As Swift As Swordfish
(September 8th, 2016) Back in 1969, young PhD student, John Videler, started to wonder how do fish swim. Throughout his career, he collected many pieces to the puzzle. The latest comes from the mighty swordfish.
Swordfish (Xiphias gladius) are the fastest fish in the sea. They are able to speed up to over 100 km/h because of their streamlined body, lateral keels and the crescent-shaped tail fin. But are these three adaptations all that is needed for high-speed swimming? No, thought John Videler and his team at the University of Groningen – there must be more. To fully understand fish swimming dynamics, the group successfully secured themselves a 1.6 m long swordfish from Corsica and put it into a magnetic resonance imaging (MRI) machine. Combining MRI with electron microscopy, they identifed and characterised a completely new and unique organ in the swordfish's head, the glandula oleofera. This oil-producing organ reduces friction drag and increases swimming efficiency.
The gland, located directly behind the onset of the sword, is connected via a specialised capillary network to tiny 0.1 mm diameter pores in the skin of the swordfish head. Videler stresses that “the capillaries have nothing to do with the sidelines found in many fish”. By using a simple hair dryer, the researchers mimicked the action of heat-producing cells in the swordfish eye muscles, hypothesising that this heat warms up the oil to keep it fluid. In addition, the concave shape of the head induces a subambient dynamic pressure around the head. “During swimming at high speed, this underpressure sucks oil out of the tiny pores, covering with a film of oil the front part of the head,” explains Videler.
The team also did some mass spectrometry to study the oil's chemical composition and found that it is composed of fatty acids called methyl esters, which generate a hydrophobic layer on the head's surface. Additionally, the tiny oil-excreting pores are surrounded by little bulges, called denticles, which, together with the hydrophobic oil film, create a super-hydrophobic environment that reduces drag.
“Only the last couple of years, I have tried to find out what the benefits [of studying how fish swim] for the society could be. Drag reduction techniques used by this extreme fish could be used in many cases where fluid flows along a solid surface, in order to save a lot of energy,” Videler comments on the practical applications of his research.
Image: Freshwater and Marine Image Bank - Frederick Whymper (1838-1901)