“Ubagai” or more commonly called “Hokkigai” when served as sashimi or sushi have many names in English.
Member of the Trough Shells Groups, they are called Surf Clams, or more precisely, as pertains to the varieties eaten in Japan, either Japanese Surf Clams or Sakhalin Surf Clams as they are collected both along Japan and Sakhalin Islands shores

8,000 tonnes are caught in Japan every year. 94% of fresh/live Surf Clams are collected in Hokkaido, Aomori, Fukushima and Miyagi Prefectures.
About 4,000 tonnes are imported frozen from Canada.
They are popular dried, in soups or cooked with vegeatbles and rice.

Their “tongues” can be appreciated as sashimi, but are most popular lightly poached and cooled down.

That is the way they are usually processed before being served as sushi nigiri, either straight, or with a few small incisions for better effect!