Spiced Crabs and Shrimp With Potatoes

Name: Spiced Crabs and Shrimp With Potatoes
Ingredients: 2 tablespoons dill seeds; 2 tablespoons dried dill; 2 tablespoons mustard seeds; 2 tablespoons chili powder; 2 tablespoons sweet paprika; 2 tablespoons coriander seeds; 2 tablespoons white peppercorns; 2 teaspoons celery seeds; 2 teaspoons red pepper flakes; 3 bay leaves; 10 sprigs thyme; 1/2 bunch parsley; 1 head garlic, cut in half horizontally; 4 leafy stalks celery, halved crosswise; Kosher salt; 12 live jumbo blue crabs or 6 Dungeness crabs; 1 1/2 pounds new potatoes; 2 pounds extra-large shrimp (unpeeled); Melted butter, for dipping