Cook the spaghetti in a large saucepan of boiling, salted water until 'al dente' usually 8-10 minutes. Meanwhile, heat the olive oil in a large saucepan over a moderate heat and saute the garlic for 1 minute. Add the chopped tomatoes, sugar and seasoning and bring to a simmer.
Lower the heat and cook for 6-7 minutes, stirring occasionally. Add the mussel meat halfway through the cooking, then the baby capers, chopped parsley and anchovy. Drain the spaghetti when ready and reserve some of the cooking water.
Add the the spaghetti with a little of the cooking water to the sauce and toss well. Adjust the seasoning if necessary and transfer to serving plates. Garnish with the capers and serve.