Tuna are high up in the food chain. They eat animals that ate animals. Their diet leads to the accumulation of heavy metals in their flesh.
The flesh of the tuna is pink to dark red. This is because tuna muscle tissue contains greater quantities of myoglobin. Myoglobin is a molecule that binds oxygen. Some of the larger tuna species such as the Northern bluefin tuna can raise their blood temperature above the water temperature with muscular activity. Yellowfin tuna is often marketed as ahi.

What kind of tuna you use for sushi?
When Japanese say tuna for sushi, it usually means, Maguro, tuna with red flesh. There are mainly three kinds of Maguro used for sushi: Yellowfin, Big Eye and Blue Fin. Albacore Tuna is also used for sushi, and they are called white tuna.

Mercury levels can be relatively high in some of the larger species of tuna such as bluefin and albacore. As a result, guidelines recommend pregnant women, nursing mothers, and children to limit their intake of tuna and other types of predatory fish . Canned tuna is usually made from albacore, while fresh tuna is usually yellowfin, or bluefin. However, most canned light tuna is skipjack tuna and is very low in mercury.

The Best Canned Tuna In North America - packed in water for Sandwiches and Salads
The three most common types of canned tuna you'll find are skipjack, albacore, and yellowfin. According to the National Fisheries Institute, the average American eats 2.2 pounds of canned tuna annually. Currently, the Best Canned Tuna are:
Wild Planet
Pole & Line at Whole Foods if you live near a Whole Foods Market.
OTHER CANNED TUNAS ARE
Crown Prince Solid White Albacore Tuna in Spring Water
Safe Catch Wild Albacore Tuna
Trader Joe's Albacore Tuna in Water, Salt Added
Bumble Bee Solid White Albacore
StarKist Solid White Albacore Tuna
American Tuna with Sea Salt