How to Make It
In a small saucepan, combine the sake, mirin, soy sauce and sugar and boil over moderate heat until reduced to 1/2 cup, 15 to 20 minutes. Transfer the dressing to a bowl to cool.
Bring a large pot of lightly salted water to a boil. Add the shrimp and simmer until they are pink, curled and just cooked through, about 5 minutes. Using a slotted spoon, transfer the shrimp to a bowl of cold water to cool. Drain them thoroughly and pat dry.
Return the water in the saucepan to a boil. Add the asparagus and cook until just tender, about 2 minutes. Refresh under cold water, drain and pat thoroughly dry. Cut the asparagus on the diagonal into 2-inch lengths.
Peel the shrimp, leaving the last section of the tail shell on. Using a thin sharp knife, slice each shrimp into thirds lengthwise, stopping when you reach the tail. Remove and discard the dark intestinal veins. Refrigerate the shrimp until chilled.
Soak the wakame in a large bowl of cold water until pliable and tender, about 5 minutes. Drain and rinse the wakame, squeezing out as much water as possible. Toss the wakame with all but 2 tablespoons of the dressing and mound it on 8 salad plates. Fan out the shrimp on top of the wakame and arrange the some of the asparagus alongside. Drizzle with the remaining dressing. Garnish each plate with a small spoonful of wasabi and mayonnaise and serve.