In a blender, combine citrus juices, shoyu, and cilantro. Pulse until cilantro is minced. Combine the rest of the ingredient except rice and cucumber in a glass bowl. Add citrus mixture and toss. Refrigerator for 1 hour.
To assemble sushi, place 1/4 cup of sushi rice into a round cookie cutter. With damp fingers press rice firmly and evenly, forming a 1-inch disk. Layer sliced cucumbers on top rice and finish with 1/4 of the ceviche. Remove cookie cutter and serve immediately. Makes 4 servings.
Approximate Nutrient Analysis per serving (not including salt to taste):
190 calories, 0.5 g fat, 0 g saturated fat, 0 mg cholesterol, 250 mg sodium, 36 g carbohydrate, 1 g fiber, 6 g sugar, 10 g protein