A marine biology research team spearheaded by University of Massachusetts Dartmouth scientist Diego Bernal and his colleagues at Scripps Institution of Oceanography at the University of California, San Diego, has concluded that salmon sharks, one of the planet's fastest predators, may be rendered "paralyzed" in the cold Alaskan waters if they stop swimming and their body temperature decreases even by small amounts. The findings of the team's research study will be published in the October 27 issue of Nature.
We view our findings as an example of how specialized these sharks have become," said Bernal. "They need to continuously swim around, not only to ventilate their gills and find their prey, but also to keep warm. If salmon sharks in Alaskan waters stopped swimming and cooled down, our results suggest that that they may not be able to start swimming again.'
The four-member research team of Bernal; Jeanine Donley (now at MiraCosta College in San Diego) and Robert Shadwick of Scripps Institution of Oceanography; and Douglas Syme from the University of Calgary traveled to the University of Alaska Marine Center in Seward to work with the warm bodied salmon shark, a relative of the white and mako sharks.
Movement is an important element to salmon shark survival, Bernal said. They can continue to thrive and hunt provided they continue to swim and maintain a warm body core temperature. However, once they stop, then the muscles in their bodies "stiffen" and makes them dormant. In the much-colder Alaskan waters, that may cause trouble for these sharks, as their bodies will tend to cool down much faster if they become inactive even for short periods of time.
"Our previous studies of warm bodied tunas led us to investigate physiological and biomechanical parallels with the lamnid sharks, also known to have elevated core temperatures," said Shadwick, now at the University of British Columbia. "Bernal and Donley both worked on mako sharks for their Ph.D.s at Scripps and this led us to questions about muscle function in the salmon shark, a cold water member of the same shark family."
Alaska proved to be the best place to study salmon sharks as they are found in the relatively cold and shallow coastal waters during the summer months. What they discovered was that these animals may be susceptible to "muscle freeze" should they stop swimming and allow their body temperature to cool down.
"Unlike mammals and birds, which can use other sources of metabolic heat during times of inactivity and thus do not depend solely on muscle activity or swimming to warm their body core, salmon sharks appear to only be capable of warming their body core during swimming," Bernal continued. "For this reason, salmon sharks live in a physiological extreme in that if they stopped swimming, and ceased to produce heat, their muscles would fail to function and possibly could not recover."
The salmon sharks used in this study were caught off Prince William Sound in the Gulf of Alaska. The research team sampled small amounts of muscle tissue and further processed them back on land at the University of Alaska's Seward laboratory.
The temperature of the swimming muscles normally operates at 26 degrees Celsius, which is about 20 degrees warmer than the surrounding water. If the temperature of the swimming muscles decreased below 20 degrees Celsius, the shark's ability to move around freely would be severely impeded.
"(Our) observations indicate that the red muscle is extremely sensitive to temperature, particularly at colder temperatures, and would probably be incapable of contracting in a way that would result in effective swimming if allowed to cool even to 15 degrees Celsius," Bernal said.
Researching warm bodied fishes has been a lifelong pursuit for Bernal. He said his interest began in high school and intensified during college. Bernal said this subject continues to fascinate and motivate him.
"I have always been interested in fishes that are not normal," said Bernal. "Most all of the 25,000 or more species of fishes did not evolve the capacity to retain their body heat, but there are some unique fish that have (tunas, billfishes, lamnid and one thresher shark). It also turns out that these warm fishes are highly active swimmers and the majority are apex predators in the open ocean. How is their unique physiology related to the ability to be a faster or more efficient swimmer when compared to other fish? I am interested in why these fish groups have independently evolved these unique features."
The research was supported by funding from the U.S. National Science Foundation, University of California San Diego Academic Senate and Canada's Natural Sciences and Engineering Research Council.