Thaw shrimp, if frozen. Preheat broiler. Rinse shrimp; pat dry with paper towels. Coarsely chop shrimp; set aside. In a medium bowl combine 1 tablespoon of the oil, the lemon juice, chives, basil, snipped mint, and garlic. Add shrimp, crabmeat, tomatoes, and onion; toss gently to coat.
Cut bread into 48 thin slices. Arrange bread slices on a large baking sheet. Brush one side of slices with the remaining 2 tablespoons oil; sprinkle lightly with pepper. Broil 3 to 4 inches from the heat for 2 to 3 minutes or until toasted, turning once.
Using a slotted spoon, spoon seafood mixture onto oiled sides of toasted bread. If desired, garnish with mint sprigs. Serve immediately.