Cockles are one group of bivalve (twin-shelled) molluscs. Mussels are another, oysters another, razorshells another.
Generally, bivalves living buried in mud or sand are called clams -- and this includes some with other special names too, such as cockles, venus shells, razorshells etc. There are some which are just called clams.
They all taste somewhat different, but I think that will just make your recipe interesting.
I am a clam and I knew a cockle once, pretty little thing on baker street, when I was just a young clam, never forget that little cockle on baker street................