The lean meat of tuna has less fat so it is the part that easily demonstrates the quality in its taste. Top quality lean meat has a smooth texture, an elegant acidity, and there is a subtle sweetness and rich aroma in the fat. Whether it is raw and unaltered or soaked in soy sauce, you will get a sense of the chef’s artistry.
Trivia : Around 1840, a place called Ebisu-zushi in Edo / Bakurocho area sold nigiri "Zuke" which is red meat having been put in soy sauce. This became popular, and spread quickly throughout the city.
learn more about Zuke
learn more about soy sauce