Spun up a quick image showing just where akami, chutoro and ohtoro are cut from a Bluefin Tuna.
Akami is what many sushi lovers outside of Japan know as “maguro” or simply tuna. Not so expensive and is used in a variety of non-sushi dishes.
Chutoro is the fattier variant of akami, with noticeably lighter color (light red to pink). Often consumed as sashimi (sliced raw fish) as well as sushi.
Ohtoro is the fattiest part of the tuna with a clearly marbled appearance (similar to how Kobe beef and other domestic steaks are marbled in fat) and allegedly can only be taken from the fattiest bellies of Kuromaguro and Minamimaguro Tunas. Being expensive and sweet, sushi shops normally don’t stock a lot of Toro or Ohtoro since it doesn’t pay off as well as cheaper-to-procure high-selling fish like kohada, buri, sake, etc. Sushi eaters who start with bland sushi and end with sweet or strong tasting sushi will finish their meal with ohtoro, so economics tell sushi chefs not to procure it in high volumes.
In the diagram below, assume it is of a Kuromaguro which has a thick layer of fatty flesh between its akami and chutoro innards and the ocean (separated by the white dotted line). In real life however, the appearance of these three representative cuts of tuna can be very different depending on the type of tuna they come from – for example, if I see a piece of tuna sashimi labeled chutoro with white fatty fibrous stripes, I can pretty much tell it is not a Kuromaguro a.k.a. Honmaguro (Pacific Bluefin Tuna) but possibly a Mebachi “”Bigeye Tuna or Kihada “Yellowfin” Tuna. Of course, one could always ask the seller or look at the label…
Sometimes you may hear use of the term “Toro” in general (with the leading “Oh” dropped). This refers to fatty maguro meat with the gelatin-like sinews that are rich in collagen. Toro is available in most sushi shops, grocery stores and fish markets.
But, the popularity of tuna cannot be disputed as akami and chu-toro are often included among the Japanese’ favorite sushi (let’s assume they are used in several sushi variants including tekka-maki, negitoro-maki, etc).
(Edit: Otoro is cut from the underside of the fish (the belly) while chutoro is cut from fatty parts closer to the dorsal region. Please refer to the image below more like a color guide, not as an anatomically correct cross-section of a Bluefin maguro.)
Author’s note: “Chiai” is the darker rather bloody meat that you don’t normally see served. The akami referred to in the image above is pretty dark but please don’t think I forgot about chiai.