As well as parasite destruction, super-freezing also prevents oxidation of the blood in tuna flesh, thus preventing the discoloration that happens at temperatures above −20 °C. Some forms of sushi, notably those containing pufferfish fugu and some kinds of shellfish, can cause severe poisoning if not prepared properly. Particularly, fugu consumption can be fatal. Fugu fish has a lethal dose of tetrodotoxin in its internal organs and, by law in many countries, must be prepared by a licensed fugu chef who has passed the prefectural examination in Japan. The licensing examination process consists of a written test, a fish-identification test, and a practical test that involves preparing the fugu and separating out the poisonous organs. Only about 35 percent of the applicants pass. Sustainable sushi is sushi made from fished or farmed sources that can be maintained or whose future production does not significantly jeopardize the ecosystems from which it is acquired. Concerns over the sustainability of sushi ingredients arise from greater concerns over environmental, economic and social stability and human health. Traditionally, sushi is served on minimalist Japanese-style, geometric, mono- or duo-tone wood or lacquer plates, in keeping with the aesthetic qualities of this cuisine. Many sushi restaurants offer fixed-price sets, selected by the chef from the catch of the day. These are often graded as "shō-chiku-bai" (), "shō/matsu" (, pine), "chiku/take" (, bamboo) and "bai/ume" (, plum), with "matsu" the most expensive and " ume" the cheapest. Sushi restaurants will often have private booth dining, where guests are asked to remove their shoes, leaving them outside the room; However, most sushi bars offer diners a casual experience with an open dining room concept. Sushi may be served "kaiten zushi" (sushi train) style. Color-coded plates of sushi are placed on a conveyor belt; as the belt passes, customers choose as they please. After finishing, the bill is tallied by counting how many plates of each color have been taken. Newer "kaiten zushi" restaurants use barcodes or RFID tags embedded in the dishes to manage elapsed time after the item was prepared. Some specialized or slang terms are used in the sushi culture. Most of these terms are used only in sushi bars. Unlike sashimi, which is almost always eaten with chopsticks, "nigirizushi" is traditionally eaten with the fingers, even in formal settings.