MARTINEZ, G.; DRUCKER, E.G.; SUMMERS, A.P.; University of New Hampshire; University of California – Irvine; University of California – Irvine: Under pressure to swim fast

High speed swimming requires a stiff body to maximize thrust delivered along the main axis of the fish. Swiftly swimming bony fishes have few vertebrae (i.e. 22 in marlins), and have significant bony zygapophyses that span the intervertebral joints. The net result is a very stiff vertebral column which provides the requisite whole-body stiffness. Some sharks are remarkably fast swimmers, yet they have a large number (180 precaudal in the mako shark) of discoidal vertebrae that seem ill-suited to resisting flexion. We tested the hypothesis that sharks can dynamically change their body stiffness by pressurizing their thick, inextensible skin. We implanted pressure transducers in the epaxial musculature of spiny dogfish sharks and swam them in a flow tank at speeds from 0.25 to 1.75 body lengths per second. The myomeric pressure varied sinusoidally over the course of a tail beat cycle, from subambient to superambient. The average pressure increased with increasing speeds, and contralateral pressures were 180 degrees out of phase. Our choice of spiny dogfish as a study animal was dictated by availability and willingness to swim in a flume. They are not very fast swimming sharks and we have evidence that they can swim a good deal faster than we could run our flume (3+bl/s). We would expect that faster swimming sharks such as makos and great whites would be pressurize to a greater extent.