About "Katsuo"
"Katsuo" is Striped tuna, Skipjack tuna, Oceanic bonito or technically called Katsuwonus pelamis or Linne.
This salt-water fish is found in the coast of Japan and the tropical and temperate sea zones across the world. "Katsuo" of "Katsuobushi" refers to the fish used in Japanese. "Katsuo" is widely known as the mother of "Dashi" in the Japanese food culture.
Katsuo's gill cover is stationary; therefore, it must swim to capture oxygen by running the sea water through its gill slits. "Katsuo" is a great swimmer because of its relatively high body temperature, as a result of its active metabolism. And this is largely due to how the Katsuo's blood vessel functions. Oxygen from the sea water that it captures through the gills is cold in temperature. Its blood vessel can transfer heat from its metabolism-warmed blood to the cold oxygen-rich blood; therefore, Katsuo's blood temperature can stay constant.