If you find that cooked bivalves often turn out a bit tough, you might want to try this, which worked well on smaller chicken venuses (Chamelea gallina) collected near Plocton on the recent Skye field meeting - delicious and tender.
A) Scrub and rinse the shells.
B) Place them in a layer only one or at most two shells deep in a pan or bowl.
C) Pour on boiling water about three times as deep as the shells (i.e. lots of water in relation to the amount of shell, so that they heat up quickly).
D) Leave for 10-15 minutes.
E) Pour the hot water off and eat.
It's probably even more important than usual to make sure that the place you collect the shells isn't polluted, as the shellfish are cooked relatively lightly; and for the same reason it's probably best not to use it on very large shells as they may not cook right through.
The same method worked well also with winkles (see photo); and even better on another occasion with some fresh fairly small carpet shells (Venerupis phillipinarum) from Poole Harbour, when we started by lightly frying an onion in butter in a saucepan, pouring on a little white wine, then turning the plate off, adding the shells as in B above, and then pouring on boiling water etc. as in C-E.
Enjoy ...