Freshwater clam. シジミ。Cyrenoidea (Corbicula).
Mainly used as an ingredient in miso-soup (like asari, short-necked clams, but never mixed). Also used as an ingredient in tsukudani (small seafood simmered in soy sauce and mirin and eaten on top of the rice). The meat of shijimi is small.
Shijimi are dark-colored bivalve molluscs with two halves of the shell equally sized. They are found in both fresh and brackish waters. The largest are 2-3 cm.
In fact there are three types native to Japan: Yamato-shijimi (brackish water), Ma-shijimi and Seta-shijimi (both fresh water), The last type come exclusively from Lake Biwa near Otsu, the other two types can be found all over Japan. Yamato-shijimi are most common, the Seta-shijimi are said to be most tasteful. Most important production sites are: the Jusanko and Ogawarako lakes in Aomori, the Kitakami River in Miyagi, the Hinemagawa and Tonegawa rivers in Ibaraki and - perhaps most famous - Lake Shinji next to Matsue in Shimane.
Japanese waters have, however, been invaded by foreign species as the Taiwan-shijimi and it is often unclear which type one is buying. Also, the number of imported shijimi has risen considerably.
Folk wisdom says shijimi are good against a hangover, and after a night drinking, many Japanese would the next morning have miso soup with shijimi for breakfast. Also said to help against tiredness in summer (natsubate).