Heat the oven to 375°F and spread 1 cup marinara in a 9-by-13-inch baking dish.
Boil the shells for 2 minutes less than the package directs, drain them and separate them on a baking sheet.
Fold the ricotta, crab, artichokes, spinach, egg, lemon zest, salt, pepper and 1/2 cup mozzarella together without breaking up the crab too much.
Fill each shell with about 2 tablespoons of the crab mixture and arrange the shells open-side up in the dish.
Spoon the remaining marinara around and lightly over the shells and add the remaining mozzarella.
Cover with foil and bake for 20 minutes, then uncover and bake for 10 to 15 minutes, until bubbling and 165°F in the filling.
Rest for 8 minutes before serving.