Its All About the Tesserae

Student: Hey Sarah, if sharks’ jaws are made of cartilage, and that’s relatively flexible like you said our noses are, how can they bite into anything? Their teeth are sharp but how are they supported?

Me:
Well.. I’m honestly not sure.

High school kids ask the best questions. If sharks are catilaginous fish, and cartilage in the human body is flexible and almost squishy – used to pad joints and form noses and ears – then what physiological adaption gives shark bodies the stiffness required to propel them forward in the water? How do their jaws support the forces created when they bite down into prey, if there is no true bone produced in the body?

Hmmm.

Tesserae, my friends. Its all about the tesserae. I honestly did not have this answer for my inquiring friend earlier today, but I’m glad I have it in my back pocket now. Tesserae are tiny hexagonal calcium salt crystals inlaid across the jaws and some of the backbone cartilage supports in many shark species. They are the bracing points that stiffen out the cartilage and give it the ability to support massive forces exerted by the muscles in the jaw when attacking prey.

Even more intriguing: larger sharks of the Bull, Great White, and Tiger species, tend to have multiple layers of tesserae fortifying their jaws. Elasmo-research.org reports back from a 1991 paper (Gingerkus et al) that two standout white sharks in particular – 16 and 18 ft animals – had five layers of tesserae to stiffen up their jaws.

Elasmo-research.org is an exceptional website for all things shark, stingray and skate. If you ever have conundrums concerning these animals definitely consider visiting it in the future. I sure will.

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