Because we are bony, we tend to dismiss soft, squishy cartilage as an inferior imitation. Many fishes, notably sharks and rays, have skeletons made almost entirely of cartilage, and they manage very well. They demonstrate a pedigree of success going back hundreds of millions of years, but human prejudice looks at the antiquity, rather than the success of cartilage, and thinks it primitive, compared with bone.

The cownose ray (Aetobatus narimari). Image © NOAA.

Now, Adam Summers of the University of California, Berkeley, speaks up for cartilage in the Journal of Morphology, by showing how the cartilaginous jaws of some stingrays have been modified into effective nutcrackers able to crush the hard shells of clams.

These stingrays are members of the family Myliobatidae, which includes several species of hard-jawed clam-crusher, including the spotted eagle ray (Aetobatus narimari) and the cownose ray (Rhinoptera bonasus), as well as a few plankton-feeders such as the giant manta ray (Manta).

Before he gets into stingrays, Summers lays to rest some old myths about the alleged squishyness of cartilage. Among the cartilaginous fishes, he says, are animals capable of swimming at 60 kilometres per hour, and giants weighing more than 10 tonnes. Though cartilage is neither as strong nor as stiff as bone, it can perform in physically demanding conditions. "But how," Summers asks, "can cartilaginous jaws be used to crush prey that is harder than the jaws themselves?"

The answer lies in another myth about cartilage. On its own, cartilage is about as stiff and strong as Turkish Delight (a gelatin-based confection). But cartilaginous fishes are all built of a stiffer, somewhat mineralized version, called 'calcified' cartilage. Each of their skeletal elements is composed of a thin peel of fibrous cartilage surrounding a rind of 'prismatic' cartilage, made of mineralized tile-like sections or 'tesserae'. This sheaths a softer core of so-called 'hyaline' cartilage.

The jaws of clam-crushing stingrays are no exception. And Summers shows how stingrays have introduced some design innovations that help them squash seashells. First, the tesserae in their jaw cartilages -- which support their crushing, pavement-like teeth -- may be several layers thick. This is analogous to the thickening seen in bones that bear large loads.

Second, the soft, cartilage cores of the stingrays' jaw elements are braced by thin, hollow mineralized struts, like the bracing struts in an airframe, or the struts inside the hollow bones of birds. This design combines lightness with strength. Stingrays are born with these bracing struts in place: they do not, it seems, develop them later in life as a response to stress. Summers demonstrates this with a specimen of an unborn cownose ray (Rhinoptera bonasus) bearing a complete set of bracing struts, even though the animal had never taken a single bite.

Even plankton-feeders such as the manta have jaws braced by internal struts, suggesting that they evolved from a clam-crushing ancestor. However, it could be that the metre-wide jaws of the manta could use a little internal bracing to prevent them buckling as the fish swims, mouth agape, through the water.

The loose jaws of the southern stingray (Dasyatis sabina) -- a stingray, but not a myliobatid -- allow great freedom of movement. In contrast, the ligament-bound jaws of clam-crushing myliobatid stingrays have only a very restricted range of movement. The upper and lower jaws are, basically, horizontal bars that can move only a short distance up and down. They cannot move from side to side to chew, nor can they rotate.

But these jaws can work asymmetrically. When a stingray holds a clam on one side of its jaw, the muscles on the opposite side operate the jaws as a second-order lever, using the clam side of the jaw as the fulcum and delivering a magnified crushing blow. Just like an old-fashioned nutcracker.