The active tuna, the only fish whose body temperature is higher than sea temperature, swims constantly, because its body is heavier than seawater. Tuna seem to be able to swim indefinitely at nine miles (about 14 kilometers) per hour.

Flying fish are said to work up to a speed of thirty-five miles (56 kilometers) an hour before vaulting out of the water to glide for a distance through the air. The tuna, the dolphin and the blue marlin are believed to be even faster.

They have researched such questions as: How is it that fast fish like the tuna actually achieve greater speed than mathematicians say they should be able to? How do the tuna and the shark slip through the water so smoothly and without turbulence?

Some answers are known. First of all, such fish are highly streamlined. This, submarine designers have copied. Fast-swimming fish also can fold their fins against their bodies. Scales evidently adapt to water pressure to eliminate turbulence. But the primary secret of their speed, for a long time a mystery, lies in the construction of their skin, which is elastic and flexible.