Like parvalbumins, SCPs seem to be more abundant in fast contracting muscles, but no functional relationship can be established from this distribution. SCPs from crayfish, lobster, and shrimp are polymorphic dimers. The sarcoplasmic calcium-binding proteins are abundant in the muscle of arthropods, mollusks, annelids, and protochordates. This protein has three functional calcium-binding sites; potential site 4 has lost affinity for calcium.