What mollusk is called tako when served as sushi?

Tako is octopus when it appears as a sushi ingredient.

The Japanese word tako, written 蛸 or たこ, refers to octopus. In sushi restaurants it may appear as sliced octopus nigiri, octopus sashimi, or as a topping in other preparations. Unlike many sushi fish, tako is commonly cooked before serving, often by boiling or simmering it until the flesh becomes tender.

Octopus is a familiar ingredient throughout Japanese cuisine. It is used in sushi and sashimi, but also in takoyaki, karaage, stews, sour salads, and akashiyaki. Takoyaki—literally associated with grilled octopus—is a popular Osaka snack made with batter and pieces of octopus.

The most common mix-up is squid. Squid is ika in Japanese, while tako means octopus. Both can have a firm, springy texture and appear in sushi assortments, but they are different animals. Cuttlefish and squid may also be listed separately, making the distinction useful when reading a Japanese menu.