Finely chop shallots.

Finely grate lemon zest to measure 1 1/2 teaspoons. Juice lemons to measure 3 tablespoons.

Fill a 6-quart pasta pot three fourths full with salted water and bring to a boil.

In a deep 12-inch heavy skillet cook shallots in butter with salt and pepper to taste over moderate low heat, stirring, until soft, about 5 minutes.

Stir in cream and zest and simmer, stirring occasionally, until slightly thickened, about 10 minutes.

Stir in 2 tablespoons lemon juice and remove skillet from heat.

Cook pasta in boiling water, stirring occasionally, until al dente.

Drain pasta in a colander and add to sauce with crab and remaining tablespoon lemon juice, and salt and pepper to taste. Heat mixture over low heat, gently tossing (and add a small amount of warm water if mixture becomes dry), until just heated through.

Serve pasta garnished with crab and lemon zest.