mussels
Blue mussels, black-shelled sea mussels with tender orange to pale flesh and a mild, briny-sweet flavour.
Caution: Mussels that are open before cooking and do not close when tapped, or that have cracked shells, are dead and should be discarded.
Fresh mussels are sold in season, roughly autumn to spring, at well-stocked supermarkets and fishmongers; outside that window they are mostly found frozen or vacuum-packed and pre-cooked. Buy fresh mussels alive and cook them the same or the following day.
Substitute:
Clams such as carpet shell or Manila clams work in most Spanish and Portuguese dishes and give a similar broth, though they are smaller and more delicate; frozen mussels will do but release less of their own liquor.