Cockle soup is now usually cockles in a creamed base, commonly with celery, onions, parsley, and very often with mushroom. There is a tradition that the soup should be made with either seawater or the depurating water in which the fresh cockles have been soaked, but this is not now recommended. (British Food Trust, etc)

The very large number of literary references from the 18th Century onwards suggest that this soup was formerly much more popular. Charles Dicken's magazine "Household Words" of 1882 has; "who does not know cockle soup?".

Original Receipt in Domestic economy, and cookery, for rich and poor, by 'A Lady' (1827);

Oyster, Cockle or Muscle Soup with Mushrooms
Prepare meat vegetable or fish stock and season it well without salt as the juice of the shell fish generally makes it salt enough. Boil the parings and stalks of the mushrooms in it where oysters cannot be afforded to be rubbed down for a thickening rub down the beards with some mushrooms and if necessary some white meat or fish and panada farce may also be made of the fish. If the oysters are very large they ought to be cut in two. Nothing is more vulgar than large mouthfuls and every thing in the soup should be nearly the same size. Rub the thickening through a tammy with a little of the soup. Every quart of soup will require half a pint of oysters more or less according to the size and the quantity of mushrooms used. Iif they are not ail buttons of the same size they ought to be cut and should be cooked white in butter or cooked slowly in the stock as they take a long time All fish soups may be heightened with ketchups anchovies lemon pickle soy &c