After about 8 – 9 minutes not only should the pasta be just about al dente, but the mussels should be just about opened. Drain the pasta, reserving a little water just in case, before dumping it into the pan with the mussels. Toss everything to combine, adding a bit of pasta water as need to help coat the spaghetti. The mussels will finish opening in the residual heat of the pan (although, any that haven’t opened at all should be discarded).