“Crushing clams with cartilaginous jaws is like trying to fell a tree with a sock full of custard.”
That was a bit of an exaggeration on Mr. Gee’s part, but the point still stands – how do certain rays (shark relatives with skeletons made of cartilage instead of bone) manage to crack the shells of shellfish in order to eat them? This cartilage is the same soft material that your ear and the end of your nose are made from.
I wrote a few weeks ago about how bone grows and what it looks like at a microscopic scale, but I didn’t talk about how it is formed in the first place.
In many cases bones replace cartilage which is laid down as a template. First, the cartilage itself has mineral crystals added to it, then this mineralised cartilage is broken down and replaced by true bone. In the cartilaginous fishes, which includes rays, sharks and skates, the mineralised cartilage is what forms the hardest parts of the skeleton. It is never replaced by true bone, which means that their skeletons are not as hard or rigid as ours.
To make up for this, rays’ skeletons, and especially their jaws, undergo thickening of the mineralised part of the cartilage to mimic cortical bone and even have an equivalent to the spongy bone found in true bones.
In the rays, cortical thickening is achieved by producing many layers of mineral crystals just inside the edge of the bone. These mineral crystals are also used to build the struts which look like spongy bone. Whereas in true bone each strut is solid, the cartilaginous fishes create hollow struts, which is a very efficient way of creating strong structures with minimal effort and material (think of a hollow scaffolding pole). But even this doesn’t approach the hardness that true bone achieves.
The real magic for breaking into shellfish happens thanks to the way their jaws are put together. In most rays the upper and lower jaw cartilages are split into two pieces each, whereas the rays which specialise in shellfish have the two top cartilages and the two bottom cartilages fused and highly mineralised, leaving them with something more similar to our mouth – one top part and one bottom part.
The top and bottom cartilages are joined by two very tough ligaments which means the ray can’t open its mouth very far, but also means the jaw muscles can make the jaws act as very effective levers.
The ray positions shellfish in roughly in the middle of its mouth and clenches the jaw muscles on the side that is furthest from the shellfish to get the greatest lever action. The tough ligaments keep the other ends of the jaw cartilages in place, like the hinge of a kitchen nutcracker.
In this way, the force exerted on the shellfish is generally 2-4 times the amount generated by the muscle.
And if that’s helped you to cut down a tree using a sock full of custard, you’re a smarter person than I am.
 Summers, A P (2000). Stiffening the Stingray Skeleton — An Investigation of Durophagy in Myliobatid Stingrays (Chondrichthyes, Batoidea, Myliobatidae). Journal of Morphology. Vol. 243, pp. 113-126.
Wikimedia Commons, John Norton. Available from http://en.wikipedia.org/wiki/File:Spotted_Eagle_Ray_%28Aetobatus_narinari%292.jpg. Accessed: 01/11/13, 16.48pm.