Friday, March 16, 2012

Sharks are good swimmers

It's hardly surprising that sharks are good swimmers, given their more than 400 million year evolutionary history in the ocean. But, it's the way that they achieve their swimming prowess that is surprising. Two recently published papers show how sharks achieve more thrust than their muscle movements alone would suggest that they should. One involves their tail and the other involves their skin.

Vortices generated by the water jets from a swimming shark's tail (image Brooke E. Flammang)
The main function of a sharks tail is, obviously, to provide thrust and drive the animal forward through the water. As the tail moves to and fro it creates jets of water that travel backwards, propelling the shark forwards. For most fish, just one jet is created at the end of the swing. The sharks in the first study, however, generated two by stiffening and slightly changing the shape of the tail mid-swing. So, the sharks generated thrust at the extent of its tail-stroke and at the midpoint.

The shape of a shark's tail tells you something about how it swims.
The study looked at two species of dogfish, which have similar tail shapes. Many sharks, like the dogfish, have 'heterocercal' tails. That is, the upper lobe of the tail is larger than the lower lobe. An extreme example of this tail shape is the thresher shark. Other sharks, like the great white, have more 'homocercal' tails, where both upper and lower lobes are roughly equal in size. It would be interesting to know whether all sharks use the same 'tail-stiffening' strategy as the dogfish, or whether different tail shapes require different strategies.

The second study looked at the skin of the sharks. Sharks have placoid scales, which are also known as dermal denticles ("skin teeth"). Dermal denticle is often the preferred term because they are structurally similar to vertebrate teeth and likely have the same evolutionary origin. Dermal denticles have a flat plug inside the skin, passes through the skin as a narrow neck, then flattens into a more elaborate crown-like form. In most sharks the points of the crown are directed backwards, so the skin feels smooth running a finger from head to tail, but sandpaper-rough running a finger from tail to head.

Scanning electron microscope image of the dermal denticles of a bonnethead shark, Sphyrna tiburo. The green scale bar indicate 50 um (photo Oeffner & Lauder).
The orientation of the dermal denticles was long thought to decrease the drag on a shark as it swam. But, the authors noted that this hadn't been properly tested. They attached shark skin to a rigid metal plate and moved it through water to simulate a swimming shark. Then they sanded back the dermal denticles and repeated the test. Curiously, the skin performed better without the dermal denticles.

The authors decided that the rigid metal plate did not adequately mimic the body of a shark, which flexes as the shark swims. So, they removed the metal plate and repeated the tests of the skin, with and without denticles. This time the skin with dermal denticles performed better then than sanded skin. Further analysis showed that the reason for this is that as the denticles move on the flexing skin they create a leading-edge vortex that sucks the shark forward. The denticles, therefore, not only reduce drag, but increase thrust.

The articles:
Flammang, B. E., Lauder, G. V., Troolin D. R.,  and Strand, T. (2011) Volumetric imaging of shark tail hydrodynamics reveals a three-dimensional dual-ring vortex wake structure. Proceedings of the Royal Society B: Biological Sciences 278 (1725), 3670-3678

Oeffner, J. and Lauder, G. V. (2012) The hydrodynamic function of shark skin and two biomimetic applications. The Journal of Experimental Biology 215 (5), 785-795.

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