I love tuna. It is delicious in any way, shape or form. But I know very little about the wonderful fish that provide me with such tasty meals, and I’d like to change that. So today we’ll look at these mighty creatures of the sea.

There are eight species of true tuna, all belonging to the genus Thunnus. I will focus on the Atlantic bluefin tuna, because it is one of largest species of tuna, and big things are cool. I probably don’t need to tell you that Atlantic bluefin tuna live in the Atlantic Ocean, but I will anyway. They can also be found in the Mediterranean Sea, and used to live in the Black Sea before we wiped them out there.

All species of tuna can get reasonably large, but the Atlantic bluefin outdoes them all. They can reach lengths of 3.7 metres, and weigh over  450 kilograms (the largest specimen ever caught weighed 679 kg). Bluefin tuna are countershaded for camouflage, being dark coloured on top and white on their undersides.

A school of bluefin tuna, swimming around.
Image by Danilo Cedrone (United Nations Food and Agriculture Organization), Public domain, via Wikimedia Commons

Tuna are built for speed, and are among some of the fastest fish in the sea. Bluefin can travel at speeds of 64 km/hour. To do this, the fish possess a number of adaptations that make them speed demons. The first dorsal fin of tunas can be folded into a special groove along the fish’s back to reduce water resistance. The bluefin also has a different way of propelling itself than most other fish: it moves its tail back and forth while keeping its body rigid. Most other fish move their bodies along with their tails, but the bluefin’s way is more efficient.

To power themselves through the ocean at high speeds, bluefin tuna have to be able to supply their muscles with lots of oxygen, and be able to keep their muscles warm. Thus, bluefin tuna have extremely efficient circulatory systems. Their blood is packed with hemoglobin, allowing for increased oxygen carrying capacity. They are also able to uptake oxygen extremely quickly, thanks to a super thin blood-water barrier.

And now on to the second part of the equation: how do these fish keep their muscles warm? Aren’t fish cold-blooded? Answer: no, not all fish are. All members of the tuna family are warm-blooded, but the bluefin tuna are better at it than any other fish. Blood is transferred from arterial blood into venous blood very quickly, to prevent heat loss. This allows bluefin tuna to exploit cooler waters more efficiently than other fish, because they can keep their bodies warmer than the surrounding water.

This is a sad picture of a tuna caught in a net, but also a great picture showing how big these guys can get! Image by Danilo Cedrone (United Nations Food and Agriculture Organization), Public domain, via Wikimedia Commons

Bluefin tuna spawn in two major areas of the world: the western Mediterranean Sea, and the Gulf of Mexico. Tuna born in one of these locations will return to that location. During spawning, bluefin tuna will gather in large numbers, and females can lay up to 30 million eggs. By fifteen years of age, tuna can reach lengths of 2.5 m, and it is believed that larger specimens could be up to 50 years of age.

Of course, as a big, tasty fish, bluefin tuna have been fished heavily, and are now a very endangered species. Tuna carcasses can be unbelievably valuable — the record for the sale of one tuna was about 1.7 million dollars, for a Pacific bluefin tuna sold in Japan. Hopefully sometime soon we’ll realize tuna are much more than just a tasty meal and a way to make money, and we can save these guys.

Cover image by Takashi Hososhima, CC BY-SA 2.0, via Wikimedia Commons