Are Bluefin Tuna Warm-Blooded? How Fish Heat Their Bodies

What does warm-blooded really mean?

“Warm-blooded” and “cold-blooded” are everyday words for two separate ideas. An endotherm warms its body with heat from its own metabolism, while an ectotherm relies mostly on heat from its surroundings. A homeotherm keeps its body at a nearly constant temperature, while other animals let their temperature change with conditions.

Mammals and birds are both endotherms and homeotherms: a person’s body stays close to 37 °C (98.6 °F) whatever the weather. Most fish, like most reptiles, are ectotherms whose bodies match the temperature of the water. The ideas are explained further in Are reptiles cold-blooded?

Bluefin tuna sit in between. The Atlantic bluefin tuna produces and keeps its own heat, so it is an endotherm, but only in certain parts of the body, and its temperature still varies over time. That is why scientists describe it as a regional endotherm rather than truly warm-blooded.

How do bluefin tuna keep warm?

Every working muscle produces heat. In most fish, that heat is carried away in the blood and lost to the water at the gills, where blood passes very close to cold seawater to pick up oxygen. Bluefin have three adaptations that stop this happening.

First, their red muscle, the dark, oxygen-rich muscle that powers steady swimming, lies deep inside the body near the backbone, insulated by surrounding white muscle and fat. Second, blood reaches this muscle through vessels running just under the skin, rather than through a central artery. Third, and most important, these vessels pass through dense bundles of tiny arteries and veins called retia mirabilia, Latin for “wonderful nets”.

The retia work as countercurrent heat exchangers. Warm blood flowing out of the muscle in the veins runs right beside cold blood flowing in from the gills in the arteries, in the opposite direction. Heat passes from the warm vessels to the cold ones, so the incoming blood is warmed before it reaches the muscle, and the heat stays in the body instead of escaping at the gills.

Bluefin have similar heat exchangers that warm the stomach and intestines, which speeds digestion, and others that warm the eyes and brain. Oddly, the heart is left out: it receives blood straight from the gills and works at close to the temperature of the water.

How warm does a bluefin get?

In classic studies from the late 1960s, American scientist Francis Carey and colleagues found that bluefin tuna could keep their muscles well above water temperature, by up to about 20 °C (36 °F) in cold water. Bluefin have been found with core muscle temperatures of 25 °C (77 °F) or more while swimming in water as cold as about 6 °C (43 °F).

Their temperature is not fixed, though. It rises with activity and after a big meal, and research suggests that bluefin can adjust how well their heat exchangers hold in heat, warming quickly when they rise into warm surface water and conserving heat when they dive into the cold.

Why does being warm help?

Warm muscles contract faster and more powerfully than cold ones, so a bluefin can swim hard for long periods. A warm body also lets the fish leave the tropics and hunt in cold, food-rich waters such as the Gulf of Maine, the Gulf of St. Lawrence and the seas off Norway, where other large predatory fish would be sluggish.

Warm eyes and brain improve vision and reaction times when chasing fast prey in deep, cold water. A warm stomach digests large meals quickly, so the fish can feed again sooner.

There are costs. A warm, nonstop swimmer burns a great deal of energy and needs to eat a lot. Bluefin may also risk overheating in warm water: in the Gulf of Mexico, where surface water can approach 30 °C (86 °F) during spawning season, tagged bluefin spend much of their time in deeper, cooler water.

Other warm-bodied fish

Only about 0.1% of fish species are known to warm any part of the body, and they belong to a handful of groups that evolved the ability separately:

• Mackerel sharks: the great white shark, the shortfin mako and longfin mako, the porbeagle and the salmon shark, which hunts salmon in North Pacific water only a few degrees above freezing. Some thresher sharks also warm their swimming muscles.
• Billfish: swordfish, marlins and sailfish warm their eyes and brain with special heater tissue that evolved from eye muscle, as explained in How do swordfish heat their eyes?
• The opah: a large, round fish that keeps its whole body slightly warm, shown in a 2015 study.

The similarity between tunas and mackerel sharks is a famous case of convergent evolution. The two groups are only distantly related, yet both evolved deep red muscle, countercurrent heat exchangers, stiff bodies and crescent-shaped tails, a design suited to fast, long-distance swimming in cold seas.