typically, varieties of fish. For culinary, sanitary, and aesthetic reasons, the minimum quality and freshness of fish to be eaten raw must be superior to that of fish which is to be cooked. Sushi chefs are trained to recognize important attributes, including smell, color, firmness, and freedom from parasites that may go undetected in a commercial inspection. Commonly used fish are tuna (), Japanese amberjack, yellowtail (), snapper (), mackerel (), and salmon (). The most valued sushi ingredient is , the fatty cut of the fish. This comes in a variety of (often from the bluefin species of tuna) and , meaning "middle toro," implying that it is halfway into the fattiness between and the regular cut. style refers to nigiri sushi, where the fish is partially grilled (topside) and partially raw. Most nigiri sushi will have completely raw toppings, called . Other seafoods such as squid (), eel ( and ), pike conger (), octopus (), shrimp ( and ), clam (, and ), fish roe (, , and ), sea urchin (), crab (), and various kinds of shellfish (abalone, prawn, scallop) are the most popular seafoods in sushi. Oysters are less common, as the taste is thought to not go well with the rice. , or imitation crab stick, is commonly substituted for real crab, most notably in California rolls. Pickled daikon radish () in , pickled vegetables (), fermented soybeans () in , avocado, cucumber in , asparagus, yam, pickled (), gourd (), burdock (), and sweet corn (sometimes mixed with mayonnaise) are plant products used in sushi. Tofu, eggs (in the form of slightly sweet, layered omelette called ), and raw quail eggs (as a topping) are also common. Condiments Sushi is commonly eaten with condiments. Sushi may be dipped in (soy sauce), and is usually flavored with wasabi, a piquant paste made from the grated stem of the Wasabia japonica plant. Japanese-style mayonnaise is a common condiment in Japan on salmon, pork, and other sushi cuts. The traditional grating tool for wasabi is a sharkskin grater or . An imitation wasabi (), made from horseradish, mustard powder, and green dye, is common. It is found at lower-end restaurants, in bento box sushi, and at most restaurants outside Japan. If manufactured in Japan, it may be labelled "Japanese Horseradish." The spicy compound in both true and imitation wasabi is allyl isothiocyanate, which has well-known