Akami is a part of bluefin tuna that refers to lean red meat. This is the main part of the fish, so it's more readily available than the other parts, called chūtoro and otoro. It has the least fat out of the whole tuna, which makes it the most affordable.

Akami is typically used atop rice in sashimi or nigiri. In a sushi restaurant, if you order maguro (a broad term for tuna), you will get akami from whichever species of tuna they have on hand. This part of tuna also has the most umami tuna flavor of all other, fattier cuts.

Otoro is a part of bluefin tuna that is the most desirable. It comes from the inside of the belly and it's the fattiest part of the fish. The texture is often described as intensely rich, marbled, and having melt-in-the-mouth properties. Due to these facts, it's traditionally the most expensive part of bluefin tuna.

It's often used in sashimi and nigiri, giving the dish a sweeter flavor. Otoro is the best during winter, once the tuna acquires additional body fat. It should always be consumed fresh as it doesn't last too much when frozen. This part of tuna is rarely found outside of Japan.

Unagi is the Japanese word referring to the freshwater eel, commonly used in Japanese cuisine. It is mainly consumed in the form of fillets, which are always grilled due to the presence of harmful toxins. The fillets are then spread with a sweet and salty sauce made with soy sauce, mirin, and sugar.

Although unagi is consumed in the entire country, there are slight regional differences in the way the eel is cut and prepared. In Kanto region, the cuts are made from the back, and the fillets are grilled and steamed, while in Kansai the eel is cut from the stomach and the fillets are merely grilled. 

Tarako is a type of roe mostly harvested from Alaska pollock, but since the fish is closely related to cod, it is often labeled as cod roe. It is a type of salted roe with a light pink color, incredibly tender texture, and a mild and neutral flavor.

This roe can be eaten plain or incorporated into various dishes. It is often used as a topping on rice dishes and sushi, but it can also be cooked and used in sauces. When salted and marinated in spices, it goes under the name mentaiko.

Chūtoro is a part of bluefin tuna that refers to medium fatty cuts when served as a part of nigiri or sashimi. The tuna yields otoro (fatty parts from the belly), akami (red meat), and chūtoro, which is a combination of both that's found near the skin on the back and belly.

It's a mix of otoro's sweetness and a deep, slightly bitter flavor of akami. This part of bluefin tuna is usually a bit more on the expensive side, and it's typically served on special occasions or festivities. Specifically, the part of the back near the head is called sekami, the center of the back is senaka, and the part near the tail is called seshimo.

Ikura (also known as salmon roe) are round, almost transparent salmon eggs, an unusual ingredient which is especially popular in Japan. After they have been removed from the sack, creating individual plump orbs, the eggs undergo the curing process which usually includes salt or brine.

During the Taisho period in Japan, which started in 1912, the Russians introduced the tradition of curing salmon roe to the Japanese. However, the Japanese adjusted the recipe to their tastes and started using fragrant soy sauces, sake, and rice wine for curing, giving the roe a deeper and distinctively Japanese flavor. 

One of the essential ingredients in Japanese cuisine, katsuobushi or okaka refers to dried, fermented, and wood-smoked skipjack tuna. It is typically available pre-packaged in the form of delicate shavings which are used for imparting not only an intense umami flavor to various dishes but also the so-called kokumi - another taste sensation that can be described as heartiness and mouthfulness.

The production of katsuobushi is known to be complex and time-consuming, which contributes to its price tag. Also, the expensive skipjack tuna is sometimes substituted with its less pricey relative called bonito fish, which is why katsuobushi is often dubbed bonito flakes, and even though real katsuobushi is a far more superior product, both names are used interchangeably. 

Uni is a Japanese name for sea urchin gonads, an organ which produces the animal's roe. It is the only edible part of the sea urchin, commonly consumed in various European, South American, and Asian countries. The texture of uni should be firm, creamy, and smooth, while its flavor is rich and briny.

Uni is usually found at local markets or restaurants, and it can be easily incorporated into a variety of dishes. However, it should always be consumed fresh, preferably just out of the shell. In most countries, it is eaten raw, usually with a splash of lemon juice. 

Mentaiko or myeongnanjeot a term describing whole sacks of marinated Alaskan pollock roe, commonly eaten in Japan and South Korea. Quite often, it is mistaken for tarako (salted version), and since Alaska pollock is closely related to cod, the product is often labeled as cod roe.

The marinating process contributes to its savory, salty, and somewhat spicy flavor, which perfectly combines with its creamy texture. It originated in South Korea, where it has been enjoyed since the 17th century. After WWII, it became available in Japan, particularly among the traders in the region of Hakata, and soon it became a prevalent ingredient nationwide.