In many vertebrates cartilage supplements a bony skeleton, as its squishy nature absorbs stresses and strains better than bone, which provides the strength and rigidity. Sharks, however, have skeletons made of cartilage. This provides great flexibility but may not be sufficient to resist the forces generated by sudden changes in movement which heavily compress the backbone. Instead, strength and rigidity comes from minerals deposited in distinct ordered patterns around and within the cartilaginous vertebrae. So far, each shark species studied has its own unique pattern: `You can measure the age of sharks from the rings of mineral as it's laid down – like aging a tree using tree rings,' reveals Marianne Porter. Having previously compared the details of mineral deposits in vertebrae across shark species, Porter focused on cartilage properties in the smooth-hound shark in her latest study(p. 3319).

Working with Adam Summers at the University of California, Irvine, and Thomas Koob of University of South Florida, Porter extracted vertebrae from these small sharks. They soaked a number of vertebrae in the chemical EDTA to demineralize them, so that they could calculate the mineral content. Porter determined that smooth-hound shark vertebrae contain 49.5% mineral on average,which is comparable to other species previously studied. But to her surprise she found that this content varied significantly within smooth-hound sharks,whereas normally there is little variation within a particular shark species. Smooth-hound sharks have a short life span of 10 years and mature quickly, so Porter suggests that the maximal rate of mineral deposition may not have yet been reached in her specimens, resulting in this variation.

To find out how mineral strengthens vertebrae, Porter subjected both mineralized and demineralized vertebrae to compressive forces between two flat plates. From the results the team calculated material properties of stiffness,or resistance to compression, and strength, the maximum stress before breaking. Porter found that, as with other sharks, more mineral correlated with stronger and stiffer vertebrae.

At the same time, Porter took the opportunity to examine how cartilage responds to crushing forces applied at different rates. Cartilage is a viscoelastic material; because it contains fluid it responds differently depending on the rate at which it is crushed. Bone, on the other hand, behaves as an elastic solid, reacting the same way whether it is crushed slowly or quickly. The rate of compression had little effect on smooth-hound vertebrae meaning that at the biologically relevant forces Porter used, shark vertebrae cartilage in fact behaves like a bony elastic solid.

However mineral content is not the only story. `There seems to be a premium of how much the mineral will do,' explains Porter. By combining her results with those from other species, she showed that a set increase in mineral content didn't account for all the increases seen in stiffness and strength,and that the greater part of this increase had to come from the mineralization pattern. Mineral infiltrates all shark vertebrae like a web, but there are distinct additional mineral deposits around the vertebrae that are unique to each species, possibly because their musculoskeletal systems are arranged differently. While the details remain hidden, these mineral deposits essentially prevent the shark backbone from collapsing under the forces generated by the muscles as the sharks swim.

Porter, M. E., Koob, T. J. and Summers, A. P.(
). The contribution of mineral to the material properties of vertebral cartilage from the smooth-hound shark Mustelus californicus.
J. Exp. Biol.