The swordfish is famed for its speed, and its rapier-like bill is just one adaptation that allows the fish to slice through the water at an extraordinary pace. The sandpaper-like texture and an array of small holes near the tip of the bill help to reduce turbulence.
But another recently discovered adaptation is a fist-sized oil-producing gland in its skull, allowing the fish to lubricate itself and reduce drag even more.
John Videler from Leiden & Groningen University and student Roelant Snoek made the discovery while searching for the purpose of the strange gland.
Ed Yong reports on National Geographic:
While taking photographs of a swordfish head, he accidentally dropped a lightbulb onto it. The bulb illuminated a web of tiny blood vessels inside its skin, and Snoek showed that these were connected to the gland. The vessels then open out into the fish’s skin via tiny pores, each just a fraction of a millimetre wide. Snoek proved this by heating the gland with a hair-dryer; once hot, the congealed oil became liquid and oozed out the fish’s pores. So Videler thinks that the gland is yet another drag-reducing adaptation. Its oil repels water and allows incoming currents to flow smoothly over the surface of the bill. That depends on the oil staying warm, but swordfish have a solution for that, too. They have modified some of their eye muscles into heat-producing organs that warm their blood and sharpen their vision as they hunt. This same heating effect could liquefy the drag-reducing oil, allowing it to ooze out of the glands just as the fish have the greatest need for speed.
While more research is needed to understand exactly how the whole system works, Leiden and Snoek have uncovered yet another extraordinary adaptation that allows the swordfish to compete for the title of world's fastest swimmer.