Fast-Swimming Swordfish Automatically Lubricate Themselves

Swordfish steaks frequently appear on menus and dinner plates around the world. But even though many people have hooked, hacked apart, and devoured these majestic fish, few truly understand their bodies. Indeed, until John Videler from Leiden & Groningen University started studying swordfish, no one knew that they had a fist-sized gland in their heads, which slathers lubricating oil over their famous pointed snouts.

Videler has been studying the physics of swimming fish for most of his career, and swordfish were particularly intriguing to him because they’re such superlative swimmers. It’s commonly said that they can reach speeds of 100 kilometres per hour (62 miles per hour), and although the provenance of that estimate is dubious, there’s little doubt that they are really, really fast. So in 1994, while teaching a diving course in Corsica, he bought a swordfish bill from a local fisherman and started studying it.

When a swordfish swims, layers of water flow along the surface of its bill. As it picks up speed, these currents threaten to break away, creating swirling areas of turbulence that increase the drag upon the animal.

But Videler found that the bill is rough, like sandpaper. This limits any turbulence to a thin layer close to the bill, and prevents the larger, destabilising eddies from forming. The bill is also pitted with small, interconnected holes near its tip, which stop water pressure from building up at the fish’s front end—again, this reduces drag by preventing turbulence.

By then, Videler was hooked. He got two more swordfish from the same fisherman, and persuaded Ben Szabo—the head of radiology at Groningen University—to put them in a medical MRI scanner. The team scanned the fish heads between 2 a.m. and 5 a.m., when the machine was available.

At first, the images were confusing and hard to interpret. But when Videler dissected the heads themselves, he noticed a large oily gland above the base of the bill and between the animal’s eyes. And sure enough, there it was on the scans.

He thought nothing of it until 2005, when a student named Roelant Snoek came to him with an interest in swordfish. Videler told him about the gland, and suggested that it might connect to the fish’s olfactory system, influencing its sense of smell. But Snoek couldn’t find any such connections.

After much frustration, he finally worked out the gland’s true purpose by accident. 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.

The oil might explain another weird feature of swordfish anatomy. They are among the only fish with a concave hollow at the front of their heads—an slight inward-curving bowl that, counter-intuitively, ought to increase drag. “I’ve been puzzling about that for years,” says Videler. He now thinks that the hollow is shaped so that water flowing past it creates an area of low pressure, which sucks the oil out of the fish’s gland.

If he’s right, it means that a fast-swimming swordfish automatically lubricates itself.

This makes a lot of sense, but it’s still a hypothesis. “We still have to find some way of doing experiments to visualise the flow of water [over the bill],” Videler admits. “We can’t do that on live swordfish,” since these animals are impossible to keep in captivity. But he hopes that other scientists could run fake swordfish—sandpaper skin, pores, oil, and all—in water tunnels to see how they perform.