In a large bowl, combine egg and bread until the liquid is absorbed.
Add mayo, ginger, scallion, and soy sauce; stir.
Gently fold in crab and season with salt and pepper.
Shape mixture into eight 2 to 2 1/2 inch patties; cover and chill 15 minutes.
In a large nonstick skillet over medium heat, cook crab cakes in sesame oil, turning once, for 8 minutes or until brown and crisp (if necessary, cook in two batches, using 2/3 of the oil for the first batch and the rest for the second).
Make dressing by whisking together wasabi paste, vinegar, sesame oil, sour cream and soy sauce.
Toss arugula with 3 tablespoons dressing.
Divide greens onto four plates and top with two crab cakes.