From http://www.epicurious.com/recipes/food/views/Thai-Spiced-Watermelon-Soup...

For soup
5 cups coarsely chopped seeded watermelon (from a 4-lb piece, rind discarded)
1 fresh lemongrass stalk*
3 tablespoons finely chopped shallot
1 1/2 tablespoons finely chopped peeled fresh ginger
1 tablespoon finely chopped garlic
1 1/2 tablespoons mild olive oil
1 small hot green chile such as Thai or serrano, finely chopped (including seeds), or to taste
2 tablespoons fresh lime juice, or to taste
3/4 teaspoon salt, or to taste

For crab