Swordfish Lube up Their Heads for High-Speed Swimming

July 7, 2016 | Erica Tennenhouse

Swordfish, Xiphias gladius
Photo credit: Citron/Wikipedia ((CC BY-SA 3.0)

One of the fastest swimmers on the planet secretes oil to stay slick.

Swordfish, known for their iconic long bills resembling a sword, are reputedly one of the fastest swimmers on Earth.

Reporting in the Journal of Experimental Biology, researchers have discovered that their incredible speeds (they can allegedly reach up to 100 km/h) are enhanced by a large oil gland situated at the base of the bill that lubricates the fish's head to reduce drag.

Lead author John Videler, from Groningen University in the Netherlands, first acquired swordfish specimens in 1995 when he was teaching a diving course for biologists in Corsica. He discovered that the roughness at the tip of the bill generates local microturbulence, reducing the amount of drag pulling on the fish as it pushes through the water, and improving performance.

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20 years later, Videler returned to the MRI scans he had taken on the swordfish and noticed something astonishing. "I saw this gland,” he said in a press release. "It was so big there was hardly any room for bony structure and the bone around it was very thin". however, the purpose of the gland wasn't initially clear.

It wasn’t until his colleague Roelant Snoek accidentally dropped a light bulb on a specimen’s skin that the function of the gland became apparent. “All of a sudden he saw this network of vessels that were connected to the oil gland,” said Videler.

By further examining the surface of the skin with a powerful microscope, they discovered that the head was covered with tiny holes that were connected to the gland by the capillaries. Oil seeping out of these holes, along with the microscopic rough spots on the skin, might produce a water-repellent surface that could reduce the drag on the fish by over 20 percent, according to the researchers.

Testing the theory poses a challenge, as swordfish cannot be kept in captivity. Videler hopes that physicists will take on the next phase of this research by testing simulated swordfish skin to find out exactly how slick a swordfish head can get.

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