Heat the oil in a large pot over medium-high heat.
Add the ginger, garlic and chiles and stir-fry until fragrant, 1 to 2 minutes.
Add the curry paste and ketchup and fry until slightly browned, 3 to 4 more minutes.
Add the lime juice and 3 cups water, cover and bring to a boil.
Add the clams; cover and steam until they open slightly, about 3 minutes.
Add the mussels; cover and steam until they open slightly, 3 to 4 minutes.
Stir in the bell pepper and half of the cilantro and basil.
Cook, covered, until the pepper is crisp-tender, about 2 minutes. Transfer the clams and mussels to serving bowls and ladle the broth on top.
Sprinkle with the remaining cilantro and basil and garnish with lime. Serve with crusty bread, if desired.