Like swordfish, marlins, and other such flashy billfish, sailfish have long, pointy bills that look like deadly weapons, Musketeer-style. And new research shows that they are!
Nobody has ever showed exactly how the fish use their bills: Some say it’s for foraging, others say there are hydrodynamic benefits (like aerodynamic, but in the water). Peter Domenici of Italy's National Research Council in Torregrande, Andrew Wilson of Carleton University and colleagues show the first direct and “unequivocal evidence” that a combination of stealth and swiftness makes the sailfish bill an extremely effective adaption for feeding on schooling prey.
The team captured high-speed, high-resolution videos of Atlantic sailfish (Istiophorus albicans) -- named such for their long, prominent dorsal fin -- brandishing their bill and injuring their prey in open water in the Yucatan Channel, Mexico, over the course of six days in 2012. To analyze the video, they slowed the original 240 frames per second down to just 30.
The bill is so thin, the fish stealthily sneaks into a school of round sardines (Sardinella aurita) without eliciting any evasive maneuvers. Then the sailfish whips its head and swipes the bill side to side through the school, producing an effective slash that moves faster than sardines can swim – giving them no opportunity to avoid the gashing thwacks.
The powerful sideways slashing motion has one of the highest accelerations ever recorded in aquatic vertebrates. It’s about the same acceleration as the tip of a swinging baseball bat, even in the water, Domenici tells Nature. The large sail keeps the animal stable while it slashes with the bill.
Here’s the video of them swashbuckling with a school of sardines. It starts with stalking, the second segment shows the slash, and then there's the real-time attacking (with the 10 main states labeled). It’s a pretty savage scene, with those shiny scales and bits of skin being sliced right off.
By following birds of prey, the researchers managed to track sailfish in the open ocean -- where they’d find balls of several hundred sardines surrounded by up to 40 sailfish. They hunt in groups, taking turns to prod the school with their bills. “It’s a very orderly process,” Wilson explains. “They don’t want to risk breaking their bills.”
Weakening the prey first increases the hunt’s success. But the sailfish’s specialized weapon not only exerts great force, it can also delicately nudge prey. Using its bill, the predator singles out and destabilizes an individual target by tapping it, then flicking it away from the school. Then the sailfish corrals the sardine into its mouth with the help of small projections on its bill (called denticles). Sailfish use this energy-efficient technique about half the time.
Sailfish are one of the fastest creatures in the ocean, capable of swimming more than 110 kilometers per hour (or 60 knots), and they hunt for as long as necessary, taking turns until every individual in the prey school is consumed. “The poor sardines are just being chased for hours and hours,” Wilson says. “There’s no reprieve.”
The work was published in Proceedings of the Royal Society B this week.
Video copyright: A. Wilson and the sailfish team