It may be a little odd sounding, but tuna are very cold hearted creatures. No, they aren't unnecessarily cruel or stoic in life. Instead, they just can literally have cold hearts, with the organ somehow able to keep functioning even when deep-diving chills it to temperatures that would stop a human heart. Now researcher think they know how the fish is capable of this amazing feat.
"Tunas are at a unique place in bony fish evolution" researcher Barbara Block at Stanford University explained in a recent statement. "Their bodies are almost like ours - endothermic (warm blooded/bodied), but their heart is running as all fish at ambient temperatures. How the heart keeps pumping as the fish moves into the colder water is the key to their expanded global range."
As detailed in a new study published in the journal Proceedings of the Royal Society B, Block and her colleagues looked to bluefin tuna to learn more about this fascinating ability. A top predator of the Pacific Ocean, the bluefin are renown for their epic migrations, traveling far in search of prey and diving chillingly deep - up to 1000 meters below the ocean surface.
"When tunas dive down to cold depths their body temperature stays warm but their heart temperature can fall by 15°C within minutes," added Holly Shiels, from the University of Manchester. "The heart is chilled because it receives blood directly from the gills which mirrors water temperature. This clearly imposes stress upon the heart but it keeps beating, despite the temperature change. In most other animals the heart would stop."
Shiels, Block, and Manchester researcher Gina Gali reportedly used electromagnetic tags to monitor bluefins in their lengthy migration from the waters of Japan all the way to the Californian coast. The tags allowed them to measure the depth at which each fish swam on its journey, its internal body temperature at any one point, and the ambient water temperature surrounding it. This data was then reapplied in lab simulations using single tuna heart cells to see how they beat.
The trio found out that rushes of adrenaline during dives helped keep essential calcium circulating in the tunas' hearts, which kept them pumping their chilled blood.
"We were recording the fish swimming down into colder depths only to resurface quickly into the warmer surface waters, a so called 'bounce' dive," said Block. " From work at sea and in the lab we now know the fish hearts slow as they cool and as they resurfaced it sped up. Our findings suggest adrenalin, activated by the stress of diving, plays a key role in maintaining the heart's capacity to supply the body with oxygen."
Now the researchers are wondering if this is a new and unique mechanic among only tuna, or if other species have taken on this adaptation as well.
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