VIDEO: Lethal rapier

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Marine scientists long have known that billfish use the long spear on their upper jaw to slash at and stun prey during feeding, although some experts have suggested that it also helps them navigate.

New data from a team of European researchers reveal the techniques the fish use in wielding their lethal weapon, and new underwater video offers a front-row view of the kill.

Suspecting that the bill's main purpose is to catch prey, the researchers mounted an expedition to Cancun, Mexico, and learned from local fishermen how to find sailfish where seabirds congregate. When hunting, sailfish push some of their prey to the surface, making those fish easy pickings for birds. The prey in this case was sardines, which band together in ball-shaped schools.

The team spent nearly a week on the water, using high-speed cameras. What they saw was that groups of sailfish surrounded a school. Each sailfish took turns pushing its pointy bill into the mass of fish, a move the sardines didn’t seem to notice.

Then the sailfish suddenly swung their bill back and forth very sharply — strong enough to catch the sardines off guard. Some of them were stunned by the blows; others were cut. That allowed the sailfish to grab their prey and gulp them down.

It’s an impressive bit of surgery to watch.