At the core of the shark's hypnotic swimming motion, there is an elastic secret. Marianne Porter, from Florida Atlantic University, USA, explains that the fish's cartilaginous skeleton allows energy to be stored in the vertebrae as the vertebral column bends, compressing the bulky centra structures that stack together to build the spinal column. This energy is then released at the end of a tailbeat to power them on their way. According to Porter, the elastic vertebral column could store and release as much as 10% of the shark's energy, providing a substantial advantage over the rigid skeletons of bony fishes. However, Porter and colleagues Randy Ewoldt and John Long suspected that shark spinal columns were more than simple springs because of their complex material properties; ‘[the vertebral column] is a composite of different materials’, they say. So they decided to investigate how sections of the vertebral column (comprising 9 or 10 vertebrae) from spiny dogfish (Squalus acanthias) exert force as the team systematically wiggled them to and fro over realistic swimming curvatures and tailbeat frequencies ranging from one tailbeat every 4 s to two tailbeats per second.
Measuring the displacement of the section of spinal column as it flexed and the force exerted at the end, the trio was then able to calculate how the stiffness of the vertebral column (energy storage) and energy dissipation (braking power) varied. ‘Based on its mechanical behaviour, the vertebral column may serve as both a spring and a brake’, say Porter and colleagues. They suspect that the shark's remarkably elastic spinal column may become more springy to provide more propulsion as the animal increases speed, while increasing braking power as the shark continues beating the tail over a wide amplitude but at a slower rate when slowing down. And the team suggests that the vertebral column's unique combination of material properties provides ‘continuous variable power transmission’ to smoothly power sharks on their way.