Had to look up the plant that Nao reported on.....
It is called 慈姑 / クワイ/ Kuwai or Kwai.
arrowhead Sagittaria trifolia var. edulis
This plant, which grows in shallow water, has a bulb 3 to 5 cm in diameter, with a pointed protuberance about 4 cm long. To eat, it is peeled, boiled, and seasoned. It is often served as nimono (food that is boiled and seasoned) and features in the New Year o-sechi ryori. Kuwai is rich in protein and has a somewhat waxy texture.