_________, also known as omusubi, nigirimeshi, or rice ball, is a Japanese food made from white rice formed into triangular or cylindrical shapes and often wrapped in nori. Traditionally, this is filled with pickled ume (umeboshi), salted salmon, katsuobushi, kombu, tarako, mentaiko, takanazuke (pickled takana) or any other salty/sour ingredient as a natural preservative. Fill up the blank.