Sushi toppings are not as easy to classify as “fish means raw” or “a different color means it has been cooked.” Japanese sushi can be served raw, boiled, steamed, simmered, seared, cured with vinegar, or marinated in sauce. Many of these differences are difficult to recognize just by looking.
Tuna and salmon are often served raw, but the name of the fish alone does not tell you how it has been prepared. Zuke-maguro is tuna that has been marinated in sauce, while aburi salmon has been seared on the surface. Shrimp and octopus may be boiled or steamed, but they can also be served raw. In other words, you cannot tell whether a topping is raw or cooked simply from the name of the seafood.
Cured toppings such as shime-saba can be even more confusing. Shime-saba is usually salted and then cured with vinegar. To many Japanese people, it feels different from both fish served raw without being cured and fish that has been cooked. But curing with vinegar does not cook the fish. “Cured” should not be mistaken for “cooked.” The same applies to marinated fish: having flavor added does not necessarily mean that heat has been used.
When trying to understand a sushi topping, look not only at the name of the fish but also at words such as boiled, steamed, simmered, seared, cured, or marinated. These give you a better idea of what has been done to it. Even then, it may be impossible to tell just by looking whether a seared topping is cooked all the way through or whether a marinated piece of fish is still raw inside.
If you need to avoid raw food completely, even a Japanese customer may simply ask the staff, “Is this raw?” Sushi can include raw toppings, cooked toppings, vinegar-cured fish, marinated fish, and pieces that have only been seared on the surface. It is difficult to identify every one of them by appearance alone.
