Metallic blue above, silver below, with fins so yellow they look painted and a row of small finlets running to the tail. Adults reach 2.4 m (8 ft) and 200 kg (440 lb), and in big fish the second dorsal and anal fins stretch back into long sickles.
A tuna is a fish that runs warm. Heat from the swimming muscles is trapped by a counter-current network of blood vessels instead of being lost through the gills, so the muscle, the eyes and the brain run several degrees above the surrounding water. Warm muscle contracts faster, which is what lets this animal cruise all day and sprint at over 70 km/h.
Every detail follows the same logic. The fins fold into grooves in the body to leave the profile clean, the tail is a stiff crescent on a narrow stalk built to transmit force rather than bend, and the fish cannot stop swimming: it must move to push water over its gills, and it dies if held still.
The cost of all that machinery is oxygen and food. A yellowfin eats several percent of its body weight a day, grows to spawning size in two years, and is fished globally at a scale that makes it one of the most economically important animals in the ocean.
The species. Thunnus albacares (Bonnaterre 1788) - from Albicores. ETYFish
- metallic dark blue back, silver belly, bright yellow fins and finlets
- second dorsal and anal fins drawn out into long sickles in big fish
- a row of small yellow finlets along the tail stalk
- length up to 2.4 m
