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."
Name: Salmon Shark (
Lamna ditropis)
Primary Classification: Chondrichthyes (Sharks, Rays and Relatives)
Location: Northern Pacific Ocean
Habitat: Cold, open or coastal waters
Diet: Mainly pelagic, bony fish, such as salmon, herring, cod and mackerel
Size: Up to 12 ft in length and 700 lbs in weight
Description: Dark blue-gray to black in color; white underbelly with dark blotches; short, blunt, conical snout; large gill slits; heavy, spindle-shaped body; large dorsal fin
Cool Facts: Able to increase its internal body temperature 36°F above the surrounding sea water, it has the greatest thermoregulatory ability of any shark. It is one of the fastest fish in the oceans, and has been clocked at faster than 50 knots.
Conservation Status: Data Deficient
