Bring a canning pot filled with water to a boil, about 30 minutes. Using tongs, grasp rear of crab (claws away from you). Lower crab into pot; cover. Repeat with 1 more crab; cover. Bring water back to boiling, then cook 8 minutes for 1 1/2- to 2-lb. crabs, 10 minutes if larger. Or, in a large stockpot, boil 1 crab at a time (5 minutes for the smaller size, 8 for the larger).
In a small saucepan, melt butter with Old Bay over medium-high heat and bring to a simmer. Reduce heat to low and skim foam until only clear liquid is left. Stir in lemon juice and salt.
Using tongs, transfer crabs to a colander to drain and cool; meanwhile, cook remaining crabs. Set cooled crabs belly up on a rimmed baking sheet, then clean and crack.
Serve on a platter with citrus, parsley, and the seasoned clarified butter.