Shellfish
Ark shell (Akagai)
【Nigiri sushi: Kai】
Ark shell (Scapharca broughtonii, known as akagai in Japanese) is found from Hokkaido to Kyushu and the Korean Peninsula. They live buried in shallow sandy-mud bottoms at depths of 10 to 50 meters in inner bays. Their shell length ranges from 7 to 12 cm. The shell has about 42 deep grooves and is covered with bristles. As the name “red clam” suggests, the flesh and adductor muscle inside the shell are a vivid reddish-orange color. This is rare among marine organisms and is due to the presence of blood containing hemoglobin within their bodies.
What does Akagai (Ark shell) nigiri sushi taste like?
Akagai was once abundant in the waters of Edomae (present-day Tokyo Bay), and those harvested from Kemigawa, Chiba Prefecture, were regarded as the finest and were highly prized. For this reason, until the early Showa period, akagai was sometimes referred to as “Kemigawa.” Its lustrous, vivid red color is considered a sign of freshness, and it is regarded as a premium sushi topping for its distinctive briny aroma and satisfying texture.
However, because akagai begins to develop eggs around the time of the spring equinox, its flesh is said to become leaner and its flavor to decline.
The finest akagai are large and meaty. It is essential to keep them alive and leave them in their shells until just before preparation. After opening the shell, the flesh is removed and separated from the shell, and the edible portion is also known as “tama.” Just before making the sushi, the flesh is gently slapped against a cutting board to firm up the still-living, tender meat, creating a pleasantly resilient texture.
In addition, kazari-buocho (decorative knife cuts)—which resemble a butterfly—are made to gently open the flesh. This allows it to adhere better to the rice and enhances the harmony between the topping and the sushi rice when eaten. With akagai that are no longer alive, the flesh does not swell at the cut surface, resulting in a less attractive presentation.
On the other hand, the mantle (also called “himo”) is often valued even more highly than the main flesh itself, and many sushi connoisseurs appreciate its firm texture and intense briny aroma. Both the flesh and the muscle are rubbed with salt and briefly dipped in vinegar just before being prepared as sushi to remove any undesirable flavors that develop after handling. However, some sushi chefs deliberately serve them raw to preserve akagai’s natural aroma.
It is precisely this distinctive flavor—a subtle iron-like aroma intertwined with the freshness of the sea—that makes akagai so appealing, and it is also why opinions about it tend to be divided.
Also called Bloody clam or red clam.
【Temperature of vinegared rice (shari) and sushi toppings】
36℃/18～20℃
【Substitute fish】
Half-crenated ark: Anadara kagoshimensis (Tokunaga, 1906)
Bloody clam: Anadara satowi (Dunker, 1882)
Atlantic bay scallop: Argopecten irradians irradians (Lamarck, 1819)
【Related contents】
What is Ishigakigai sushi?
What is Himokyu-maki?
List of Shellfish (Kai)
(Revision date: June 17, 2026)