What makes dolphins and swordfish such fast swimmers?
Speedy dolphins Dolphins are known to be very fast swimmers with a low energy expenditure. In the past this has puzzled marine biologists because they believed that the muscle power of the dolphin was not sufficient to overcome the big ‘drag’ or resistance caused by the body moving through the water. The puzzle was even called the Gray’s paradox after the British Zoologist Sir James Gray.* Gray thought that the dolphins skin probably would have special anti-drag properties. In 2008, 75 years, later Timothy Wei with Frank Fish, a biologist at West Chester University in Pennsylvania and Terrie Williams, a marine biologist at the University of California, Santa Cruz solved the paradox using cutting edge technology **
A factor that was overseen by Gray in 1937 was that muscle power of the dolphin is largely used to undulate its body, more specific to produce powerful up and down sweeps of its tail ( ‘flukes’) causing hydrodynamic vertical forces and ‘drag’ turbulence behind the tail. So the great propulsive forces of the dolphin seemed to resulted mainly from movements of its tail. But up to recently there were no ways to measure directly the force exerted by a dolphin and the forward thrust of its body. The method Wei a.o. used method came from aerospace and is known as Digital Particle Image Velocimetry (DPIV). DPIV measures were based on a analysis of video recordings of micro air bubbles injected in a pool with swimming bottlenose dolphins (Tursiops truncatis). DPIV can capture up to 1,000 video frames per second. They found that the dolphin exerted approximately 200 lb of force every time it thrusts its tail – much more than Gray hypothesized – with at peak force between 300 to 400 lb. Without going into all the technical details: ‘dorsoventral oscillations of the flukes of the dolphin produced pairs of counter-rotating vortices during each propulsive cycle. One vortex was generated for each upstroke and each downstroke of the tail’( (see picture, upper panel).
Speedy swordfish. Swordfish (Xiphias gladius) belong to the Billfishes that also comprise Sailfish and Marlins (see picture, lower panel). Swordfish swim at even higher speeds and accelerations than dolphins, and alleged to reach speeds of 100 km ph. What makes swordfish such fast swimmers? Here the anti drag properties of its body, in particular the head seem to play an important role. Emeritus professor John Viderel from the Groningen University in the Netherlands already suspected that the rapier shaped head (‘bill’) of the swordfish might reduce the amount of drag pulling on the fish as it sweeps through the ocean***. Roughness at the tip of the bill could generate microturbulence in the water, to make it ‘thinner’ and reduce the drag, which improves performance. Later, he took two dead swordfish he got from a Corsican fisherman home, and placed them in a medical MRI scanner. He then discovered a network of vessels in a thin spot near the nose that were connected to an oil gland. Videler believes that the oil, in combination with microscopic rough projections on the skin, might produce a surface that is super water-repellent and could reduce the drag on the animal by over 20%. So according to Viderel the oil layer, in combination with the denticles, ‘creates a super-hydrophobic layer that reduces streamwise friction drag and increases swimming efficiency.’
*Bale et al. (2014) Gray's paradox: A fluid mechanical perspective" Nature: Scientific Reports 4: Article
**Fish, F. E., Legac, P., Williams, T. M. & Wei, T. Measurement of hydrodynamic force generation by swimming dolphins using bubble DPICV. J. Exp. Biol. 217, 252–260 (2014)
***Videler, J. J., Haydar, D.,Snoek,R.Hoving, H.-J. T. and Szabo, B. G. (2016). Lubricating the swordfish head. J. Exp. Biol. 219, 1953-1956.
***Lee, Hsing-Juin; Jong, Yow-Jeng; Change, Li-Min; and Wu, Wen-Lin (2009) "Propulsion Strategy Analysis of High-Speed Swordfish" Transactions of the Japan Society for Aeronautical and Space Sciences