Combine the flour with 1 teaspoon of the rub in shallow bowl. In another shallow bowl, combine the bread crumbs with the remaining 4 teaspoons of rub. In a third bowl, mix together the eggs with the water.
Dredge the crabs in the flour, dip into the egg mixture, then dredge in the bread crumb mixture, coating evenly.
Heat the shortening to 360º F.
Hold the body of the crab with the claws and legs hanging down over the hot fat and carefully drop it in. Repeat with the remaining crabs. Fry until golden brown, flipping the crabs with tongs 2 to 3 times to brown evenly. The crabs will float to the surface of the oil when they are cooked. Drain on paper towels.
To serve, place a spinach cake in the center of each dinner plate. Put a crab on each cake, then top with the hollandaise.