"As apex predators in the ocean these sharks are important and becoming rare," said study coauthor Robert Shadwick, a professor in the Marine Biology Research Division at Scripps. "In this study we have found that lamnid sharks have diverged from their shark ancestors in the mechanical design of their swimming apparatus, just as tunas diverged from their bony fish ancestors in much the same way over the last 50 million years or so."

Donley and her colleagues used several research methods during the study. These included analyzing video recordings of mako sharks in a swim tunnel (see video). They also used a device called a sonomicrometer to precisely record muscle shortening and lengthening during swimming activity. And they used an array of computer-based techniques to explore the three-dimensional characteristics of shark tendons and how they connect to muscles.

The combined results, which remarkably matched similar studies in tunas, displayed a unique biomechanical design in which powerful red muscles in the front of lamnid sharks transfer energy to the tail region. This high-performance muscle system serves for powerful swimming propulsion, not unlike a natural, thrust-producing hydrofoil.

"It's interesting because the area of the body that is producing this motion is not the same area of the body that is moving back and forth--it's physically separated," said Donley. "It's exactly like tuna in that respect."

The authors say these characteristics distinguish lamnid sharks and tunas from virtually all other fish and arose independently in each, most likely the result of evolutionary selection for fast and continuous locomotion. Shadwick says in this respect lamnids and tunas are more like each other than they are to their closest relatives.

The authors believe the study shows that not only have the physical demands
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Contact: Mario Aguilera or Cindy Clark
scrippsnews@ucsd.edu
858-534-3624
University of California - San Diego
5-May-2004