“It came as more of a surprise than it should have when Wainwright, Vosburgh, and Hebrank (1978) showed that sharks utilized a hydroskeleton. Sharks, after all, have conventional skeletons, albeit somewhat less calcified ones than that of most other vertebrates. Shark skin is sturdier stuff than fish skins generally…and it has the crossed helical fiber array typical of these hydrostatic arrangements, shown in figure 20.5b. The system, though, is more complex than those described previously–muscles attach directly to the skin, which thus acts both as external, pressure-resisting membrane and as an external, whole-body tendon. Fiber angles, not unreasonably, vary with location on the fish–the unstretchy skin must transmit the forces generated by the body musculature back toward the tail. During locomotion, the pressure inside the body of a shark rises to as much as 200 kilopascals–twice atmospheric and as high as that inside an automobile tire. So sharks are just shark-shaped balloons with teeth.
“In sharks, the peculiar resistance of crossed helical fibers to torsion may have functional significance, at least of a negative kind. Sharks lack swimbladders and thus sink if inactive. Swimming must produce a little lift in addition to thrust, and a shark gets that lift, in part, by beating a tail fin that extends farther dorsally than ventrally, as you can see in the figure. That asymmetry might make a shark uselessly twist lengthwise, reversing twist twice during each full tailbeat–were it not for a torso that, while flexible in bending, resists twisting.” (Vogel 2003:415-416)