When you cook a piece of fish conventionally, the heat causes a number of physical changes in the flesh. The proteins in the muscle tissues coagulate, like an egg in a skillet, tightening and giving the fish a firm texture. White-fleshed fish change from translucent and pearly to opaque and white, while salmon changes from a vividly dark hue to a milder pink. Acidity affects the muscle tissues in exactly the same ways. A piece of fish or shellfish soaked in lime juice or other acids will become firm and opaque, just as it would if cooked by heat.

This pseudo-cooking method gives fish a lighter, more delicate texture than conventional cooking. That makes it an ideal technique with shrimp and scallops, which are easily -- and all too frequently -- marred by overcooking. A relatively brief bath in this marinade complements the sweetness of the shellfish with the bracingly clean acidity of the limes. Small shrimp can be marinated whole, while larger shrimp are often butterflied to speed the action of the lime juice. Similarly, small scallops can be marinated whole, but large specimens are usually sliced into thinner discs.

Marinating shrimp or scallops at home is simple enough for even inexperienced cooks, yielding a sophisticated appetizer or light meal. Mix lime juice and other marinade ingredients in a glass or stainless-steel bowl, which resists chemical reactions that might otherwise discolor the fish. Most ceviche marinades include some salt and other spices, such as crushed chili flakes or fresh hot peppers. For small shrimp or sliced scallops, marinating time can be as little as 10 to 20 minutes. Larger pieces might require as long as two hours. Taste a sample piece periodically, and drain your shellfish when they reach your preferred texture and intensity of flavor.

The relatively high acidity in ceviche marinade provides a modest level of protection against bacteria and foodborne illness, but it's not as effective as cooking. Always buy the highest-quality fish you can find, either very fresh at seaside markets or sushi-grade frozen fish. Wash your hands, utensils and work surfaces scrupulously before you start, and keep the fish and the prepared marinade in your refrigerator until you combine them. Once the shrimp or scallops are in the marinade, refrigerate them again until mealtime. To serve, drain the marinade and arrange the seafood on a serving platter or individual plates. Discard any that's uneaten after two hours.