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Color bursts may serve not only to unsettle prey but also to warn other sailfish to stay back, helping avoid collisions. "Given their pointy noses and swimming speed, this would be important," Fritsches says. Indeed, sailfish bills—elongated upper jaws that the hunters whip left and right to batter prey, and likely wield against sharks, marlins, and other enemies—are dagger sharp. Yet despite their rapid-fire strikes, reports of sailfish skewering one another are hard to find. The fish take turns—and it appears no one loses an eye or goes hungry in the frenzy.

The sardines, too, work in concert. Detecting each other's proximity and movement, they shift in synchrony, each fish both leader and follower. The fish mass slides like a drop of mercury, mesmerizing, with a shimmer that may help to confuse predators.

But no hypnotic dance can fully protect the sardines, which will hide in a squirming mass under any bit of flotsam—even a snorkeler. The sailfish simply wait within striking distance for their prey to be exposed. Soon the hunt is back on, predators again corralling, swatting, swallowing. After a rush to mop up the leftovers, the deadly game is over, and the sailfish retreat. In their wake, drifts of glinting sardine scales fall slowly into the blue.

An iridescent flash along the body, often in silvery blue stripes, adds to the effect. Darkly pigmented cells called melanophores are "like venetian blinds," says neurobiologist Kerstin Fritsches of the University of Queensland, in Australia. Ordinarily the animal appears dull, but "during stress or excitement, the cells contract their pigment to expose gorgeous metallic colors in the skin below."

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