Many shark species, including the great white and mako, swim at speeds greater than their morphology should allow. Dr. Adam Summers (University of California) has discovered they achieve this speed by changing the stiffness of their body by pressurising their thick, inflexible skin. “This study could help answer questions about the evolution of the shark, especially why sharks abandoned a bony skeleton in favour of a cartilage structure,” said Dr. Summers who will be presenting his findings on Friday 4 April (Session A7.33).

The latest research reveals that shark species overcome flexibility by pressurising their outer skin. Dr. Summers took pressure readings, using pressure transducers implanted in side muscles, in the spiny dogfish shark as it swam at varying speeds in a flow chamber. “The results are the first pressure data gathered from free swimming fish” said Dr. Summers. The experiment revealed that as swimming speed increased, the average pressure in the skin of the shark also increased.

High-speed swimming is normally dependent on a stiff body, able to produce the greatest thrust through water. Stiffness requires a backbone with relatively few vertebrae. For example, as Dr. Summers states, “Marlin have very few vertebrae, tied tightly together. This provides their spine with an almost broom like stiffness, enabling them to generate great thrust.” Yet sharks, including many that reach impressive swimming speeds, have a large number of vertebrae (a 180, in the case of the mako shark). Large numbers of vertebrae means low stiffness, and low thrust.

Spiny dogfish sharks are not an example of particularly fast swimming sharks. They were selected for their availability and the practicalities of flow tank testing. Dr Summers hopes to move the study onto shark species with faster swimming speeds, for example the mako shark, where pressurisation is expected to be much greater. However, as Dr. Summer mentions, “This would involve measurements taken in the wild, which has difficulties. We are currently working on techniques that can overcome the considerable problems associated with such a study.”