Why a Swordfish’s Sword Doesn’t Break

A swordfish’s “sword” is its most prominent feature, but scientists have only now discovered the unusual properties that keep the sword strong and ready to slash.

A study published last Monday in the Proceedings of the National Academy of Sciences reveals that the fish have an unusual way to repair their bone, keeping it strong and stiff.

Billfish like marlin and swordfish are known for their characteristic protruding upper jawbone (also called a rostral bone), which they use to help stun and catch their prey.For the bone to remain strong, it needs to not only withstand a large amount of force, but also be repaired when it is damaged. In mammals, this requires two different types of bone cells: one to break down and absorb damaged bone and another to add new, healthy cells. This process, known as remodeling, leaves telltale marks within the bone that biologists can detect.
Swordfish, however, don’t have either of these cell types in their bone. If the swordfish can’t repair its sword, wondered Ron Shahar, a biologist at the Hebrew University in Jerusalem, how does it remain strong enough to help the fish catch its dinner?
Stiff Upper Lip
To study billfish bone, Shahar needed samples—no easy task considering that many species of billfish are protected. Maria Laura Habegger, a Ph.D. student at the University of South Florida, in Tampa, routinely attended fishing competitions to obtain any castoffs for study. Shahar convinced her to collaborate, so she traveled from Florida to Spain and on to Israel, all while lugging a suitcase full of billfish bones through some of the world’s strictest airport security.
“No one said a thing, but it was probably a very long 12 hours of travel,” Shahar said.
As soon as Shahar placed his first sample of billfish rostral bone under the microscope, he saw something unusual. The bone showed distinct signs of remodeling. He looked at several other samples, and all showed the same thing. To be sure, Shahar used several different kinds of microscope to study the bone, and all revealed signs of remodeling, despite the billfish not having the usual types of bone-repairing cells.
“I was really surprised to see this. I didn’t think it was possible,” Shahar said.
The distinctive marks left by the bone remodeling process in billfish, however, were one-tenth the size of those typically seen in mammal bone. Shahar wanted to see whether these differences affected the bone’s strength. The rostral bone of billfish was very stiff (comparable in strength with horse bones) and required a significant amount of force to break.
The new study is “very original,” said Roger Bouillon, a retired professor of endocrinology at the University of Leuven in Belgium, who has studied bone remodeling. Although Shahar found circumstantial evidence of bone remodeling, Bouillon pointed out that the researchers weren’t able to document the process in action.
“It’s like looking at a snapshot of a horse galloping, and you infer that it’s moving,” he said.