Bioengineers could take inspiration from squid beaks, Waite says. For example, prosthetics such as hip replacements are a lot harder than the surrounding bone tissue, and a smoother transition could help prevent long-term wear and tear.