Mussel power, muscling into the clam's territory. Maine mussels now an economical food

Clams - steamed, on the half shell, fried, and in chowders - have always been a favorite American food, but mussels, recently discovered by a lot of people, may soon surpass clams in popularity.

In San Francisco, seafood markets are rushing to keep up with the demand. Wholesale distributor Sal Balestrieri of Fisherman's Wharf says his volume of fresh mussels increased 100 percent last year.

Chris Heisler, who teaches clambake cooking at the University of Rhode Island every summer, says his students like mussels better than clams even though clams are traditional in this outdoor feast.

Roger Berkowitz of Legal Seafoods, Boston, says his restaurant has had mussels as an alternative to clams on their clambake menu for a couple of years.

''The popularity of mussels has skyrocketed, with sales about 75 percent to that of clams,'' he said.

Japanese-Americans like them, too. At the Nippon Club in New York City recently, a platter of mussels on the half shell disappeared completely before members, including children, took any interest in a platter of steamed clams at the same buffet table.

And in Cleveland, at Pamela's Restaurant, chef-owner Pamela Grosscup says customers who order mussels want them because they know they like them.

But at special parties Pamela caters, many people try her Smoked Mussels, or Mussels Vinaigrette, or the ones she makes in tiny puff pastry shells. These people are usually eating mussels for the first time, and they really enjoy them , she says, noting that these mussels also disappear quickly.

Why such a sudden increase in the popularity of this shellfish?

It's because blue mussels are now being cultivated commercially and are more available. They are uniform in size, low in price, and they arrive at market clean and scrubbed, a big improvement over wild ones.

Straight off the rocks, wild mussels are covered with barnacles and contain gravel, sand, and rock pearls. There's also a bunch of stringy fibers called the ''beard'' that must be removed with a quick tug.

For the home cook, cleaning is a problem. But for the restaurant owner, rock pearls are anathema. Having one customer bite on a tiny stone can mean trouble.

Mussels grown on aquatic farms produce a far greater quantity and quality of ''meat'' than those growing wild on the rocks of the nearby ocean.

Until recently, most aquatic farming in the United States has been for lobster, shrimp, trout, oysters, and salmon - all luxury foods.

Mussels, along with the catfish that are also now flourishing on aquatic farms, are a seafood that's available to everyone at a reasonable price.

Chip Davidson and Frank Simon, founders in 1978 of the Great Eastern Mussel Farms in Tenants Harbor, Maine, raise the edible blue mussel (Mytelus edulis), from deep-water beds along the Maine coast.

Selling mussels was not easy at first, they told me. Highly regarded in many other countries, mussels have been neglected here except among some ethnic groups. But the company's sales have increased from 120 bushels to 3,000 bushels a week.

Farm-raised mussels are grown on specially prepared beds in deep rocky gorges off the coast of Maine, where moving tides help keep the water from freezing. More than 150 acres of ocean is leased from the state by Great Eastern Mussel Farms.

After a two-year growing period, the mollusks are harvested. The company will ship more than 5 million pounds of mussels this year.

Mussels are also farmed in Washington, California, and Rhode Island, as well as several areas in Maine.

But many people still don't know what mussels are, what to do with them, or how they'll taste. Here are some of the answers.

Compared with clams, they provide in excess of three times more meat per pound. Mussels are firmer overall, but they do not have the tough necks of clams , and there is no neck covering to be removed. They're available both frozen and fresh in most supermarkets and are plumpest and best from September through May.

Mel Pell of Great Eastern Mussel Farms says the price is low, about $1.19, although wild mussels are often lower. There are about 20 mussels in a pound.

Pell has cooked them for groups of restaurant chefs, fish-market owners, wholesalers and distributors, and others who are interested but unsure what mussels are really like.

''Cooking them is surprisingly easy,'' he says. ''They open by themselves during cooking and are good either hot or cold. They can be substituted for clams in clam recipes.

''You can serve just 10 mussels per person for a first course or allow 1 pound per person for a main course.

''Mussels are highly perishable. Keep in the refrigerator with temperatures between 32 and 40 degrees F. and use within 24 hours,'' he says.

Mussels can be steamed and served plain with butter, with an herbed broth, or lavish with cream and curry. They can be baked; pickled; or stuffed with seasoned crumbs. Here are some recipes: Steamed Mussels 2 pounds mussels 1/4 cup chopped onion 2 cloves finely chopped garlic 2 tablespoons olive oil, butter, or margarine 1 cup chicken broth or water Freshly chopped parsley

Rinse mussels, remove beards with a sharp tug or with scissors. Saute onion and garlic in oil in a large kettle over medium heat for about 1/2 minute.

Add mussels and liquid, then cook over high heat about 4 minutes, or until shells open. Shake pot so mussels cook evenly. Serve garnished with parsley in soup plates with some of the juices and plenty of crusty bread for dipping.

Serves 4 to 6 as an appetizer, or 2 to 3 for a main course.

Variation: Add 2 cups drained, chopped fresh or canned tomatoes during cooking to make a sauce for spaghetti, linguini, or rice.

Add thyme, oregano, basil, or hot red pepper to taste. Sprinkle with parsley before serving. Mussels With Chinese Dipping Sauce 2 pounds mussels 1/4 cup chopped onion 2 cloves finely chopped garlic 2 tablespoons olive oil, butter, or margarine 4 tablespoons soy sauce 4 tablespoons rice wine vinegar 2 teaspoons chopped ginger root 2 teaspoons chopped garlic 2 teaspoons chopped scallion

Steam mussels as in above recipe. When cool, remove meat from shells and save half the shells for decoration around edge of serving plate.

Combine remaining ingredients and serve in individual dipping saucers. Offer food picks for dipping mussels. Pamela's Mussels Vinaigrette 5 pounds steamed mussels 1 egg yolk 1 tablespoon lemon juice 1 or 2 tablespoons balsamic vinegar 1 tablespoon mustard, Dijon style 1 teaspoon finely chopped garlic 1 tablespoon finely chopped shallot 1/4 cup hazelnut oil 1/2 cup olive oil, approximately 2 tablespoons half-and-half cream 1 tablespoon finely chopped fresh tarragon, or a pinch of fresh thyme 1/8 teaspoon salt (optional)

At Pamela's Restaurant, the mussels are poached with bay leaf, a little carrot, celery, and shallots. The broth is saved for another use.

Combine and mix egg yolk with vinegars, lemon juice, mustard, garlic, and shallots until it thickens slightly, then add oils in drops according to taste. The proportion should be about 3 parts oil to 1 part vinegar. Add cream.

Serve mussels on lettuce with a sprig of watercress, spoon vinaigrette over top of mussels.