Pour oil to a depth of 1 inch into a large Dutch oven, and heat to 350°.
In a medium bowl, combine flour and salt.
Stir yolk into soda water in a 2-cup measure until blended; pour over flour mixture, and, using a large fork, gently combine (the batter should remain slightly lumpy and should have the consistency of heavy cream).
Dip 2 crabs into batter, letting excess batter drip back into bowl, and carefully slip into the hot oil, top-side down. Fry 1 1/2 minutes per side, until golden brown and crisp. Drain on paper towels, then transfer to a baking sheet, and keep warm in a 200° oven. Repeat with remaining oil and crabs.
Garnish with toasted sesame seeds and serve with Asian Slaw and Crab Dipping Sauce.