The mechanics of how fish use their complex muscle systems is a tantalizing puzzle in animal physiology. These muscles are the fundamental sources that fish use to power steady swimming and bursts of speed to elude predators and to capture prey. Scientists have long predicted that tuna, with their highly streamlined body and elevated internal temperatures, are equipped with a "high performance" muscle system. Tuna, researchers suspected, power their swimming by projecting muscle force from the mid-body, where the muscle is concentrated, back to the tail, which essentially acts as a natural, thrust-producing hydrofoil.
Now, through a study sponsored by the National Science Foundation and conducted at Scripps Institution of Oceanography at the University of California, San Diego, and the National Marine Fisheries Laboratory in Honolulu, researchers have for the first time documented this muscle action in motion. Stephen Katz, Douglas Syme, and Robert Shadwick report their results in the April 12 edition of the journal Nature.
"The anatomy has been known for a long time, especially the idea that the connective tissue architecture in tunas allows muscles to focus their action further down the body," said Shadwick, a professor in Scripps’s Marine Biology Research Division. "We’ve taken measurements directly from swimming fish to show it working this way."
In other fishes, such as trout and mackerel, swimming muscles are distributed more uniformly along the body. When their muscles shorten and produce power, the burst is seen as a wave of contraction that causes the entire body to undulate.
Tuna, however, contain swimming muscles located primarily in the central part of the body. Tendons that angle to the backbone link the muscle with the tail.
Using ultrasound technology, Shadwick and his colleagues attached tiny transducers directly to tuna muscles to record the muscle electrical activity and contraction as tuna swam in a large water tunnel. A device called a sonomicrometer measured the muscle shortening by timing the ultrasound signal between pairs of transducers.
"When we went inside the fish with ultrasound, we saw that the muscle contraction caused bending to occur further down the body," said Shadwick. "We now know that because the muscle tunas use for cruising is close to the backbone–not adjacent to the skin as in other fish–it is allowed to do large amounts of shortening, which means more work and more power production. That’s the essence of how this fish is different from others. Hydrodynamically, that’s a more effective way to swim. If all the middle segments throughout the body were undulating, it would create much more drag. Tunas have a more streamlined body and the motion at the tail acts almost like a propeller."
Shadwick says the results of the study hold implications for research in comparative physiology and the evolutionary biology of fishes. The results also could be important for the design of robotic, self-propelled autonomous underwater vehicles that mimic biological design.
The results have prompted Shadwick to move to other species. With new support from the National Science Foundation, he and Scripps researcher Jeffrey Graham have launched a new study to search for the same results in lamnid sharks.
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