Oct. 31, 2005 — Salmon sharks from the Gulf of Alaska have heat-hungry swimming muscles more like those of mammals than their scaly kin, say marine scientists.

A new study of the large sharks caught in chilly Alaskan waters has revealed that their innermost swimming muscles, which are conspicuously red from more blood flow, generate and warm the fish to 79° Fahrenheit (26° Celsius), which is 36°F (20°C) higher than the surrounding water.

More surprising, however, is that when the red muscle was tested under laboratory conditions, it did not work at lower temperatures. That means the salmon shark must maintain a certain elevated core body temperature at all times in order to swim with any speed.

"We didn't really expect to find this," said shark metabolism researcher Robert Shadwick of the University of British Columbia in Vancouver.

Most fish, including most sharks, have only "white muscles" that operate at the same temperature as the surrounding water. The most likely — but so far unproven — advantage of maintaining warmer muscles is that it could give the predator greater speed and agility, plus freedom to venture into colder waters than their cold-blooded prey, he said.

A paper by Shadwick and his colleagues on the discovery appears in the current issue of the journal Nature.

Salmon sharks can run to about 350 pounds and "are the biggest, meanest guys on the block," said Shadwick.

Swift predators, they have evolved contours and even a fin shape very similar to blue-fin tuna, another big, hot-blooded, aggressive, cold-water predator.

In fact, it was the similar lifestyles and physical similarities in these two fish that got Shadwick and his colleagues interested in whether salmon sharks had also hit on the same internal heating trick as tuna.

"We had this idea they would be sort of the blue-fin tuna of the shark world," said Shadwick.

And despite hundreds of millions of years of evolutionary distance between sharks and tuna, it appears that salmon sharks and blue-fin tuna have undergone what's called convergent evolution — adapting in remarkably similar ways to a similar ecological niche.

One major difference, however, is that a tuna's red muscles can operate well at lower temperatures, whereas the salmon shark's innermost red muscles, which are surrounded by white muscles, really need the heat to function.

That's what makes these shark muscles so surprisingly mammal-like. The muscles and most other organs in most mammals have evolved to operate well only within a comparatively narrow range of high temperatures, as anyone knows who has suffered from hypothermia or heatstroke.

"The ability to elevate the body temperature is very unusual," said shark metabolism researcher Kathy Dickson of California State University, Fullerton.

And because these large fish — tuna and sharks — aren't easy to put on a treadmill or to monitor as they hunt, it's very difficult to gather precise information, she said.

"It's an amazing thing," said Dickson of mammal-like salmon shark muscles. "We really don't fully understand why (they are warmer), but we'd like to."