Sharks differ from most other fish in that their skeleton is made of cartilage instead of bone. By examining whole body curvature of the spiny dogfish (Squalus acanthias), and strain placed on the vertebrae of sharks during swimming, a team of researchers have discovered that the entire vertebral column bends during locomotion. This indicates that the spine is behaving like a spring and providing power to the sharks when swimming.
The morphology of the spine of cartilaginous fish appears to allow for spring loading. The vertebral column of S. acanthias displayed an open, intracentral canal, covered by a velum layer which effectively strengthens the vertebral column and could facilitate elastic recoil of the entire spine. This recoil in the cartilaginous vertebral column of sharks assists undulatory swimming by providing springback power to the motion of the tail. A shark requires 6% of its metabolic power to flex the tail and load the spring, but this investment is a relatively inexpensive cost for the recoil power generated.
The results indicate that the bodies of sharks have been positively selected for speed, which is in keeping with the lifestyle of many large, metabolic sharks.
This method of swimming differs from bony fishes, whose spines consist of hardened vertebrae and flexible joints, and may provide a clue for one reason why sharks diverged from the ancestral state of having a bony skeleton.
M.E. Porter, Candido Diaz Jr., Joshua J. Sturm, Sindre Grotmol, A.P. Summers, John H. Long Jr., Built for speed: strain in the cartilaginous vertebral columns of sharks, Zoology, Volume 117, Issue 1, February 2014, Pages 19-27, ISSN 0944-2006, http://dx.doi.org/10.1016/j.zool.2013.10.007
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