We call it "Kuwai"（慈姑）in Japanese, and it's a popular ingredient in Japanese cooking. We eat the tuber of it, by peeling and cooking it. There are different ways of cooking the tuber, including simmering and deep-frying.
I cut off the bottom stem and peeled the skin by a pairing knife. The tip of bud should be cut off by leaving about 1/2" (1.2 cm) long. I blanched them for a few minutes (to remove the bitterness), then cooked in a broth - 180 ml dashi stock, 1 tablespoon sake, 1/2 tablespoon sugar, 1 tablespoon miring, and 1 1/2 tablespoon light-color soy sauce ("usukuchi shoyu") in small classic-style donabe, "Hakeme" for about 15 minutes.
These cute tubers came out so tender and delicious. "Kuwai" is one of the traditional ingredients for Japanese New Year dishes ("osechi ryori") because it has the but shooting upward which symbolizes good luck. So, I'm glad I got to make myself a good luck dish this new year.
Happy donabe life.