Cruising the planet's oceans, bluefin tuna are true giants of the deep;Barbara Block describes them as `majestic fish'. Yet very little was known about the physiology or habits of these gentle giants, as working with the fish in captivity seemed impossible. But being situated in Monterey allowed Block and her lab to establish a collaboration with their next-door neighbour,the Monterey Bay Aquarium, to build a unique facility housing bluefins weighing up to 50 kg. For the first time, Block and her students could begin studying these enigmatic creatures in ways they had only dreamed of before.
One of the main questions that puzzled Block about the bluefin was the fish's ability to forage and breed in waters ranging from the tropics to the poles (p. 881). Block explains that unlike other teleosts, whose body temperature is in equilibrium with their surrounding water, tunas are significantly warmer as they retain heat in many tissues thanks to complex counter-current heat-exchanging blood vessels. Yet the fish breathe through gills that constantly chill their blood and hearts to ambient temperature. How could the fish's heart function over such a wide thermal range?
Keen to know how bluefin's hearts weather the enormous temperature range,Block and her team set out to sea in a fishing boat to collect juvenile fish. Sailing 500 miles into the Pacific Ocean, the scientists successfully caught more then 20 juvenile bluefins, transferring them to an onboard storage tank,ready for the long journey back to Monterey and their new home in the Tuna Research and Conservation Center's massive tanks.
Back on land, Jason Blank, Jeffery Morrissette, Thomas Williams and Block prepared to begin testing the fish's cardiac function. Block explains that`keeping the heart going is a challenge' and adds that the experiments are limited by diffusion, so they must be done on small fish. It took the team almost a year to master the experimental techniques they would need to successfully test the fish's cardiac function, but by then the fish had grown too large for their hearts to be sustained by the perfusion technique. The team had to wait another year before the juvenile bluefins returned to the Mexican coast, and then they were ready to continue making cardiac recordings.
Gently dropping the temperature from 30°C to 2°C, Blank and Morrissette saw the fish's heart rate decline from over 150 beats min–1 at 30°C to 13 beats min–1 at 2°C. While the strength of the fish's maximal heart beat was relatively unaffected by the massive temperature change, the dramatic fall in heart rate caused the bluefin's maximal cardiac output to plummet from 106 ml kg–1 min–1 at 25°C to 18.1 ml kg–1 min–1 at 2°C. Most surprisingly,the bluefin's heart continued functioning at temperatures where other tuna hearts failed. And when Block and her team began investigating the cellular basis of the heart's thermal tolerance, Ana Landeira found that the heart tissue expressed high levels of SERCA 2, one of the pump proteins responsible for regulating the heart's contraction, `fitting nicely with the organismal study' Block adds.
Block suspects that it is the bluefin heart's exceptional thermal tolerance that permits the creature's nomadic lifestyle, allowing it to exploit environments that are simply too cold for other tuna species. And it could also explain some of the fish's intriguing diving behaviour. The fish seem able to descend to enormous depths where the temperature is barely above freezing. Block suspects that even the bluefin's heart can't tolerate such low temperatures for long, so the fish `bounce' back to warm in waters at the surface.