Sea urchin sushi or uni sushi is usually made in to a ship roll (gunkanmaki). The reason for this is that the sea urchin is very loose and fragile that it can be disfigured if squeezed into a nigiri. The ship roll is not officially a traditional Edo style sushi, and it was only a solution to hold loose sushi ingredients. Many people say that the strong ocean scent of the seaweed (nori) interfered with the aromas of the sea urchin which totally goes against the traditions of Edo style sushi which is to intensify the flavors and aromas of the single and main ingredient.
A radical sushi chef thought of a way of making a ship roll with thinly sliced cucumbers(right image). This was a very good alternative to the regular ship roll since cucumbers have less aroma than seaweed. However, cucumber is considered an independent ingredient in Edo style sushi, and the water content slightly diluted the sea urchin’s flavors which created a different concern. The crisp texture from the cucumber was also another concern. Chefs continued to think of a way to make the sea urchin sushi without extra ingredients that can alter its taste and the best way was to divert back to the basics.
The only way to make the sea urchin into a nigiri (left image) was to constrict its flesh (sea urchin is a gonad) and the best way to do it was to use a mild acid. A marinade made of sudachi juice diluted with kombu broth and reduced rice wine was used to immerse the sea urchin for a few minutes and this helped tightening the sea urchin just enough to retain its creaminess and natural flavors. It is still loose, so the nigiri is quickly made by the tategaeshi (vertical flip) method.