Unagisaki hocho
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A Unagisaki hocho (鰻サキ包丁?) is a knife specialized for filleting eel. The sharp tip is pushed into the eel near the head, and then slid along the body of the eel to open up the entire length of the fish. Besides the standard version as shown in the picture, there are also local styles that differ significantly for Nagoya, Osaka, and Kyoto.
See also Japanese kitchen knives and the list of Japanese cooking utensils.