FISH AND SHELLFISH salads used to be unglamorous concoctions of leftovers. Today, however, with the growing popularity of light, fresh-tasting dishes, seafood salads are often the main attractions on the menu. Chefs seem to be trying to outdo each other, with such creations as Michel Guerard's salad of prawns with snow peas, asparagus and morel mushrooms, or the Troisgros brothers' crayfish salad with a creamy vinaigrette flavored with crayfish essence. Almost every chef has his treasured version of lobster salad, usually served simply on a bed of seasonal greens with a dressing accented by fresh herbs.

Yet great chefs should not be allowed to monopolize elegant seafood salads.. They are quick and easy to prepare at home, which makes them ideal for summer meals, whether casual or festive. Depending on the size of the portions, they can be served as an appetizer or a main course. Best of all, the "fruits of the sea" (as the French call them) are rich in flavor but low in calories.

An endless variety of salads can be composed from fish and shellfish because they are good partners for rice, pasta and nearly all vegetables. In France, steaming is the preferred technique for cooking seafood for salads because it keeps in its delicate, natural taste. A favorite dressing is the traditional oil-and-vinegar vinaigrette, because it is light and clear and allows the colors of the ingredients to show. Good quality olive oil, walnut oil or hazelnut oil adds a special touch to the dressing.

There is a reason for the current fancy for warm seafood salads. Fish and shellfish served immediately after cooking taste best and provide an exciting contrast to crisp, cool greens. The seafood can also be served at room temperature but loses flavor when refrigerated. Rice salads containing seafood are an exception and benefit from being chilled.

When creating your own seafood salads, try to add at least one colorful vegetable. If the vegetable is to be cooked, keep it slightly crisp. Be especially careful not to overcook the fish or shellfish. Drain both seafood and vegetables thoroughly so their cooking liquid won't dilute the dressing.

Most of all, remember that salad ingredients cannot hide behind thick sauces or behind anything else. There is no way to disguise less than fresh seafood in a salad. Buy the best quality and the freshest fish and shellfish possible, and cook and season it with care. SCALLOP SALAD WITH WALNUTS AND WALNUT OIL DRESSING (4 appetizer servings or two main course servings) 12 leaves leaf lettuce 2 teaspoons walnut oil 1/2 cup walnuts Salt and pepper 2 carrots, peeled 3/4 pound sea or bay scallops For the vinaigrette: 2 tablespoons white wine vinegar 6 tablespoons walnut oil Salt and pepper

Wash the lettuce and dry thoroughly. Chill until ready to use.

Heat 2 teaspoons walnut oil in a small frying pan over low heat. Add the walnuts and a pinch of salt and saute' lightly for 2 minutes over low heat. Be careful not to let them burn. Transfer to a bowl and keep at room temperature until ready to use.

Cut the carrots in thin lengthwise strips. Put in a small saucepan and cover generously with water. Add a pinch of salt and bring to a boil. Boil 1 minute. Drain thoroughly. Keep at room temperature until ready to use.

To make the dressing: In a small bowl, whisk the vinegar with the walnut oil and salt and pepper to taste.

Just before serving, tear the lettuce leaves in two or three pieces and arrange them on four plates or on a platter.

Put the scallops in a steamer above boiling water. Season lightly with salt and pepper. Cover and steam about 3 minutes or until just tender. Drain quickly on paper towels.

Whisk the dressing again and spoon about one third over the lettuce. Arrange the scallops on top in a ring and spoon all but 1 tablespoon of the remaining dressing over them. Put the carrots in the center of the ring and spoon the remaining dressing over them. Sprinkle the walnuts around the ring of scallops. Serve immediately. SHRIMP AND RICE SALAD WITH CELERY (Makes 4 appetizer or 2 main course servings) 1 cup long-grain white rice Salt and freshly ground pepper 2 cups water 10 ounces medium raw shrimp, unshelled 4 celery stalks 1 tablespoon chopped parsley 2 tablespoons white wine vinegar 6 tablespoons olive oil 2 tablespoons mayonnaise, preferably homemade

Heat a large saucepan of salted water over high heat to boiling. Add the rice and boil, uncovered, until just tender but still firm, about 15 minutes; taste a few grains to check. Drain and rinse under cold running water. Drain about 15 minutes.

To cook the shrimp, heat 2 cups water in a medium saucepan until simmering. Add a pinch of salt. Add the shrimp and simmer until pink, about 1 minute. Drain thoroughly. Shell the shrimp and devein if necessary. Cut half the shrimp in three pieces each; leave the remaining shrimp whole.

Using a vegetable peeler, peel the strings from the celery. Cut the celery in thin slices.

Combine the rice, shrimp pieces (not whole shrimp), celery and parsley in a large bowl; mix well.

Make vinaigrette dressing by combining the vinegar, olive oil and salt and pepper to taste in small bowl; whisk thoroughly. Mix 1 tablespoon dressing with the whole shrimp.

Add the mayonnaise to the remaining dressing. Add to the rice mixture; mix gently. Taste for seasoning. Cover and refrigerate at least 1 hour. The salad may be prepared to this point up to two days in advance; refrigerate, covered.

To serve, transfer to bowl or platter and decorate with whole shrimp. Serve cold or at room temperature. RED SNAPPER AND PASTA SALAD WITH RED PEPPERS (4 appetizer or 2 main course servings) 1 red bell pepper 1/4 pound fresh or dried fettuccine 2 tablespoons chopped parsley 3/4 pound red snapper fillets, in 4 pieces Salt and pepper Garlic Dressing: 2 large cloves garlic 1 egg Salt and pepper 1/2 cup olive oil 2 tablespoons white wine vinegar

To make the dressing: In a food processor, chop the garlic until very fine. Add the egg, salt, pepper and 1 tablespoon olive oil and process until blended. With the blades of the processor turning, pour in the remaining olive oil in a very fine stream. Last, pour in the vinegar gradually. Taste for seasoning.

Halve the pepper lengthwise and remove the core and the thick ribs. Halve the pepper again crosswise and cut it in short strips. Put the pepper strips in a medium saucepan of boiling salted water and boil for 1 minute. Drain, rinse and drain thoroughly.

Add the pasta to a large pan of boiling salted water. Cook uncovered over high heat about 2 minutes for fresh pasta or 7 minutes for dried, or until just tender but still al dente or slightly firm to the bite; check by tasting. Drain, rinse under cold running water and drain thoroughly.

Reserve about 1/4 cup red pepper strips for garnish. In a large bowl, toss the pasta with the remaining pepper strips and the chopped parsley. Add half the dressing and taste for seasoning. Transfer to a serving platter.

Bring water to a boil in the base of a steamer. Season the red snapper pieces lightly with salt and pepper on both sides. Put them in one layer in the top part of the steamer above the boiling water. Cover and steam about 3 minutes or until just tender. Drain quickly on paper towels.

Set the pieces of snapper on top of the pasta and coat them with the remaining dressing. Garnish them with reserved pepper strips. Serve warm or at room temperature.