The Salmon sharks in the frigid waters of Alaska possess red muscle bundles that are visible due to blood vessels, resembling those of mammals: they can elevate and maintain their body temperature by up to 26 degrees Celsius, which is 20 degrees higher than the surrounding water temperature.
However, what is even more surprising is that when these red muscle bundles were examined in laboratory conditions, they did not function at low temperatures. This indicates that salmon sharks must maintain a high internal body temperature at all times, enabling them to swim at any speed.
“We did not expect this,” said Robert Shadwick, a researcher specializing in shark metabolism at the University of British Columbia in Vancouver, Canada.
Most fish species, including sharks, only possess “white muscle” that operates at temperatures equal to that of the surrounding water. The most significant potential benefit (although not yet proven) of maintaining warm muscle bundles is that it allows predators to achieve greater speed and agility, along with the ability to swim freely in colder waters compared to their cold-blooded prey, Shadwick analyzed.
Salmon sharks can weigh up to 160 kg. As agile predators, they have evolved a body shape and even a fin that closely resembles that of the bluefin tuna—a large, aggressive, warm-bodied predator that inhabits cold waters. In fact, the similarities in lifestyle and physical characteristics between these two species led Shadwick and his team to investigate whether salmon sharks have an internal “warming” mechanism akin to that of tunas. And they found that to be true.
The significant difference between these two species is that the red muscle of the tuna can operate even at low temperatures, while the red muscle bundles in salmon sharks (which are surrounded by white muscle bundles) require heat to function.
This surprising similarity makes the muscles of salmon sharks akin to those of mammals. Muscle and most organs in most mammals have evolved to function only within a narrow range of high temperatures.
“The ability to elevate body temperature in sharks is very unusual,” noted Kathy Dickson, a shark metabolism researcher at California State University, Fullerton.
T. An (according to Discovery)
