Bring a large pan of salted water to the boil. Cook the spaghetti according to the packet instructions.
While you are waiting for the spaghetti to cook, in a large frying pan, heat the oil and add the garlic, fennel seeds, and cook over a medium heat until the garlic starts to turn golden brown. Remove from the heat and add the chilli, crab, lemon zest, juice and parsley.
Once the pasta is cooked, drain thoroughly and add to the pan with the crab sauce, season with sea salt and plenty of black pepper, mixing well. Divide between 4 warm bowls, sprinkle over the radishes and serve at once.