For thousands of years, populations living close to the sea have found shellfish an easily obtainable and convenient source of protein and trace minerals. Shellfish is the general term for crustaceans (shrimp, crab, lobster) and mollusks (clams, oysters, squid and octopuses). All of these shellfish (kokakurui or kai) have been an important part of the Japanese diet for centuries and to this day remain a highly prized component of the national cuisine.
Bivalves and gastropods together account for more than 70,000 species of mollusks; their habitats range from the deep ocean floor to the tops of the highest mountains. Bivalves are organisms with twin shells and are principally marine. Washoku incorporates dozens of bivalves from hard clams (hamaguri), short-neck clams (asari) and cockles (torigai) to oysters (kaki), scallops (hotategai) and Japanese mussels (igai).
The mussels traditionally eaten in the West (murugai) are not commonly used in traditional Japanese food, but they are nonetheless enjoyed in Japan and are served in some restaurants. Scallops and oysters are eaten raw on occasion, but as a general rule most bivalves are cooked — albeit briefly — before being eaten.