How fish swim: study solves muscle mystery.

Bolting like startled gazelles to avoid predators or trekking across oceans on seasonal migrations, fish use different kinds of muscles for different kinds of locomotion. Now, researchers have shown for the first time how fish use one of those muscle types to cruise as effortlessly through water as a hawk riding a canyon updraft.

The new study contradicts a widely accepted theory of how fish convert muscle contractions into steady-pace swimming, says biologist Lawrence C. Rome of the University of Pennsylvania in Philadelphia. Scientists have come up with theories to explain how fish swim, he says, "but nobody had done the definitive experiment."

It comes down to power: where in its musculature a fish generates swimming power and how this power is transmitted to the tail. Fish use two strips of blood-rich "red" muscle - one on either side of their bodies - to power the wavy motion of steady swimming.

Conventional wisdom holds that contractions in the front part of red muscle generate a fish's cruising power. The middle and rear portions function much like the rigid driveshaft of a truck: They transmit power from the engine (the front of the red muscle) to the rear wheels (the fish's tail).

Rome says he has disproved this theory by making the first direct measurements of where and how fish generate swimming power. His experiments show that most of the locomotive power for steady swimming comes from the fish's driveshaft - the area of the red muscle nearest the tail. Rome and colleagues report their findings in the July 16 SCIENCE.

The researchers first monitored the mechanical motion and electrical activity of red muscle in scup (porgy) swimming in an aquatic "treadmill." They used this information to set up a laboratory simulation of red-muscle motion with actual scup muscle tissue. This enabled the team to attach a force-measuring device directly to the red muscle, which cannot be done in a whole, living fish.

Although some biologists find the physiology of fish locomotion fascinating in its own right, Rome has bigger scientific fish to fry. His research could help resolve a 50-year-old puzzler known as Gray's Paradox: The horsepower of a fish's muscle "engine" is consistently less than calculations indicate the creature needs to cruise the deeps so fast and far. For instance, a 200-pound ocean tuna can swim steadily at 20 knots and reach 40 knots in a pinch - with only one-half horsepower of muscle power.

Mark A. Grosenbaugh, a researcher at the Woods Hole (Mass.) Oceanographic Institution, is part of a team of scientists building a mechanical tuna to test their theories of how marine creatures propel themselves through water so efficiently. In designing this "tuna sub," as some wags have nicknamed it, the team will use Rome's research to make sure the device swims in a truly fish-like manner, says Grosenbaugh.