Why do swordfish have swords? The adaptation probably has more to do with speed than swashbuckling, and a new study reveals a bone-hidden oil gland that supports that theory.
But first a little context: You won't find swordfish at your local aquarium. These saber-headed speed freaks thrive in the wild, open ocean. Pursued by relentless sport fishermen and prized as wall-mounted trophies, the creatures very occasionally return the favor with a lethal skewering, but for the most part remain elusive and on the move.
So why does this particular billfish resemble one of humanity's all-time favorite murder weapons? Heck, the scientific name Xiphias gladius is just the word "sword" repeated in Greek and Latin, as if to punctuate our murderous enthusiasm.
But swordfish are creatures of speed, not confrontation. They lose all scales and teeth by adulthood, likely prefer a diet of squid and certainly did not evolve for use in one of Lew Zealand's "Muppet Show" fish-throwing performances.
In fact, as noted in a 2015 Journal of Experimental Biology study, a weak spot where saber-meets-skull makes the swordfish a rather impractical lawn dart. Sure, they might slash with their long bills, but they're just not made to stab at their few natural enemies.
In a new study published in the same journal, Dutch biologist John Videler reveals the "weak spot" in question to be a specialized oil-producing gland hidden beneath a thin layer of bone. Videler made the discovery after loading a dead swordfish into an MRI scanner and then using scanning electron microscopy to observe the secretion of oil through a network of capillaries to tiny holes in the swordfish's head.
The theory here is the oil helps to lubricate the creature's head to make it super water-repellent — perhaps reducing drag by up to 20 percent. Videler's previous research highlighted the drag-reducing qualities of the creature's long, sleek bill, so this discovery merely ads to our picture of a migratory species bent on speed, not swordplay. However, given the swordfish's elusive nature and aversion to captivity, physics-based simulations will have to fill in the details of their marvelous, streamlined physiology.
In the meantime, Michael Phelps, if you're reading this, have you considered installing a lube-spurting gland behind your nose? You might want to beat the Russians to the transgenic punch.