Unagi-sabaki Eel knife
In Japanese, 'unagi' is eel, 'sabaku' means to separate, sort or 'deal with'.
There are very different knives under the unagi-sabaki label, most are associated with a region of Japan and suit the way the fish is prepared there. The primary influence is whether the eel is opened along the spine or along the belly in the regional cuisine. Any eel knife will be effective for either cut, the differences are most relevant for a specialist preparing eels all day long.
A quick summary:
Edo-saki eel knives are the most common style. The familiar triangular-tipped, dual-edge unagi-sabaki bocho is a professional tool for Kanto-style preparation of this fine delicacy, where the fish is deftly sliced along the spine from end to end and the head, fins and tail removed. 'Kanto' refers to the region of the capital Tokyo, or Edo, hence Edo-saki.
Kyo-saki or Kyoto-gata eel knives are associated with the city and culinary centre of Kyoto. These knives have a tall, short blade and a proud spine that acts as an angle-guide for sharpening. Some Japanese websites state that the spine is useful for tapping the eel-pin into the cutting board. Hmmm.
Nagoya-saki eel knives look much like a bamboo knife - small, slender and thick - and are said to be the most traditional form of Japanese eel knife. The handle is comparatively long, the blade is rounded for safety. Those eels are slippery little things.
Kyushu-gata eel knives appear to vary in profile from smith to smith, generally they are short and solid and shorter and taller in the blade compared to the Nagoya style. Some combine the rounded point with an Edo-saki-like two-stage edge. Interesting.
Osaka-saki eel knives look a lot like a beefy kiridashi. No handle.
There's one more, the dojou eel knife. This is a compact version of the Edo-saki with a blade as short as four or five inches. Not sure if it's for cutting dojou (pond loach) or is so named because it's around the same length sans handle. Dojou are the little eel-like fish that are placed swimming in a pot and then burrow into cool tofu as the water is brought to the boil.
Some eel trivia for you. Leonardo da Vinci was a fan, apparently. His grocery receipts and the inclusion of eels with slices of orange on the table of The Last Supper support the theory. We like the idea that in 1498, while Leonardo was painting his masterpiece in Milan, throughout Japan the forebears of today's blade smiths were creating masterpieces of their own, in steel.