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 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 such as this broadbill swordfish can repair minor damage to the upper jawbone that forms their sword, scientists have found. Photograph by Brian Skerry, National Geographic

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.

Follow Carrie Arnold on Twitter and Google+.

Carrie is a freelance science writer living in Virginia. When she's not writing about cool critters, she's spending time outside, drinking coffee, or knitting. You can visit her website at http://www.carriearnold.com
  • dave schein

    if scientists are able to identify billfish remodeling processes, might that have repercussions in microgravity bone density loss as well as earth-bound osteoporosis?

  • Doug

    This is an interesting question, but I dont think the article answered the question of why the sword doesnt break. If swordfish do not have the type of cells that conduct the remodeling, than what do they have that does?

  • Dan

    I’m not sure why this is a surprise to any of the researchers, aside from the fact that the specific remodeling cells cited in mammals aren’t present in these fish. Countless swordfish have been sampled that had bills that had obviously suffered fractures or other injuries that distorted their shape. I have a bill on my porch that grew thick and curved because the fish’s bill speared a piece of thick, cylindrical plastic and the plastic shaped the bill’s growth, kind of like what happens to the necks and shoulders of Kayan women in Burma.

  • koustavjana

    THEY ARE VERY STRONG………………………….

  • lars

    Article doesn’t answer the question at all. “Why a swordfish’s sword doesn’t break: because they rebuild the bone just like us humans, but we don’t really know since they don’t have the cells we have.” ? Way to go ngc.

  • Steve

    …THIS is what scientists are wasting time and money on? Who cares?? Unless there is some practical knowledge to be gained…there are FAR more useful “mysteries” to pursue.

    … it can’t just be me :-/

  • jean

    This article revealed nothing and makes us ask more questions that have gone unanswered

  • Danielle

    In response to Steve in AZ’s comment…

    You can’t generalize that this is what scientists are doing. These are a few scientists, not all in general. It’s not really a big deal. To add to that fact, you can’t assume that it has no practical use just because they don’t tell you or you don’t understand it. Maybe if they knew they would be able to find something that they can somehow utilize to help with osteoporosis. You never know until research is done to find out.

    A different examples is researchers who studied how fish school. Seems pretty useless, right? Who cares how they all swim together really close without bumping into each other. Well…you know how Google is developing a self-driving car? Knowing how to have a bunch of cars on the road together without bumping into each other and causing wrecks is kind of an important thing if no one is driving them.

    Just because you don’t know everything doesn’t mean there is nothing to know. Just saying….

  • Keith D.

    Has the U.S.’s reading comprehension really fallen this far? How can so many of you commenters say that this article and these scientists revealed nothing about what makes these bones so strong? Just because they don’t know HOW the remodeling process works in these fish doesn’t mean they found nothing, or that NatGeo’s article revealed nothing or answered no questions. It’s right there in the article, folks!

    “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.”

    Why does nobody know how to science anymore? We just keep getting dumber and dumber. It makes me sad, because we’re better than this!

  • Gryphon of Oregon

    I fully agree with Dave, this is a VERY valid topic (contrary to some beliefs outlined in the comments). The article itself seems… lacking. Nat.Geo, I understand we’re a silly country, but kindly avoid dumbing down science for the rest of us…

  • Hunter

    I am doing a report on a swordfish thanks for helping hoping for a A+

  • Ryan Taylor

    LOL, just wanted to know small details on “Swordfish” an became very interested..in the article!
    Question: Can these new found “cells” repair human bone?? Is there scientific research being done..to see if these “cells” can be applied to human bone for quicker bone repair?..an or human bone recovery. (Making broken bone/bones in human applications heal faster..stronger?)

    -would like your feedback. Thank-you

  • Ryan Taylor

    Also, can the swordfish’s “bone marrow” be used for human applications? If the “cells” would accept.. the human anatomy? Wow..I’m so excited about this article..so many questions..lol.
    I say…let’s make history! “A cure possibly?)

    -would like your feedback. Thank-you

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