Heat a medium saucepan over medium heat. Coat pan with cooking spray. Add shrimp shells to pan; cook 3 minutes, stirring frequently.
Add 1/2 cup celery, 1/2 cup carrot, and 1/2 cup onion to pan; cook 1 minute, stirring occasionally.
Stir in 2 1/2 cups water, peppercorns, and bay leaf; bring to a boil. Reduce heat, and simmer 30 minutes, stirring occasionally.
Strain mixture through a sieve over a bowl; discard solids.
Heat oil in a large saucepan over medium heat. Add remaining 1/2 cup celery, 1/2 cup carrot, and 1/2 cup onion to pan; cook 3 minutes, stirring occasionally. Add bell pepper; cook 1 minute, stirring occasionally. Stir in tomato, garlic, curry paste, and rind; cook 2 minutes, stirring frequently. Stir in broth mixture and coconut milk; bring to a boil.
Add clams and mussels. Cover, reduce heat, and cook 2 minutes or until clams and mussels open. Remove from heat; discard any unopened shells.
Stir in shrimp, basil, and remaining ingredients except lime wedges. Cover and let stand 5 minutes or until shrimp and halibut are done. Discard lime rind.
Place 2 clams and 2 mussels in each of 6 bowls. Divide shrimp and fish evenly among bowls. Ladle 2/3 cup broth mixture over each serving. Serve with lime wedges, if desired.