The ark shell , or akagai (red clam) is a very rare clam these days. They were abundant in the Edo (Tokyo)bay in the early days but they are decreasing in numbers. The ark shell is valued by its color, which is due to high hemoglobin content. Some of them are caught in the Miyagi prefecture (southern Japan) which are all bought by high-end sushi restaurants. The majority of arc shells are replaced with similar clams of the same genus, which are imported from China, Russia,and South Korea. They are similar in appearance, and the only way to distinguish between them is to count the vertical slits on the surface of the arc shell. The original ark shell used for Edo style sushi has 42 lines, and the other similar clams have 38. The imported versions are yellow in color, and the taste is lighter, but good all around.
Arc shell tastes the best during late winter and spring.