What Do Swordfish Use Their "Swords" For?

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It's easy to envision: A sleek swordfish, many meters long and massive, flaps its powerful tailfin, rapidly accelerating through the ocean water at breakneck speed, until it spears its helpless prey. Wait a second... Then what? How does the swordfish get the fish off its "sword" and into its mouth? It doesn't make any sense!

That's because none of this actually happens.

Turns out, the "swords," or bills, of billfish like marlin, sailfish, and swordfish, are indeed used for hunting, but not as devices to impale prey. Rather, they are wielded as scythes to swipe at larger prey or through schools of smaller fish, knocking them senseless so they can be easily gobbled up.

A few years ago, a team of researchers filmed sailfish underwater swinging their bills at sardines at dizzying accelerations, some of "the highest... ever recorded in an aquatic vertebrate," they remarked. What's more, the merciless attacks were relatively covert – sailfish inserting their foot-long bills into fish schools often didn't elicit an evasive response from their targets.

While the study was only about sailfish, a close relative to swordfish, it's likely that other billfish employ similar tactics, although perhaps not as refined. The sailfish bill sports serrated teeth on the sides that can stab into fishy victims. The swordfish bill, on the other hand, is smooth and horizontally flattened, better suited perhaps for whacking larger prey like squid.

Beyond their "swords," all billfish come equipped with a remarkable internal heating system for their eyes, keeping them ten to fifteen degrees Celsius warmer than ambient water temperatures.

"Warming the retina significantly improves temporal resolution, and hence the detection of rapid motion," University of Queensland neuroscientist Kerstin Fritsches explained. "Depending on diving depth, temporal resolution can be more than ten times greater in these fishes than in fishes with eyes at the same temperature as the surrounding water."

The better to slash at you with.

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