Photograph by Norbert Wu, Minden Pictures/Nat Geo Image Collection
Read Caption

Swordfish, like this individual off Sardinia, like to eat squid.

Photograph by Norbert Wu, Minden Pictures/Nat Geo Image Collection

Natural Lube Powers One of World's Fastest Fish

A newfound oil-producing organ may help accelerate swordfish as they prowl the seas, a new study says.

With a hydrodynamic rapier for a nose and over 1,000 pounds (454 kilograms) of fin-pumping muscle, the swordfish can reach speeds of over 60 miles per hour (97 kilometers per hour)—making it one of the fastest fish on Earth.

Now scientists say they’ve found a new, never-before-seen organ that may be responsible for some of that wave-cutting speed.

According to new experiments published today in the Journal of Experimental Biology, the swordfish has an oil-producing gland at the base of its bill, or sword. As the animal swims, this gland pumps a cocktail of fatty acids to its skin through a network of tiny capillaries and pores. (See "Why a Swordfish’s Sword Doesn’t Break.")

The scientists believe the oil creates a water-repelling layer across the front of the swordfish’s head, thereby reducing drag and allowing the predator to slip through the sea more easily.

It's amazing that such a large sport fish consumed worldwide has managed to hide such speed-enhancing gadgetry for so long, says study leader John Videler, a retired marine zoologist at Groningen University in the Netherlands.

Slippery Secret

It all started when Videler read a recent study investigating the strength of the swordfish's bill. "They found a weak spot and I realized that I knew what was causing it," he says.

As every cartoon character that’s ever stepped on a banana peel knows, lubrication can lead to acceleration.

According to Nicole Sharp, an aerospace engineer and author of the fluid-dynamic blog FYFD, “Coating a fish with a layer of hydrophobic fluid like oil could reduce its drag.”

Reducing drag is important for speed, because drag is the force that slows an object down as it propels itself through a liquid. “Water would prefer to slip off the oil instead of adhering to the fish's scales,” says Sharp, who wasn't involved in the new study.

But confirming this is no easy task: Swordfish can’t be kept in captivity, and their speed makes them difficult to observe in the wild.

“You have to bring your lab to the swordfish, not the other way around!” says Eric Warrant, a zoologist from Sweden's University of Lund who was not part of the new study.

So Videler and his team instead examined 20-year-old MRI scans of swordfish.

In the scans, the scientists found both the oil-transporting capillaries and tiny, scale-like denticles around the opening of each pore.

They hypothesize that these scales create microscopic air pockets between the water and the fish’s skin, another way to reduce drag.

This is what fluid dynamicists refer to as a superhydrophobic surface, Videler says.

Splitting Denticles

Sharp isn’t so sure.

“I'm willing to accept the hypothesis that this oil gland and capillary system provides lubrication for the swordfish that may help reduce its drag,” she says.

“But I'm not convinced that the fish's head qualifies as superhydrophobic.”

For one, it doesn’t seem like there are enough denticles to create the kind of turbulence you’d need for a superhydrophobic surface.

Instead, Sharp thinks it's a “liquid-impregnated surface," in which a ridged surface (the swordfish’s skin) is protected from a liquid (seawater) by another layer of liquid (the oil).

In fact, liquid-impregnated surfaces have been in the news recently, but in a very different context.

The Massachusetts Institute of Technology has been developing condiment bottles with this technology so we can shake out every last drop of ketchup.

Evolutionary Edge

Attaining high speeds isn't just amazing—it's crucial to swordfish success, adds Lund University's Warrant.

“This oil gland would give them an evolutionary edge, assisting them to outswim their prey," he says.

“The newly discovered gland would add to their advantage as a predator, and is a superb example of how species compete with each other in a kind of evolutionary arms race.”

Follow Jason Bittel on Twitter and Facebook.