Swordfish are toothless hunters with eyes that can grow as big as softballs. The prized sport fish brandishes a sharp, flat bill up to half the length of the rest of its body, and it has a reputation as the fastest fish in the ocean that often leaps spectacularly when hooked. Now the discovery of a lubricating gland behind the bill could show how swordfish manage to slice through the water so fast—and could serve as an inspiration for biomimetic low-friction surfaces.
Swordfish have proved remarkably difficult to study. They cannot be bred in captivity and are often injured during efforts to catch, tag and release them. Estimates of their incredible speed—commonly reported at up to 100 kilometers per hour—are based on flawed research from the mid-20th century, and their true maximum remains unknown. Even the purpose of the eponymous sword is debated, with some researchers arguing it is primarily a hunting weapon and others contending that it mostly serves to reduce drag on the fish as it swims.
This uncertainty is why John Videler, an emeritus professor of marine biology at the University of Groningen in the Netherlands, leaped at an opportunity to buy two swordfish in Corsica in 1996. He packed them in ice and brought them to a Groningen academic hospital with a magnetic resonance imaging (MRI) machine. Although it was the middle of the night, technicians volunteered to come in and do the scan. “After five o’clock we stopped,” Videler says. “It started to smell, because you can’t make MRI scans of a frozen swordfish. We had [an air freshener]—forest scent—and we sprayed the whole theater because the first patients were coming at 6:30, and we had to have it fresh.” On the way out of the hospital the bill of one swordfish hit a door and snapped off just above the eyes.
Twenty years later a paper published by another group used computed tomography (CT) scans of a different swordfish in a bid to understand the sword’s mechanical properties and thus infer its function. That study found a weak spot just behind the place where the bill attaches to the head. Videler heard about this, remembered how his swordfish’s bill had broken off, and returned to the MRI images. Those, together with dissections and other evidence, revealed the existence of a gland at the base of the sword that secretes lubricating oil via a network of capillaries throughout the front of the fish’s head. The oil, it is thought, may help reduce drag as the fish—which can grow up to 4.5 meters long—glides through the water.
Swordfish were already known to have several drag-reducing adaptations. The surface of the bill has a roughness that is similar in purpose to the dimples on a golf ball. The sword is also porous, which helps to equalize pressure across different regions of its surface. Videler says golf balls would fly even better if they were porous as well as dimpled, and the paper, which was published in The Journal of Experimental Biology, suggests that fully understanding the oil’s purpose could help with the creation of low-drag surfaces.
The oil gland’s link to movement seems clear, but not everyone is certain it is all about speed. “It’s extremely interesting that they have this oil gland and that this could reduce friction, but I wouldn’t necessarily see this in the context of these superhigh speeds—I would see this in the context of making locomotion more efficient,” says Jens Krause, a fish ecologist at Humboldt University in Berlin who was not affiliated with Videler’s study. “[The study is] an interesting step forward in documenting the hydrodynamic advantages” the swordfish has, Krause says.
And what about that second swordfish Videler purchased in 1996? Asked if it had become a menu option, he replies, “No, no, we didn’t eat it. I don’t like swordfish. I like fish, but swordfish is too dry.” He had his students dissect the other fish instead.