From: BBC Earth
Published August 12, 2011 01:25 PM

Unbelievable! The fastest fish in the sea!

Sometimes an animal is so fast even the record books can't keep up! In this second instalment of "Behind the speed", we journey away from the land into the ocean, to uncover the anatomical secrets of one of nature’s greatest speed demons; the Atlantic sailfish.

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In pursuit of the title for "fastest animal on the planet" is a fish whose agility and speed pips even a sprinting cheetah to the finishing line.

With its long circular bill and elongated body, the sailfish's striking profile makes it a predator created for speed. A master of stability and solidity, the sailfish achieves unbelievable speeds while remaining perfectly on target. Considering the fact that water is over 750 times denser than air, these fish are effectively overcoming greater resistance than to their four-legged feline friends. Which is quite a feat!

At first sight, their erect dorsal fin may look unwieldy, but the characteristic sail-like form can be sleekly folded into a groove in its back - hydrodynamic at the drop of a hat. This piece of apparatus is also used to prevent rolling or yawing after a burst of speed, as well as effectively striking fear into its prey, by making the fish appear much larger than it actually is. Groups of sailfish have even been observed using them tactically to herd schools of fish and squid into "bait balls".

The dorsal fin isn't the only feature that this cunning Atlantic fish can use as a hunting weapon. Its infamous coloured stripes, which rapidly change from bright blue to yellow to black send signals to its fellow predators to keep out of the way while at the same time whipping its prey into a confused frenzy.

Like most billfish, a high ratio of white to red muscle enables good acceleration, but not such good stamina. So tucked away beneath the brain and next to the eyes, the sailfish has a strategically located block of red muscle that produce serious amounts of heat. This enables them to venture into colder, deeper hunting grounds than many of their competitors.

But what about its speed, how fast can it really go? Official testing has always failed the sailfish. One speed trial reported a sailfish running out 300ft of fishing line in three seconds, which if verified would make it faster than a sprinting cheetah at 60mph (96kph)! But more conservative estimates have placed the sailfishes average speed at a 34mph (55kph).

Yet even without an official record, the magnificent sailfish that can grow to more than three meters, earns our upmost respect as the fastest fish in the ocean.

Please link to BBC Earth to see more.

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