Offer this pale, smooth soup for a first course. Although each serving contains a generous portion of succulent pink shrimp, the flavor is too delicate for a main course.
Peel shrimp; combine shells in a saucepan with the celery and water. Simmer, covered, for 20 minutes. Meanwhile, devein shrimp and halve lengthwise. Cut chayotes into large chunks, reserving one-quarter of one. Strain shrimp stock.
Heat 1½ tablespoons butter in a casserole; add shallots and stir over low heat until somewhat softened. Add flour; stir for 2 minut