• Shark Tracks
• Sharks and Tuna: High Speed Performers

IN THE FRIGID WATERS OFF ALASKA late last spring, a group of scientists spent several weeks searching for the large, powerful salmon shark. In a series of investigations headed by Scripps marine biologist Robert Shadwick, these scientists are studying the "extreme" biomechanics of these extraordinary animals.

Salmon sharks, like their great white and mako cousins, belong to a group called lamnid sharks, animals with a high-performance locomotion system and muscle design that allows them to ply the oceans with fast, continuous motion.

Scientists have been intrigued by these powerful swimmers for years. Of the more than 25,000 fish species in the ocean, only a select few, including lamnid sharks, are able to elevate their internal core body temperature for high-speed swimming. Most fish have a body temperature close to the outside water temperature.

"These sharks' interior [red] muscle, which powers steady swimming, acts at high temperature, just like a mammal's, but the rest of the muscle in the body operates at lower temperatures, like that of an ordinary fish," Shadwick said. "Even their closest non-lamnid shark relatives keep a cooler body temperature like most fish, but these animals have elevated internal temperatures, many degrees warmer than the water surrounding them—which should give them a power boost."

Shadwick, who is a member of the Scripps Marine Biology Research Division, and Scripps marine biologist Jeff Graham began investigating these issuesmore than a decade ago, first concentrating on the high-performance design of tunas.

Recent research at Scripps by Shadwick and Scripps graduate student Jeanine Donley revealed the similarities between the lamnid shark's muscle system and that of the tuna, which is a bony fish not widely thought to have much in common with sharks. This amazing parallel has prompted questions about these creatures' mutual ancestry and evolution.

"Sharks and bony fishes have been separated for over 400 million years, and yet we see one group of sharks and one group of bony fish that share a remarkable host of similarities in body form and function. It's interesting to understand how mechanical design principles influence the evolution of locomotion in these animals," said Donley, lead author of the study, which was published in the journal Nature.

 SHARKS AND TUNA: cont'd