Creature whose species include the world's largest bivalve molluscs (4)
I believe the answer is:
clam
(Other definitions for clam that I've seen before include "Large bivalve mollusc" , "Aquatic creature" , "Bi-valve with firmly-closed shell" , "Mollusc, one proverbially reticent" , "ringing noise produced by > very reticent type" .)