Place oil and whole garlic in large saucepan over low heat and cook slowly until garlic is golden, about 5 minutes. Discard garlic. Add tomatoes, tomato paste, parsley, salt and pepper and simmer, covered, for 20 minutes.
Start cooking spaghetti so it is ready by the time the seafood is cooked. Add octopus, fish and prawns and simmer 3 to 5 minutes until tender and no longer translucent. Stir in scallops and mussels and simmer, covered, for a further 2 to 3 minutes until mussels open; discard any that do not open. Toss hot cooked spaghetti with sauce and serve immediately.