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Most fish are pretty sluggish in the cold. But the Pacific bluefin tuna is one of the fastest apex predators in the frigid Pacific ocean. Their physiology has adapted to help them retain more of the heat their bodies produce, except when it comes to their cold, cold hearts.

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[♪ INTRO].

The Pacific bluefin tuna is one of the fastest apex predators in the frigid Pacific ocean. Which is odd because most fish are pretty sluggish in the cold.

That’s because most fish’s body temperatures are the same temperature as the water they live in, which comes in handy because they don't have to use precious energy to maintain their body temperature. That's especially true for warm-water fish who take on the temperature around them to move fast. But what about cold-water fish like tuna?

Why aren’t they fresh out of luck when it's nippy out? Most fish are referred to as “cold-blooded” because, like I mentioned, the water they swim in determines their internal temperature, making them ectothermic. So to maintain their body temperature, they don’t need to spend energy like us "warm-blooded" endotherms.

But don't be deceived by the term “warm-blooded.” If ectotherms' environments get hot enough, their blood can actually become warmer than so-called "warm-blooded" species. Which is why we'll refer to them as ectotherms and endotherms from now on. And ectotherms have evolved to be great at what they do, they just need a warm environment to keep their metabolic rate high and swim their merry way around the ocean.

So ectotherms that evolved to live in frigid waters can expect life to be pretty slow. But the Pacific bluefin tuna deviated from the norm in order to maintain a warmer body temperature and zip around in very cold waters. Like, cold enough to stop a human heart.

But how and why did the tuna evolve to run hot? Well, if a fish’s speed is dependent on the water temperature around them, their greatest threat in chilly waters is probably going to be an animal that isn’t playing by those same rules. So, tuna take advantage of the heat produced in their eyes, brain, muscles, and other organs while minimizing heat loss when their gills come into contact with cold water.

See, the heart receives blood that's just been directly oxygenated by the cold water flowing over the gills, so it can’t maintain warmer temperatures like other parts of their bodies. Unlike true endothermic species, the hearts of tuna are kept very chilly! To counteract that and conserve heat, tuna use a complex meshwork of veins and arteries called retia mirabilia.

These veins lay over one other, where blood flows in opposite directions, to efficiently transfer heat. So blood flowing from heat-producing parts of the body, like the muscles, warms up the adjacent vessels. Those vessels transport cooler blood in the opposite direction.

So these criss-crossing vessels recover the body’s heat instead of losing it, keeping the tuna warm and toasty in the frigid ocean. And running hot makes this tuna one of the fastest apex predators in the Pacific Ocean, even in very cold waters. It’s a huge bonus when it comes to snatching up prey or avoiding becoming a meal themselves when young.

And tolerating warm and cold temperatures opens up an ocean of possibilities! They can swim and feed in places like the polar sea or dive 500 meters down into even colder waters for prey. And juveniles can thrive in a huge range of temperatures, from 3.4 to 27 degrees Celsius.

And their cold-water adaptations also make their migrations more flexible since they don’t have to stick to warm waters and plan their travels around the strong currents. And all these perks are thanks to their unique physiology that helps them be the cold water speed demons they are. So by breaking the mold, the Pacific bluefin tuna shows us that staying toasty in icy cold waters is an incredibly effective way to becoming one of the fastest predators around.

Thanks for watching this episode of SciShow, and thank you to this month’s President of Science, Matthew Brant! We really appreciate your support in helping us create free science education content. If you’re interested in joining our awesome community of patrons, you can go to to learn more. [♪ OUTRO].