409 Dive Log Australasia December 24

Epaulette sharks. Note the enlarged pectoral and ventral fins that allow them to walk.

9 By contrast, wobbegongs swim by undulating all of the body. The dramatically different swimming style depends on a significantly different way that muscles and skeleton interact. This is an area of research that scientists are just beginning to look into. I did say that we are rewriting the text book on sharks! Gliding pectoral fins. Uplift in sharks New research has found that some accepted assumptions are not true. Shark reference texts refer to lift being created by buoyant livers, the Heterocercal tail and the thrust through the body and the gliding of pectoral fins. The cross section of the wing of a plane is shaped to create a pressure difference. The top of the wing creates a

low pressure zone and the lower wing surface creates a higher pressure. The net result is an upward vertical force, called lift. The cross section shape of a shark fin does not create a low pressure area above the fin as a plane’s wing does. Vertical lift in sharks is more voluntary. They change the angle of the pectoral fin deliberately when they want to climb. In fact, the pectoral fins provide no lift at all when the shark is cruising horizontally as previously thought. We know almost nothing of how muscles interact with tendons and ligaments in most sharks. Their cartilaginous skeletons means that the way muscles and skeletons function and interact is not the same as for mammals. Scientists are beginning to place electrodes in sharks’ muscles to see which ones are activated and when.

Grey Reef shark elevates its head, thrusts through its tail fin and glides upwards using its pectoral fins in order to climb higher into the water column.

DIVE LOG Australasia #409 - December ‘24 91

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