Red Ark Shell Clams are a specialty in Japanese cooking, prized as a sushi or sashimi ingredient. They are called "Akagai" in Japanese.
Though there are many types of Ark (or "peponita") clams found throughout the world (at least 200), Akagai in Japanese refers to the type which has not only a red-coloured shell, but reddish meat inside as well (actually more peach-coloured.) These clams have red blood, which not many clams do.
Ark clams have thick shells with many heavy ridges on them; the idea behind their English name may be that the ridges look like the timbered sides of drawings of Noah's Ark. The clams live off the coasts of Korea, Philippines and Japan. The clams lay their eggs in the late spring. The clams are best harvested in the summer when they have been able to develop a little fat to give them flavour. They are usually farmed now, as opposed to fished from the wild.