409 Dive Log Australasia December 24

As they mature, female Grey Nurse become much bulkier than males.

PIV technology is enabling us to rewrite the book on shark movement. Shark Skin is made of little denticles Shark skin is made from same material as shark’s teeth. The major difference is teeth are much larger and have added enamel. Shark skin is incredibly adaptable. Saw sharks use specialised shark skin to grow the protruding saw in front of their snouts. Shark skin is strong, almost like armour. Helical spirals made from fibres of collagen make really tough skin. It also helps to support muscles within by connecting muscles to the skeleton. Along with the denticles, this gives them enormous hydrodynamic advantages. Eddies are circular energy waves in the water that are formed as sharks propel themselves forward. These little eddies are caused by wasted energy from the forward propulsion of the shark. The shark skin denticles which are about 1/8 of a mm, the thickness of a piece of paper and actually reduce eddies, contributing positively to forward propulsion.

Shark skin has a dramatic effect on dampening pressure waves that sharks create when moving. At a microscopic level, tiny particles of water enter the cavities between the shark skin denticles creating a low pressure laminar water layer. Some of the energy that would otherwise form pressure waves is absorbed. This creates silent swimming; more energy is directed to forward thrust and less to water disturbance. In fact, PIV technology shows that the denticles create a low pressure zone called a ‘Leading Edge Vortex’ around the front. Imagine a low pressure area ‘sucking’ the shark forward in the water. That is real magic! These tiny denticles improve the flow of water over the shark’s body as laminar flow, increasing thrust and reducing friction with the water. Dr Lauder also found that ‘skin bending’ plays a large role in improving the beneficial effect of sharkskin. There is a physical property of the rigid skin that plays a big factor in hydrodynamics.

Notice the large Heterocercal tail fin and enormous Pectoral fins.

DIVE LOG Australasia #409 - December ‘24 89

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